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Cambridge A-Level Physics 2018 Report

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Cambridge A-Level Physics 2018 Report

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Cambridge International Advanced Subsidiary and Advanced Level

9702 Physics June 2018

Principal Examiner Report for Teachers

PHYSICS

Paper 9702/11
Multiple Choice

Question Question
Key Key
Number Number
1 A 21 A
2 C 22 B
3 C 23 D
4 A 24 C
5 C 25 B

6 A 26 A
7 B 27 B
8 A 28 D
9 B 29 B
10 A 30 C

11 D 31 A
12 D 32 D
13 C 33 A
14 C 34 C
15 D 35 B

16 B 36 B
17 C 37 D
18 B 38 C
19 B 39 D
20 B 40 D

General comments

Candidates should take particular care to double-check differences between, for example, 60 mm and 60 cm,
and between 4.5 × 103 Hz and 450 Hz. An error between, say, 4.6 m s–1 and 4.6 km s–1 will give an unrealistic
value for something like the speed of a cyclist. It should be possible to see from this that an error has been
made. Similar mistakes can occur when using a calculator. It is always advisable to put any calculation into
a calculator twice; if different answers are found, one of them must be wrong. This only takes a few seconds
and it will usually confirm that no mistake has been made in using the calculator.

Candidates found Questions 6, 20, 21, 22, 24, 32 and 37 difficult. They found Questions 1, 7, 13, 31 and
38 to be relatively straightforward.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Comments on specific questions

Question 4

A common incorrect answer to this question was B. This scale has the right direction but the spacing of the
markings is incorrect. At low temperatures, the resistance changes rapidly with temperature so the markings
on the ammeter need to be far apart. As the temperature increases, the resistance changes less and the
markings need to become closer together.

Question 6

Answers B and D were commonly chosen by weaker candidates. Both of these show a ball that has a non-
zero speed at time t = 0 which then decreases as t increases, so these cannot represent the motion of a ball
falling freely in air.

Question 20

Many weaker candidates chose C and D. The horizontal axis of the graph is time so XY cannot be the
wavelength.

Question 21

This question required recall of the electromagnetic spectrum. Weaker candidates often chose C. If
candidates are more familiar with wavelengths, they could have calculated the wavelength (30 µm) and it
may then have been clearer that the wave must be infra-red.

Question 22

Just before point B the particle is moving away from the source (towards B), and just after point B it is
moving towards the source (also towards B), so B must be the centre of a compression. Point D is the
centre of a rarefaction. The other two points are midway between centres of compression and rarefaction.

Question 24

When the rate of rotation increases, the speed of the star increases both as it approaches and as it recedes.
This must increase the maximum observed frequency and reduce the minimum observed frequency.

Question 26

Candidates needed to read this question carefully. There is a path difference of 8 cm (4 wavelengths)
between S1P and S2P, and this led many candidates to think that constructive interference takes place at
point P. However, there is a phase difference of 180° between the oscillations of the two sources, so the
interference is destructive at P.

Question 32

To get the correct answer, candidates needed to realise that if the length is doubled and the volume is
constant, then the area of cross-section must be halved. Using the expression ρL / A for resistivity then gives
the answer as D.

Question 37

Each wire has a resistance of 4.0 Ω. There is a current of 0.6 A and therefore a potential difference across
each wire of 2.4 V. The required output p.d. must be 16.0 + 2 × 2.4 = 20.8 V. Candidates choosing C had not
considered that there are two wires.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/12
Multiple Choice

Question Question
Key Key
Number Number
1 C 21 D
2 B 22 B
3 D 23 D
4 D 24 A
5 A 25 D

6 C 26 C
7 C 27 D
8 D 28 A
9 C 29 C
10 D 30 D

11 C 31 A
12 B 32 A
13 A 33 B
14 C 34 B
15 D 35 A

16 B 36 A
17 C 37 C
18 C 38 C
19 B 39 B
20 D 40 B

General comments

Candidates found Questions 6, 7, 13, 28 and 33 difficult. They found Questions 2, 3, 22, 23, 31 and 37 to
be relatively straightforward.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Comments on specific questions

Question 1

Many weaker candidates chose D. These candidates used the radius of the atom rather than its diameter
when finding the number of layers.

Question 6

Candidates choosing either B or D had neglected the horizontal motion of the particle. The speed is never
zero because there is a constant component of velocity in the horizontal direction, so neither of these graphs
can represent the speed.

Question 7

Many candidates chose A. This cannot be correct because the non-zero gradient at time t = 0 shows a non-
zero velocity. Only graph C has a zero velocity at time t = 0.

Question 11

Many candidates chose A or B, but the Moon has no atmosphere so the horizontal force must be zero.

Question 13

Candidates found this question difficult. As the person moves towards the rods, the moment of the person’s
weight at both P and Q decreases, and therefore the forces exerted by the rods at both P and Q must also
decrease.

Question 24

Candidates should be encouraged to sketch graphs to help with answering this type of question.

Question 28

The vibrations at points X and Y are in phase. The pattern of movement at Y will therefore be the same as at
X, but the maximum amplitude will be lower because X is at an antinode and Y is not, so A must be correct.
The three incorrect answers were chosen by candidates with similar frequency.

Question 33

Candidates needed to look at the axes carefully. Many candidates chose A, perhaps thinking that the graph
had been plotted with voltage on the horizontal axis.

Question 37

Stronger candidates did not find this question difficult, but many weaker candidates did. A common answer
from weaker candidates was A. Candidates may find this type of question easier to answer in stages. If the
current is the same in the thermistor and resistor, then V / R must be the same for these two components.
Therefore 7.5 / RT = 4.5 / 150 and this gives RT = 250 Ω, which is C. This structured approach may be less
prone to mistakes than trying to remember more complex equations for potential dividers.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/13
Multiple Choice

Question Question
Key Key
Number Number
1 B 21 D
2 B 22 B
3 A 23 D
4 A 24 B
5 D 25 D

6 B 26 A
7 C 27 A
8 B 28 B
9 C 29 C
10 A 30 A

11 D 31 B
12 D 32 D
13 B 33 B
14 A 34 A
15 D 35 C

16 B 36 D
17 B 37 D
18 D 38 D
19 C 39 C
20 C 40 A

General comments

Candidates found Questions 14, 16, 18, 30 and 38 difficult. They found Questions 6, 12, 23 and 34 to be
relatively straightforward.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Comments on specific questions

Question 10

For candidates who understand upthrust, this question is very straightforward. The upthrust depends on the
pressure difference between the top and the bottom of the cuboid. It cannot depend on what is inside the
cuboid, so the correct answer must be A.

Question 14

Candidates found this question difficult. The pressure inside the cylinder varies in a way that cannot be
determined from the information given, and the frictional force acting on the piston is also unknown. We do
know that atmospheric pressure is constant and the work done against the atmosphere must be given by
(p∆V =) atmospheric pressure × Ar.

Question 16

It is important to read this type of question carefully. It is relatively straightforward to see that A and C
cannot be the answer. Some candidates chose D, but this cannot be the answer because some work is
done against air resistance as the ball moves through the air.

Question 18

This question required care to be taken, both with powers of ten and with the distinction between radius and
diameter.

Question 24

A loud sound is heard when there is a node at the surface of the water and an antinode at the top of the
cylinder. Between two of these loud sounds, the water level must have changed by half a wavelength so
that there is one more node in the stationary wave pattern. This gives (67.3 – 2.9) = 64.4 cm = ½λ. The
frequency is then f = 330 / λ (= 256 Hz).

Question 30

This question required care with powers of ten. Stronger candidates did not find it difficult but many weaker
candidates chose B or C.

Question 35

Answers B and D were both common. Candidates needed to take a logical approach: first, they needed to
use the information given to find the resistance R of the unknown resistor: 1 / 2.5 = 1 / 5.0 + 1 / (2R). This
gives R = 2.5 Ω. The value can then be used to find the resistance between X and Y.

Question 38

Answers B and C were both common. The resistances of the resistors must be in the same proportion as
the lengths of the resistance wire, so V1 / V2 = x / (L – x).

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/21
AS Level Structured Questions

Key messages

• Candidates should be encouraged to recall definitions in precise detail. The omission of a key word
may prevent credit from being awarded.

• The command words used in a question should be carefully considered. A glossary of command words
in provided in the syllabus. When candidates are asked to ‘state and explain’, it is essential that an
explanation is included in the answer as well as a final statement.

• Candidates should be made aware that prematurely ‘rounding off’ any intermediate answers in the
middle of a numerical calculation can lead to an inaccurate final answer.

• In numerical calculations it is important that candidates always explicitly state the subject of any
equations. If an equation is rearranged, the new subject should also be stated.

General comments

A single word can make a difference to a definition or an explanation. For example in Question 2(a)
candidates used a variety of words/phrases such as force, external force, balanced force and resultant force.
Only resultant force is acceptable as part of the answer in Question 2(a). In Question 3(b)(iv) ‘...different
relative speeds...’ is correct, whilst ‘...different speeds...’ is not. In Question 4(a)(ii) ‘distance between two
consecutive peaks’ is acceptable, whilst ‘distance between two peaks’ is not. In Question 5(a)(ii) ‘waves
with a constant phase difference’ is correct, whilst ‘waves with the same phase difference’ is not. All these
examples were seen frequently, showing that some candidates would benefit from providing greater
precision in their answers.

In Question 6, many candidates found it difficult to apply their understanding of basic circuitry to the
resistances in series and parallel. In Question 7(c), many candidates were not able to explain the variation
in path taken by beta particles in an electric field due to the particles having a range of velocities when
emitted from a nucleus.

Candidates should be encouraged to read questions carefully to ensure their answers are appropriate in
context, rather than simply repeating memorised extracts.

There was no evidence that candidates were short of time to finish their answers.

Comments on specific questions

Question 1

(a) The majority of candidates made a good start and gave acceptable explanations of scalar and
vector quantities.

(b) The majority of the candidates were able to answer correctly for at least two of the quantities listed.
There were many who incorrectly considered power to be a vector and a few gave temperature as
a vector.

(c) (i) A large proportion of the candidates drew a vector for the resultant velocity in a direction that
suggested they had given insufficient thought as to how the aircraft velocity and the wind velocity

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

would combine. In this case the aircraft velocity was the major factor when considering the effect
of the wind on the resultant direction of the aircraft.

(ii) Candidates found this question difficult. Many candidates assumed that the angle between the
wind velocity and the aircraft was 90° and used Pythagoras’ theorem. A considerable number used
the cosine rule with the included angle being 65°. There were many incorrect vector diagrams. A
significant number resolved the wind velocity into a north direction only.

A small number of candidates used a scale diagram, usually with some success. A significant
number obtained the correct answer either by using the cosine rule with the included angle being
115° or by resolving the wind velocity into two components parallel (to the west) and perpendicular
(to the north) to the aircraft velocity, and then using Pythagoras’ theorem to determine the
resultant.

Question 2

(a) The wording given by many candidates was too imprecise. Instead of including constant velocity
and resultant force, common answers included uniform motion and just force, balanced forces or
an external force. When using speed instead of velocity candidates tended to omit ‘in a straight
line’.

(b) (i) 1. The majority of the candidates attempted to calculate the area under the line of the velocity-time
graph. Many did not apply the vector nature of velocity and displacement and so added, rather
than subtracted, the two areas and then gave the direction as upwards.

A significant number misread the velocity from the graph. At time t = 0, instead of –6.8 m s–1, either
–7.0 m s–1 or –7.2 m s–1 was used, and at t = 3.0 s, instead of 3.4 m s–1, 3.2 m s–1 was used.

2. This question was generally well answered. The majority of the candidates used the expression for
the gravitational potential energy and their answer to (b)(i) part 1, and many candidates were
awarded full credit.

(ii) The acceleration was determined correctly by the majority of candidates. Many determined the
gradient of the graph. Others attempted to use more complicated equations of constant
acceleration, but sometimes made errors with the signs of the velocities used. There were some
who used too small a triangle for the gradient and this led to inaccuracy in the value of the
acceleration.

(iii) The stronger candidates were usually awarded full credit. The weaker candidates were unable to
progress from the equation F = ma or left this part blank. The difference between the tension and
the weight gives the resultant force. Some candidates did not realise that the resultant force and
the acceleration are in the same direction and gave 15 – T = 1.53 × 3.4. A number of candidates
began with an incorrect expression and then manipulated the mathematics to arrive at an answer
of 20 N, but this approach could not be awarded credit.

(iv) There were many correct answers. Common errors were due to using the weight and not the
tension when calculating the stress, or not rearranging a correct expression for the Young modulus
to be in terms of the strain.

(v) The majority of candidates found this part challenging and there were very few complete answers.
Some thought that, because the question referred to the block moving vertically, they had to
explain what happens to the horizontal motion of the block. Few mentioned that the block was in
equilibrium or that the resultant force was zero, and a smaller number realised that this meant the
block could either be stationary or moving with constant velocity.

Question 3

(a) The majority of candidates correctly referred to the quantity of matter. Some candidates used
incorrect terms such as amount of substance or the number of molecules. A significant number
described mass in terms of weight and gravitational field strength, whilst others expressed it in
terms of density and volume. These expressions are used to define gravitational field strength and
density respectively and are not appropriate here.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(b) (i) Many answers did not contain enough detail to gain full credit. For example, candidates often did
not make it clear that the force on A is equal and opposite to the force on B or that force is the rate
of change of momentum. Weaker candidates referred to ‘action and reaction’ and related these
terms directly to momentum. This is not acceptable for Newton’s third law at this level.

(ii) This question was generally well answered. A common error was to give the wrong sign for the
momentum of block B before the collision. The relationship between the total momentum before
the collision and the total momentum after the collision was known by the majority of candidates.

(iii) Common errors were to subtract the values for the speed of approach and to add the values for the
speed of separation.

(iv) Many candidates explained their answer in terms of kinetic energy without reference to relative
speeds. Some candidates referred to there being a difference in the total momentum which shows
a misunderstanding of the physics of collisions.

Question 4

(a) (i) The majority of candidates gave a correct answer in terms of oscillations or cycles. The time for a
wave or a wavelength was a common answer that was considered too vague to warrant credit.

(ii) There were many answers that were given with insufficient detail. Some candidates wrote that the
wavelength is the length of a wave. Many candidates wrote in terms of the distance between two
crests, troughs or points oscillating in phase, but neglected to state that these must be adjacent.
Some suggested that wavelength was the shortest distance between ‘similar’ points without
explaining what was meant by ‘similar’.

(b) (i) The majority of candidates obtained the correct answer. Some made an error in rearranging the
expression for the wavelength in terms of the time period and the speed of the wave. A significant
number obtained the wrong value for the time period from the graph, with 0.30 s and 0.80 s being
common incorrect answers.

(ii) Only the strongest candidates were able to determine the phase difference.

(iii) Most candidates obtained full credit. A few candidates gave an answer to only one significant
figure. A small minority did not know the correct relationship between intensity and amplitude.

(iv) A significant number of candidates gave the correct answer. Some gave incorrect answers by not
taking into consideration the vector nature of displacement. The two displacements at 0.45 s were
often added or the final sign for the displacement omitted. A further common error was to misread
the time axis for 0.45 s and give the difference in displacement as –2.6 mm (the difference at 0.50
s).

Question 5

(a) (i) Many answers would have benefited from greater detail. The majority of candidates described the
waves as spreading or said that the waves needed to pass through a slit, although very few said
both. There were some answers that described refraction with the waves bending.

(ii) Some candidates gave the correct explanation. There were many incorrect answers that gave
descriptions of waves with the same phase or same phase difference. Other answers that were
not accepted described waves that had the same wavelength, the same frequency or the same
amplitude.

(b) (i) Most candidates could give the correct diffraction grating formula. The algebra that followed to
show the ratio of the wavelengths was often incorrect or not shown in sufficient detail. One of the
important aspects of a ‘show that’ question is that candidates are expected to show all the relevant
steps of the calculation. There were many candidates who missed out key points such as labelling
which n or which λ they were using in subsequent expressions. Another common error was to
assign 90° to θ and make sin θ = 1.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(ii) This was a difficult question. The strongest candidates were able to obtain the correct answer. A
significant proportion did not give any response.

Question 6

(a) There were very few candidates who gave the correct answer. Many gave a definition of potential
difference or stated ‘current × resistance’ or ‘ampere × ohm’.

(b) (i) The correct current was determined by a significant number of candidates. Only a small number
explained their working by stating that the potential difference (p.d.) across each lamp was the
same because the lamps were connected in parallel. Some candidates ignored the value given for
the current in lamp P and determined the currents through each lamp at a p.d. of 4.0 V. Others
added the current at 4.0 V for lamp Q to that given for lamp P at 2.7 V.

(ii) Many candidates incorrectly used the e.m.f. of 4.5 V for the p.d. across resistor R.

(iii) The majority of the candidates could give the relevant symbol formula for resistivity. A smaller
number calculated the resistance of each lamp. Many candidates assumed that the resistance of
each lamp was the same and determined a ratio that depended only on the areas of the wires used
for the lamps.

(iv) Candidates found it difficult to explain the changes in the circuit due to the filament wire of lamp Q
breaking. The majority of candidates assumed that the p.d. across P would not change and stated
that the resistance and current were inversely related. Only a very small number stated that the
increase in current in P would cause the resistance of P to increase. A smaller number stated that
the p.d. across P would increase.

Question 7

(a) The majority of candidates drew an arrow in the vertical direction, although many drew it in the
wrong direction. Others drew arrows that were horizontal or along the path of the particle. A few
candidates drew symbols representing magnetic fields.

(b) Most candidates were able to link the energy in eV to the symbol formula for kinetic energy. The
majority of answers were fully correct. Some did not convert the energy given in eV into joule.

(c) A small minority of candidates described a range of paths for the beta particles due to their having
a range of kinetic energies. Many candidates misinterpreted this question: having read ‘outside the
electric field’ in the first sentence they then did not register the phrase ‘inside the electric field’ in
the second sentence and framed their answer in terms of particles not experiencing an electric
force. Another common error was to state that all beta particles have the same mass and charge
and so follow the same path, without realising that the particles have a range of speeds.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/22
AS Level Structured Questions

Key messages

• Candidates should be encouraged to recall definitions in precise detail. The omission of a key word
may prevent credit from being awarded.

• Candidates should be made aware that prematurely ‘rounding off’ any intermediate answers in the
middle of a numerical calculation can lead to an inaccurate final answer.

• In numerical calculations it is important that candidates always explicitly state the subject of any
equations. If an equation is rearranged, the new subject should also be stated.

General comments

In general, candidates would benefit from improving their understanding of the Doppler effect, stationary
waves in air columns and solving d.c. circuit problems. They found Questions 3(e), 4(b) and 6(b)
challenging.

The candidates did not have any difficulty understanding the language used in the questions and there was
no evidence of candidates lacking time to complete the question paper.

Comments on specific questions

Question 1

(a) The most common incorrect definition of force was ‘the product of mass and acceleration’. Other
incorrect definitions included ‘a push or a pull’ and ‘the rate of change of momentum per unit time’.

(b) This part of the question was very well answered. The most common incorrect response was
‘newton’.

(c) A significant number of answers incorrectly stated the base units of charge as either C or A s–1.
Care needed to be taken when combining the indices of the base units. A common error in the
working was to cancel s2 in the numerator with s–2 in the denominator.

Question 2

(a) The majority of the candidates were awarded full credit. It is important that candidates explicitly
refer to total momentum remaining constant when there is no external resultant force. Some weak
candidates confused the terms moment and momentum.

(b) (i) Different methods of calculation were possible. Almost all the candidates chose the most
straightforward method, which was to consider conservation of momentum in the direction
perpendicular to line AB. This method of calculation was usually done successfully, although some
candidates calculated components parallel to line AB when they should have been finding

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

components perpendicular to line AB. A few candidates chose to do the calculation using the
cosine rule.

(ii) The majority of answers were correct. A small proportion of the candidates stated the correct
symbol formula for the kinetic energy of an object, but then forgot to square the speed when
substituting the numerical values into the formula.

Question 3

(a) Most answers were correct.

(b) The distance moved up the slope and the distance moved down the slope were usually calculated
correctly. A common error was to add, rather than subtract, these two distances. Candidates who
made this error seemed to have overlooked that the question asks for the displacement of the
sledge, not the total distance travelled by the sledge.

A small proportion of the candidates incorrectly assumed that the acceleration of the sledge
remained constant over the entire range of time and so attempted to calculate the displacement in
one step using a single equation of uniform acceleration.

(c) The majority of the answers were correct. Most candidates chose to calculate the acceleration
from the gradient of the graph. Candidates who chose to use an equation of uniform acceleration
needed to be careful with the signs of any numerical values substituted into the equation.

(d) (i) The resultant force acting on the sledge was usually calculated correctly, although a very small
number of candidates used an incorrect acceleration of 9.81 m s–2 instead of the correct
acceleration of 0.50 m s–2. The value of the resultant force was then usually subtracted from the
component of the weight down the slope, although some candidates made the mistake of adding
these two forces.

(ii) Most candidates understood how to calculate the angle of the slope. The most common errors
were either using the wrong trigonometric function in the calculation or substituting an acceleration
of free fall of 10 m s–2 instead of 9.81 m s–2.

(e) (i) Most candidates realised that they needed to use the Doppler effect equation listed in the Formulae
sheet. Many candidates found it difficult to apply the equation. Common errors included
substituting the speed of the sledge with the wrong sign or reading the wrong magnitude of speed
from the graph. A significant number of answers confused the observed frequency with the source
frequency.

(ii) Only the strongest candidates realised that the man would hear an increasing frequency because
the sledge is moving away from him with a decreasing speed. Many did not take into account the
deceleration of the sledge and incorrectly believed that the man must hear a decreasing frequency
simply because the sledge is moving away from him.

Question 4

(a) (i) The most common correct definitions were ‘the distance between two consecutive crests/troughs’
and ‘the minimum distance between two crests/troughs’. When choosing to define wavelength this
way, it is important to include either the word ‘consecutive’ or the word ‘minimum’. Some
candidates said that it was the minimum distance between two similar points, but did not explain
what was meant by ‘similar’. The weakest candidates sometimes incorrectly defined wavelength
as wave speed divided by wave frequency.

(ii) An antinode is a position where the stationary wave has maximum amplitude. Candidates should
be encouraged to describe nodes/antinodes in terms of amplitude rather than displacement.

(b) (i) Most answers gave the correct symbol equation that relates speed, frequency and wavelength.
Many candidates did not understand the wave profile of the stationary wave in the air column and
so tended to substitute an incorrect value of wavelength, such as 4.5 cm or 9.0 cm, into the
equation.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(ii) A significant number of candidates did not attempt this question. Others did not realise that the
distance moved by the piston is equal to half of a wavelength. Very weak candidates sometimes
confused the speed of the piston with the speed of sound, or stated the period of the wave as the
final answer.

Question 5

(a) The majority of answers were fully correct.

(b) In ‘show that’ questions, credit is given for showing the working as well as the final answer.
Candidates need to carefully present all the steps of their calculation. Two methods of calculation
were seen. The most common method was to calculate the pressure due to the oil on the bottom
face of the cylinder and then to multiply this pressure by the area of the face. The other method
was to equate the upthrust to the weight of the oil that is displaced by the cylinder.

(c) (i) This question was often well answered. A minority of the candidates either did not take into
account the upthrust on the cylinder or incorrectly added the upthrust to the cylinder’s weight.

(ii) The relevant symbol equation was generally well known and usually applied correctly.

(d) (i) Many candidates of average ability were able to explain that the upthrust on the cylinder decreases
as the cylinder is raised. The stronger candidates then went on to explain successfully that the
tension or lifting force from the motor increases so the power output of the motor also increases.
Common misconceptions held by weaker candidates were that the resultant force on the cylinder
increases or that the weight of the cylinder increases.

(ii) Candidates found this question difficult. Incorrect reasons included the inefficiency of the motor
and the viscous force due to the oil.

Question 6

(a) (i) Candidates needed to refer to the sum of the currents entering a junction being equal to the sum of
the currents leaving the junction. No credit was given for statements that omitted ‘sum’. Very weak
candidates sometimes confused Kirchhoff’s first and second laws.

(ii) There were many incorrect quantities stated, such as energy and voltage.

(b) (i) 1. The symbol formula for power was usually stated correctly. However, the numerical values
substituted into the formula were often incorrect. Many candidates did not appreciate the
difference between the terminal potential difference, the electromotive force, and the potential
difference ‘lost’ across the internal resistance of a battery.

2. Many candidates found it straightforward to calculate the terminal potential difference. The most
common incorrect answer was 5.0 V, which is the potential difference ‘lost’ across the internal
resistance of the battery.

(ii) Different methods of calculation were possible. The most common method was to first find the
combined resistance of the two external resistors that are connected in parallel. The combined
resistance could then be used to find the resistance of wire Y. Common errors included either
ignoring the internal resistance of the battery or treating the internal resistance as being in parallel
with the two external resistors.

(iii) 1. Most candidates realised that the new wire could have a larger cross-sectional area or be made of
material of lower resistivity. A significant number incorrectly referred to the new wire having a
shorter length, even though the question stated that the length remained unchanged. Very weak
candidates sometimes confused the terms resistance and resistivity.

2. This question was reasonably well answered. A significant minority of candidates did not consider
the current in the battery, even though the question stem referred to current. A small number of
candidates realised that the current in the battery would increase, but then incorrectly stated that
the total power produced by the battery would decrease because more power is dissipated by the
internal resistance. Those candidates seemed to be confusing the total power produced by the
battery with the useful output power that it produces.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Question 7

(a) Most candidates were able to correctly identify the two leptons in the nuclear equation. A
significant minority either did not know that an antineutrino is a lepton or made no attempt to
answer the question.

(b) The name of the particle was correctly stated in the majority of answers. Some candidates need to
learn the precise spelling of antineutrino so that it cannot be confused with other similar words.

(d) Many candidates wrongly stated that the two nuclei were isotopes and gave various incorrect
reasons for this. Common mistakes included thinking that the two nuclei had the same number of
neutrons or thinking that nucleus X was a single neutron. Some candidates gave a general
definition for isotopes, but did not apply their definition to the question.

(e) Stronger candidates found this question to be straightforward. A small minority described the
change to the quark composition in the reverse direction. Others stated the final quark
composition, but not the change to the composition.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/23
AS Level Structured Questions

Key messages

• Candidates should be encouraged to recall definitions in precise detail. The omission of a key word
may prevent credit from being awarded.

• Candidates should be made aware that prematurely ‘rounding off’ any intermediate answers in the
middle of a numerical calculation can lead to an inaccurate final answer.

• In numerical calculations it is important that candidates always explicitly state the subject of any
equations. If an equation is rearranged, the new subject should also be stated.

General comments

The performance of the candidates taking this paper varied considerably. There were many candidates who
were able to successfully apply their knowledge and understanding to the unfamiliar situations posed by the
questions. Conversely, there were a significant number of weaker candidates who needed to improve their
fundamental knowledge in order to be able to answer questions at this level.

Most candidates had a good understanding of how to apply the equations of uniform acceleration in
Questions 3(a) and 3(b). A significant number found it difficult to determine the strain energy in a deformed
sample of a material from a graph of length against force in Question 4(b)(ii). Many candidates also found it
difficult to apply their understanding of how waves interfere to Questions 5(b)(ii) and 5(b)(iii).

Comments on specific questions

Question 1

(a) (i) The majority of the candidates were able to give an appropriate example of a systematic error.
Usually this was the voltmeter having a zero error, but a significant number of candidates also
referred to the voltmeter having an incorrectly calibrated scale.

(ii) Many answers were too vague, such as ‘not recording the right reading’ or ‘human error’. The
most common correct answer was reading the scale from different angles.

(b) (i) The power dissipated by the resistor was usually calculated correctly.

(ii) Most candidates found it straightforward to calculate the percentage uncertainty in the power from
the percentage uncertainties of the potential difference and the resistance. A common error was to
forget to double the percentage error in the potential difference.

(iii) The value of the absolute uncertainty was usually calculated correctly. However, some candidates
wrote down the final answer with an inappropriate number of significant figures.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Question 2

(a) Work done was often stated as being ‘the product of a force and displacement’. This statement is
incomplete because it does not make it clear that the displacement is in the direction of the force (a
moment is also a product of a force and a displacement).

(b) (i) The calculation of the density of the material of the sphere was generally well done, although a
small number of candidates were unable to recall the formula for the volume of a sphere.

(ii) A small number of candidates appreciated that the origin of the upthrust acting on the sphere is
that the pressure on its lower surface is greater than the pressure on its upper surface. A common
misconception is that the upthrust is caused by the density of the water being greater than the
density of the sphere.

(iii) The majority of answers correctly used the resultant force on the sphere to calculate its
acceleration. A minority did not take into account the weight of the sphere and so incorrectly
assumed that the resultant force was equal to the upthrust.

(iv) Most candidates knew that the sphere only moves with a constant speed when the resultant force
acting on it is zero. This knowledge enables the drag force and hence the speed of the sphere to
be calculated. The most common mistake was to ignore the weight of the sphere and so wrongly
assume that the drag force was equal to the upthrust.

(v) Most candidates attempted to use the Doppler effect equation stated on the Formulae sheet. This
equation was usually applied correctly to the question. Although the question asked candidates to
determine the final answer to three significant figures, some candidates incorrectly stated it to
either two or four significant figures.

Question 3

(a) The appropriate equation of uniform acceleration was usually applied correctly.

(b) The majority of the candidates correctly used an appropriate equation of uniform acceleration. A
small number of candidates chose to calculate the height of the ceiling by equating the decrease in
kinetic energy of the ball to its increase in gravitational potential energy.

(c) (i) The equation for the change in the gravitational potential energy of an object was usually recalled
and applied correctly.

(ii) Most of the candidates were able to recall the symbol formula for the kinetic energy of an object.
The decrease in the kinetic energy of the ball from the time it was thrown was sometimes
calculated instead of the required decrease in kinetic energy from the time that the ball first makes
contact with the ceiling.

(d) Candidates needed to state in precise terms how Newton’s third law applied to the collision
between the ball and the ceiling. A general statement of the law was, on its own, insufficient.

(e) The formula for the momentum of an object was correctly recalled by almost all of the candidates.
Many candidates confused the momentum of the ball when it first makes contact with the ceiling
with the momentum when it was first thrown.

(f) Most candidates appreciated that the average resultant force on the ball during the collision was
equal to its change of momentum per unit time. However, the calculation of the average resultant
force often used the time taken for the ball to travel to the ceiling (0.37 s) instead of the time taken
during the collision with the ceiling (0.085 s). Another common error was to assume that the
average resultant force on the ball during the collision was the same as the average force exerted
on the ball by the ceiling.

Question 4

(a) The Young modulus of a material was usually defined correctly. No credit could be given for just a
symbol equation without explaining the meaning of the symbols.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(b) (i) This question was generally well answered. A common error was to assume that the spring
constant was equal to the gradient of the given graph instead of the reciprocal of the gradient.
Weaker candidates sometimes confused the length of the rod with its extension.

(ii) The majority of the candidates were able to state a general symbol equation for strain energy. The
application of the equation to the graph was more difficult. A common error was to confuse the
length of the rod with its extension. Weaker candidates often stated that the strain energy was
represented by the area of the graph, but did not understand which area of the graph to calculate.
Very weak candidates sometimes confused the terms strain energy and strain.

(c) Most candidates deduced that the new graph line would be below the original line, but only a small
proportion were able to draw the new line with the correct gradient. Some candidates ignored the
instruction to draw the new line on the original graph grid and instead tried to construct a whole
new graph in the blank area of the page. This wasted valuable time and made it more difficult to
gain full credit.

Question 5

(a) This question was generally well answered. A significant minority of candidates incorrectly stated
that intensity is equal to the square of amplitude instead of the correct relationship which is that
intensity is directly proportional to the square of amplitude.

(b) (i) This was a straightforward calculation for most candidates. The most common errors were not
converting the units of the wavelength from centimetres to metres and substituting the speed of
sound instead of the speed of the microwaves.

(ii) A significant proportion of the candidates did not attempt this question. The key to answering it
was to determine that the path difference of the waves at X is exactly equal to three wavelengths.
Therefore the waves must have zero phase difference at X and produce an intensity maximum.
Many candidates were unable to apply their knowledge of interference and made vague comments
such as ‘the phase difference at X is constant’. In general, most candidates would benefit from
having a greater understanding of experiments that demonstrate two-source interference.

(iii) 1. The effect of the decrease in wavelength was not well understood. Many candidates appeared to
guess what the effect would be.

2. Many candidates wrongly thought that changing the phase difference between the waves emitted
from the sources to 180° would result in there being no detectable interference pattern anywhere
along the path of the detector. Others commented only on the effect at point X rather than the
effect along the detector’s path. A small minority confused microwaves with visible light and
inappropriately made reference to bright and dark fringes.

Question 6

(a) Most candidates were able to state the relevant symbol equation. Although there were many fully
correct answers, some were spoilt by careless mistakes such as confusing the wire’s diameter with
its radius and not converting the units of the diameter from millimetres to metres.

(b) (i) Many candidates did not understand the effect of the battery’s internal resistance on the terminal
potential difference. Some ignored the internal resistance altogether and assumed that the
potential difference across resistor R was equal to the e.m.f. of the battery (5.0 V).

(ii) 1. Different methods of calculation were possible. The most common method was to first find the
combined resistance of the two external resistances that are connected in parallel. The combined
resistance could then be used to find the resistance of wire X.

2. This question was generally well answered by the majority of candidates, although a significant
number made no attempt at an answer.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Question 7

(a) (i) Most candidates understood that the emission of an α-particle from a nucleus will cause the proton
number to decrease by two and the nucleon number to decrease by four. Some candidates
wrongly thought that the nucleon number represented the number of neutrons in a nucleus.

(ii) The cross representing nucleus R was usually plotted correctly. A common error was to assume
that the emission of a β– particle from a nucleus will cause the proton number to decrease by one.

(b) The majority of the candidates correctly identified the class (group) of particles as being leptons. A
small minority thought it was hadrons.

(c) A significant proportion of candidates did not attempt this question. The most common incorrect
answer was that an up quark was changed to a down quark.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/31
Advanced Practical Skills 1

Key messages

• Candidates are encouraged to record all their raw data (in the table and elsewhere), show this clearly
and not just state the final mean value.

• Candidates need to practise plotting and reading off graphs accurately. Graph scales should be chosen
so that the plotted points occupy at least half of the graph grid along each axis, but this should not be
achieved by selecting an awkward scale that is difficult to use. It is worth double-checking the plotting,
especially when a point appears to be anomalous.

• Many candidates find it difficult to draw lines of best fit. To practise and develop this skill, candidates
might find it helpful to draw a straight line on a transparent plastic sheet and use it as an overlay, sliding
or rotating it to find the line of best fit. They could also draw lines of best fit for data on graph paper and
then use a computer to draw the line of best fit, and compare the two. A transparent 30 cm ruler and
sharp pencil are essential when drawing lines of best fit.

• To be successful answering Question 2, candidates should be reminded that their identified limitations
and suggestions for improvement must be focused on the particular experiment being carried out.
General points such as ‘avoid parallax error’ or ‘use more precise measuring instruments’ will not
usually gain credit without further detail. Candidates should be encouraged to write about four different
problems and consequently four different solutions to address these problems, and should not try to
state four solutions to the same problem.

General comments

There was no evidence that Centres had any difficulties in providing the equipment required for use by the
candidates. It was clear that most Centres work hard to provide the equipment specified in the Confidential
Instructions. Centres were requested to provide stands, bosses and clamps. It would be helpful if Centres
could ensure all stands, bosses and clamps are tightened up so they grip well and do not wobble. A stated
common source of difficulty was loose clamps or clamps which did not grip well.

Any deviation between the requested equipment and that provided to the candidates should be written down
in the Supervisor’s Report, and this report must be sent with the scripts to Cambridge so that the Examiners
can take this into consideration when marking. No ‘extra’ equipment should be available to the candidates.
In some cases this may disadvantage candidates.

Any help given to a candidate should be noted on the Supervisor’s Report. Supervisors are reminded that
help should not be given with the recording of results, graphical work or analysis.

Successful answers showed the apparatus had been used well, and there were many very good answers.
Both questions were normally attempted and there did not seem to be a shortage of time in most cases.
Weaker answers showed that determining a periodic time was difficult. Good practical skills were
demonstrated in the generation and handling of data but could be improved by giving more thought to the
analysis and evaluation of experiments.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Comments on specific questions

Question 1

(a) Stronger candidates correctly noted a length d which was in the appropriate range, to the nearest
mm and with a correct unit. Good answers had a value of s in the range 30 cm ± 1 cm. Weaker
candidates often had values of s above 31 cm.

A unit was not given on the answer line, so for full credit candidates needed to provide a correct
unit matching the figure written down.

(b) With the rule correctly arranged, successful candidates correctly stated a value for the period T
which was in the appropriate range and had the correct unit of seconds. To find the period
accurately, several consecutive oscillations e.g. 10 needed to be measured at least twice, and the
time for one oscillation was calculated as (total time for 10 oscillations)/10. Successful candidates
showed the equation had been remembered.

Weaker answers had the total time on the answer line, inadvertently found frequency using an
inverted equation or showed that the stop-watch had been misread. It was common for weaker
candidates to measure the period by timing single oscillations.

(d) Strong candidates recorded five sets of readings and included raw values of time, along with
periodic times T and the distance s in neatly presented tables, and the table values showed the
correct trend. A number of tables in weaker answers showed confusion between period and
frequency resulting in an incorrect trend in the data.

Better answers included a large variation in s values i.e. the largest s value and the smallest s
value that could be achieved with the apparatus. These candidates varied s from less than 70 cm
to 85 cm or more. Weaker answers showed a variation in s of only a few centimetres.

Strong candidates included a correct quantity and unit in every column heading. These answers
included a separating mark, such as a solidus, between the quantity and unit e.g. s / cm, t1 / s, t2 / s,
T / s and T2 / s2. Weaker answers omitted units or did not include a separating mark between the
quantity and its unit e.g. T2 s2.

Candidates should be encouraged to record raw values of the time for n oscillations in their tables,
and not just the values of calculated period.

Consistent answers reflected the precision of the stop-watch, so all readings of raw time were
given to 0.1 s or 0.01 s.

Successful treatment of significant figures was shown when calculated values for T2 were recorded
to an appropriate number of significant figures from raw data, i.e. to the same number as (or one
more than) the number of significant figures in the raw time values.

Many candidates calculated T2 correctly. Weaker answers had errors in calculations often caused
by incorrect rounding of the final value by truncating rather than rounding.

(e) (i) Stronger candidates used axes for their graphs that were labelled with the appropriate quantities
e.g. T2 or s. Weaker answers only stated units or omitted the labels altogether. It is important that
candidates select sensible scales. Weaker candidates often used most of the graph grid but did
this by using difficult scales, e.g. one large square equal to 0.6 or 0.34. Such scales are very
difficult to work with and can cause the candidate to make mistakes in several different places.
Better answers used a scale which was easy to interpret with the value of one small square equal
to, for example, 0.1, 0.2, 0.25, 0.4 or 0.5. Weaker answers used, for example, 10 small squares
equal to 12 as then one small square with this scale was worth 1.2.

Some candidates used scales with values missing, e.g. 1.8, 2.0, 2.4 (where 2.2 is missing). Some
also had scales where the last value was ‘squashed’ to fit by changing the scale e.g. 1.6, 1.8, 2.0,
2.4.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

The strongest candidates had points spreading over more than half the grid. In successful answers
with a graph in portrait orientation, points occupied (spread into) six or more large squares in the
vertical direction and four or more large squares in the horizontal direction. Weaker candidates
often used a scale such that points were squashed into a small area, in some cases just two large
squares.

Candidates should take care to plot points within half a small square in both the x and y directions.
Many graphs could be improved by the use of a sharp pencil to draw fine points so that the points
have a diameter less than half a small square.

(ii) Some candidates were able to draw carefully considered lines of best fit, but others joined the first
and last points on the graph regardless of the distribution of the other points. There should always
be a balanced distribution of points either side of the line along the entire length. Many lines
needed rotation to get a better fit, or an anomalous point needed to be identified to justify the line
drawn. Some candidates were not awarded credit because their lines were kinked in the middle
(candidates used too small a ruler), a double line (broken pencil tip) or drawn freehand without the
aid of a ruler.

(iii) Successful answers clearly showed how to find the gradient and intercept of a straight line graph.

Good gradient calculations used a large triangle, i.e. one for which the hypotenuse was greater
than half the length of the drawn line. Weaker gradient calculations used read-offs from a small
triangle where the hypotenuse was less than half the drawn line. Good calculations had accurate
read-offs substituted correctly into (y2 – y1) / (x2 – x1) with working that was clear to follow. Some
candidates used values from the table that were not on the line rather than taking points from the
line.

In finding the intercept, stronger candidates either read off the y-intercept directly from the graph
where s = 0 or substituted values for x and y using a point on the line (not a point from the table)
correctly into y = mx + c. When rearranging this equation, candidates should obtain c = y – mx.
Some candidates incorrectly used c = y / mx. It was also common for candidates to find the
intercept by taking a read-off where the line cut the y-axis at a point other than s = 0, giving a false
origin.

(f) Candidates needed to make a direct transfer of values from (e)(iii) to (f). P was equal to the value
of the gradient and Q was equal to the value of the intercept, and no additional calculations were
necessary. Weaker answers recorded fractions for P or Q or showed fresh substitutions into the
equation and further calculations.

Some candidates did not include the units or gave wrong units.

Question 2

(a) Most candidates carefully adjusted the position of the magnet to produce a value of x close to
3 mm. Good answers had measured values of the small distance x, stating the length with its unit
e.g. 0.4 cm. Weaker answers had a value of 3 cm. Measuring the small distance x with a metre
rule was difficult and this could be stated as a source of limitation in (g).

(b) Successful candidates recognised that this measurement was difficult and noted down repeat
readings showing the experiment had been done more than once. Again this measurement could
be used as a source of limitation in (g). The measurements of y required a unit and successful
answers noted readings to the nearest mm e.g. 8.5 cm or 85 mm. Although identifying C was
difficult, the length y could be measured (on the paper) to the nearest mm. Some weaker answers
had added extra zeros e.g. 8.50 cm or noted a value only to the nearest cm e.g. 8 cm.

(c) Many candidates showed repeat readings of y reflecting the difficulties involved in identifying the
end C. Consequently, the absolute uncertainty for y was greater than 1 mm. Weaker answers
gave the smallest division on the rule, i.e. 1 mm. When readings were repeated, some successful
candidates showed the absolute uncertainty found by calculating half the range of repeated values.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(d) Successful answers showed the magnet had been raised and second values of x and y recorded.
A larger value of x should have produced a smaller distance for y if the experiment was carefully
carried out. Weaker candidates found that a larger x produced a larger y.

(e) (i) Stronger candidates showed the correct rearrangement of the equation and calculated k for both
experiments. These answers demonstrated how to rearrange the equation and used k = y√x. If
answers showed repeat measurements, successful answers used the latest measurements taking
them through into the calculation of k. Some weaker candidates rounded in the middle of the
calculations and produced inaccurate values, or did not take the latest readings through into the
calculation of k.

(ii) Successful candidates had three steps in their argument. They first state a criterion to be used for
testing the relationship. This could be a percentage uncertainty that they think is a sensible limit for
this particular experiment, e.g. 5% or 20%, or could be the percentage uncertainty found in (c).
Next they calculate the percentage difference between their values of k. Finally, they compare the
percentage difference between their k values to the percentage uncertainty chosen and decide
whether the relationship is supported or not supported. If the percentage difference between the
two k values is less than the stated criterion, these successful answers then say that the
relationship is supported. If the value of the percentage difference is greater than the value of the
percentage uncertainty stated as the criterion, then the relationship is not supported. The
candidates should make an explicit statement, e.g. ‘the relationship is not supported’.

Some candidates did not give any criterion to test against. Many statements were too vague.
Some candidates looked at the difference without determining a percentage, or gave an answer
that did not logically follow from the data. These did not gain credit.

(f) (i) Correct answers showed that the metal plate had been removed and a new paper used to record
another value of y.

(ii) Most candidates showed the correct substitution of the y value from (f)(i) and the second k value
from (e)(i) to find the x value required. Weaker candidates found the square root or inverted the
equation.

(iii) Candidates needed to be specific and relate the number of significant figures in the value of x to
the number of significant figures in the value of y. Successful answers referred specifically to y (i.e.
the relevant quantity in this question). Weaker answers stated vague quantities such as ‘raw data’
or ‘values in the calculation’, referred to the precision of the ruler or to decimal places, or gave a
bald statement such as ‘3 significant figures’.

(g) Question 2 is designed to have challenges when taking measurements. Often there is other
apparatus that could be used to ensure more accurate readings. Successful answers used
limitations found while experimenting, stating these in (i). Answers were often improved in (ii) by
stating a valid method or suggesting other apparatus that could be used to take more accurate
readings.

Successful candidates stated ‘only two distances of x were used to test the relationship and this is
not enough to draw a conclusion’. Weaker answers just stated ‘two readings are not enough’ or
‘two sets of readings are not enough to take accurate readings’. Some candidates went beyond
the suggestion of ‘take more readings’ or ‘take more readings and find the average’ and correctly
suggested ‘take more readings and plot a graph’.

Successful answers identified genuine limitations. Weaker answers had ideas that were vague or
could have been avoided by careful technique, such as miscounting oscillations, stands which
wobbled, zero errors on a metre rule, wind affecting oscillations etc., none of which were valid
problems here. Stronger candidates stated that the location of position C where the magnet
stopped for a short time at the end of the 30th oscillation as a limitation. Weaker answers simply
stated that the oscillations were ‘fast’. Successful answers stated that a video recording could be
used to more accurately locate C and needed a scale, while weaker answers suggested using a
‘video with a timer’ which was not actually useful here. Answers such as ‘record oscillations and
playback in slow motion with a timer’ missed the essential need for a scale.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Aligning the magnet with line A was a difficulty as the magnet did not stay still but moved
continually. Stronger candidates noted that the oscillations were often not regular as the magnet
was attracted to the magnetic clamp and stand, and also noted that the use of a wooden or plastic
stand would be an improvement. Weaker answers sometimes suggested the idea of covering the
stand with a material such as paper in an attempt to shield the magnet from the effect of the stand,
although this is unlikely to be effective.

The measurement of x was difficult but candidates needed to explain the reason for this, e.g. ‘it
was difficult to measure x using a meter rule because the value was 0.4 cm which was so small.’
Weaker answers just stated ‘it was difficult to measure x’ without a reason, and in the
improvements section weaker answers suggested ‘making x larger’. Better answers suggested the
use of named, more precise apparatus or described a better method e.g. ‘measure the thickness of
several sheets of paper using vernier calipers to find the thickness of one sheet, and place sheets
of paper between the magnet and strip to fill the gap’.

Candidates need to suggest detailed limitations and improvements that are specific to this
experiment. Statements of general errors e.g. ‘systematic errors’, ‘zero errors’ or ‘rules with
uneven ends’ cannot be given credit. Some weaker answers stated, for example, ‘repeat
measurements and calculate averages’ or included general statements such as ‘use an assistant’,
‘view at eye level’ or ‘look at right angles to avoid parallax’. Again these statements are not specific
enough be given credit, and candidates should be encouraged to give detail that is specific to the
particular experiment they are carrying out.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/32
Advanced Practical Skills 2

Key messages

• Candidates are encouraged to record all their raw data (in the table and elsewhere), show this clearly
and not just state the final mean value.

General comments

There was no evidence that Centres had any difficulties in providing the equipment required for use by the
candidates. Any deviation between the requested equipment and that provided to the candidates should be
written down in the Supervisor’s Report, and this report must be sent with the scripts to Cambridge so that
the Examiners can take this into consideration when marking. No ‘extra’ equipment should be available to
the candidates. In some cases this may disadvantage candidates.

The Supervisor’s Reports from many Centres included useful detail about the apparatus and about
difficulties encountered in the experiments. Supervisors rarely had to provide assistance to candidates.

For many Centres, the candidates’ work was of a very good standard. Candidates did not seem to be short
of time and all sections of the two questions were answered by almost all the candidates.

Comments on specific questions

Question 1

(a) (i) Most candidates included a unit with their value for h. A few candidates gave their value to the
nearest cm rather than the nearest mm.

(ii) Most candidates recorded a value for t in the expected range. It is probable that all candidates
took repeated timings, but some recorded only a single value. Candidates should be encouraged
to record all of their readings as this provides evidence that the measurements have been
repeated.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(b) Nearly all candidates recorded six or more sets of values of h and t. In a few cases, full credit
could not be given because each increase in h did not produce a decrease in t.

Many candidates only increased the angle of the ramp from its initial position and so all timings
were short. The strongest candidates made full use of the apparatus by also lowering the ramp to
give some longer times.

Most times were recorded to the nearest 0.01 s or 0.1 s. The calculated values of 1 / t2 were usually
correct, although there were some rounding errors.

(c) (i) Many graphs were drawn to a very good standard, with accurate and clear plotting of the correct
quantities and good use of the available grid area. Most scales were simple with divisions clearly
labelled. In a few cases, d was plotted instead of h. In some cases, the points were drawn using
dots that were too large. The clearest graphs used small crosses.

A minority of candidates set the minimum and maximum reading in the table to be the minimum
and maximum of the graph grid, leading to time-consuming work plotting and using the scales.
This type of scale cannot be awarded credit and it was very common for candidates using such
awkward scales to lose further credit later for read-offs that were incorrect.

The quality of the candidates’ data was judged by the scatter of points about a straight-line trend,
and in the majority of cases this was good and credit was awarded.

(ii) Stronger candidates drew suitable lines of best fit which had a balanced distribution of points either
side along the entire length. In some cases, a stray point was apparently ignored without
explanation. If such a point is ignored in assessing the line of best fit, the anomalous point should
be labelled clearly (e.g. by circling the point) with no more than one point being circled.

(iii) Nearly all candidates knew how to find the gradient of their line. Most took values from their line
and carried out the procedure accurately and showed their working clearly, although some used
too small a triangle. The intercept value was usually negative and so it could rarely be read directly
off the 1 / t2 axis. Candidates generally knew how to calculate the intercept using their gradient
value.

(d) Most candidates recognised that a was equal to the value of the gradient and b was equal to the
intercept. Values of a and b were sometimes given to given to only one significant figure. A large
number of candidates included correct units for their a and b values.

Question 2

(a) Very few problems were reported with setting up the apparatus. The measurement of the string
length L usually gained credit. In some cases, no unit was given or the value was recorded only to
the nearest cm.

(b) The procedure required careful and patient adjustment of the apparatus to produce a clear pattern.
Nearly all candidates gave a value for x together with its unit. Credit was also given for a value to
the nearest mm and in the expected range.

(c) There were many good answers for the percentage uncertainty in x based on an absolute
uncertainty of 2 mm or more. Some candidates looked at the spread of repeated readings and
used half the range of these values, which is another valid method that was awarded credit.

(d) Most candidates took note of the unit given on the answer line and converted their value of L to m
before calculating λ from it. A small number used their value of x instead of L.

(e) There were many good justifications given for the number of significant figures in λ. Vague
references to ‘measured values’ or ‘L and x’ were not accepted as L was the only measurement
involved in the calculation.

(f) A complicated calculation was needed to find f but the majority of candidates carried it out
successfully. In a few cases, the candidate mistakenly modified the mass per unit length µ by
dividing it by the string length.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(g) Most candidates correctly repeated the experiment using the second string.

(h) (i) The calculation of k was correct in nearly all cases.

(ii) Discussion of whether the analysis in (h)(i) supported the suggested relationship was carried out
well by many candidates. Unsuccessful answers usually did not have a numerical comparison of
the percentage difference of the k values with what would be an acceptable difference in this
experiment. General or vague statements such as ‘valid because the values are close to each
other’ cannot be awarded credit. Candidates should be encouraged to ensure their justifications
are quantitative in nature.

(i) Many candidates identified the major problems associated with carrying out this particular
experiment – the fact that the pattern died away quickly owing to damping of the vibrations, and the
difficulty of seeing the stationary waves clearly.

The descriptions of the problems were sometimes flawed – ‘fast vibrations’, for example, were not
a problem as they were necessary to generate the stationary pattern. Many candidates suggested
the valid improvement of replaying a video recording to inspect the stationary wave (although just
‘make a video’ alone was not credited, as further detail was needed). In a few cases the idea of
trying a contrasting background was suggested. Difficulty with gripping the hacksaw blade was
also commonly listed, together with the solution of clamping the second wooden block. The
difficulty of releasing the hacksaw blade with a constant force or from a constant displacement was
often suggested as a source of uncertainty, but this was not credited as it would only affect the
amplitude and not the wavelength of the stationary waves.

Credit is not given for suggested improvements that could be carried out in the original experiment,
such as ‘repeat measurements’, ‘do more readings to get an average value’, ‘ensure reading is
taken perpendicularly’ etc. Unrealistic solutions were also not given credit, e.g. ‘robotic arm’ or
‘mechanical hand’. Problems that were irrelevant or that could have been removed if the candidate
had taken greater care were not given credit. Vague or generic answers such as ‘too few readings’
(without stating a consequence), ‘faulty apparatus’, ‘use an assistant’ etc. were also not given
credit.

The key to this section is for candidates to identify genuine problems associated with setting up this
experiment and in obtaining readings. Candidates are then encouraged to suggest practical
solutions that either improve technique or give more reliable data. Clarity of thought and
expression separated the stronger candidates from those less prepared to deal with practical
situations and the limitations. Candidates should be encouraged to write about four different
problems (perhaps relating to the different measurements undertaken or chronologically going
through the experiment) stating how these difficulties impact on the experiment. Candidates
should then try to think of associated solutions that address each of these problems.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/33
Advanced Practical Skills 1

Key messages

• Candidates are encouraged to record all their raw data (in the table and elsewhere), show this clearly
and not just state the final mean value.

General comments

Any help given to a candidate should be noted on the Supervisor’s Report. Supervisors are reminded that
help should not be given with the recording of results, graphical work or analysis.

The general standard of the work done by the candidates was good, and there were many excellent scripts.
Candidates did not seem to be short of time and both questions were attempted by almost all the candidates.
They demonstrated good skills in the generation and handling of data but can improve by giving more
thought to the analysis and evaluation of experiments.

Comments on specific questions

Question 1

(a) Many candidates stated values of V1 and V2 to the nearest 0.01 V with units, and set up the circuit
correctly so that V2 was greater than V1. Some candidates omitted units, gave the wrong units (Ω
or A) or gave inconsistent readings with one voltage value to the nearest 0.01 V whilst the other
was to the nearest 0.001 V. In some cases the circuit was incorrectly set up, giving a value of V2
that was smaller than V1.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(b) Many candidates were able to collect six sets of values of x, V1 and V2 without assistance from the
Supervisor, and usually showed a correct trend in their values. Some candidates needed help
setting up the circuit. A minority of candidates took 7–10 sets of results and then did not plot all
their observations on the graph grid. Few candidates took the time to repeat their readings. This
may have helped to identify anomalous results and improve data quality. If time is limited,
candidates should be encouraged to look out for possible outliers which do not fit a general trend
and repeat these readings to double-check.

Many candidates chose a small range over which to conduct the experiment, so the values ended
up too close together. It was expected that candidates consider the whole length provided and use
at least 10.0–70.0 cm. A small minority chose an impractical range of 5 cm or 10 cm, whilst other
candidates went up to 100 cm but only started at 50 cm.

Many candidates were awarded credit for the column headings, giving both the quantity and correct
unit for each heading with both separated by a solidus or with the unit in brackets. Some
candidates omitted either the unit or the separating mark for one of the columns, in particular the
last V1 / x column. A few candidates used confused notation, giving the units as V / cm– instead of
V cm–1.

Many candidates correctly recorded their raw values for x to the nearest 0.1 cm. Some candidates
stated their measurement to the nearest cm e.g. 20 cm without considering that they can make the
measurement to the nearest mm using the ruler provided. A few candidates stated measurements
with too many trailing zeros, e.g. 20.00 cm.

Many candidates correctly recorded their calculated values for V1 / x to two or three significant
figures. Some candidates either stated too many or too few significant figures, or aimed to be
consistent in their use of decimal places at the expense of significant figures. This often increased
the amount of scatter in the results plotted on the grid.

Most candidates calculated values for V1 / x correctly. Some candidates incorrectly rounded their
answers and this did not gain credit.

Overall the table work was done well by candidates. Even those who sought the help of the
Supervisor in setting up the circuit usually gained credit for the table. Some candidates did not
state their raw values and just showed two columns of calculated values. Candidates should be
encouraged to record their raw values.

(c) (i) A few candidates plotted the wrong graph or omitted labels. Compressed scales (in either the x or
y direction) were often seen and could not be awarded credit. There were many incidences of
awkward scales (e.g. based on 3, 6, 11 etc. or having 15 squares between scale markings). A
minority of candidates set the minimum and maximum reading in the table to be the minimum and
maximum of the graph grid, leading to time-consuming work plotting and using the scales. This
type of scale cannot be awarded credit and it was very common for candidates using such
awkward scales to lose further credit later for read-offs that were incorrect.

Some candidates used irregular (i.e. non-linear) scales. Irregular scales could not be given credit,
and often the data could not be awarded credit for quality either because the error was often in the
region of the plotted points. Candidates should be encouraged to set up their graphs to make them
easy to work with in later parts of the question.

Some points were drawn as dots with a diameter greater than half a small square. Many points
were incorrectly plotted so that they were greater than half a square from the correct position. If a
point seems anomalous, candidates should be encouraged to check the plotting and to repeat the
measurement if necessary. If such a point is ignored in assessing the line of best fit, the
anomalous point should be labelled clearly (e.g. by circling the point) with no more than one point
being circled.

There is no credit specifically for identifying anomalous points, so candidates should be reminded
that they do not need to identify an anomalous point if they do not think they have one.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

needed rotation to get a better fit, or an anomalous point needed to be identified to justify the line
drawn. Some candidates were not awarded credit because their lines were kinked in the middle
(candidates used too small a ruler), a double line (broken pencil tip) or drawn freehand without the
aid of a ruler.

(iii) Some candidates used a suitably large triangle to calculate the gradient, gaining credit for correct
read-offs, and substituted into ∆y / ∆x. Other candidates needed to check that the read-offs used
were within half a small square of the line of best fit and show clearly the substitution into ∆y / ∆x
(not ∆x / ∆y). The equation m(x – x1) = (y – y1) should be shown with substitution of read-offs.

Candidates need to check that the triangle for calculating the gradient is large enough (the
hypotenuse should be greater than half the length of the line drawn). There were many instances
of incorrect read-offs, and many candidates would benefit from double-checking their read-offs.

Some candidates were able to correctly read off the y-intercept at x = 0 directly from the graph, but
a large number of candidates incorrectly read off the y-intercept when there was a false origin.
Some candidates correctly substituted a read-off into y = mx + c to determine the y-intercept.
Others needed to check that the point chosen (if it was from the table) was on the line drawn.

(d) Most candidates recognised that P was equal to the gradient and Q was equal to the intercept
calculated in (c)(iii). Some candidates recorded a value with consistent units for P (m) and Q (V).
Some candidates stated incorrect units or omitted the units.

Question 2

(a) Most candidates measured values of L in the appropriate range, with a unit and to the nearest mm.
Some candidates omitted units, whilst others stated the length measurement to the nearest cm and
a few stated a value that was not in range.

(b) (i) Many candidates stated the angle to a tenth of a degree (e.g. 13.2°) when the precision of the
protractor did not justify this, and some gave an incorrect unit (e.g. °C). Some candidates either
did not read the instruction correctly or misread the protractor, giving an angle of around 75°
instead of 14°.

(ii) Most candidates are familiar with the equation for calculating percentage uncertainty. Some
candidates made too small an estimate of the absolute uncertainty in the value of A, typically 1° or
0.5°, given that it was awkward to hold the protractor in the right place. Some candidates repeated
their readings and correctly gave the uncertainty in A as half the range, although other candidates
did not halve the range. A few candidates incorrectly divided their uncertainty by 360° rather than
their own value of A.

(c) (i) Many candidates gave a value for T in the appropriate range and with a unit, and showed evidence
of repeated timings of five or more oscillations. Some candidates did not repeat sets of readings,
measured only one oscillation, found short times (suggesting ½T was measured rather than T), or
determined frequency instead of period. There were some cases of omission of units or very small
times e.g. 0.14 s suggesting misreading of the stop-watch.

A significant number of candidates probably measured the time for 10 oscillations but did not divide
their answer by 10 (and did not give evidence that 10 oscillations were in fact measured).

(ii) Many candidates correctly calculated d. Some candidates incorrectly rounded this value. A
minority of weaker candidates incorrectly used their calculator in radian mode.

(iii) Many candidates correctly justified the number of significant figures they had given for the value of
d with reference to the number of significant figures used in A. Many candidates gave reference to
just ‘raw readings’ or ‘values in calculation’ without stating what these values were, related their
significant figures to sin A and not the raw angle A, or incorrectly related to a different reading e.g. T
or L.

(d) Most candidates recorded second values of A and T. Most candidates recorded a second T that
was greater in value than their first value at the smaller angle, gaining credit for quality.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(e) (i) Most candidates were able to calculate k for the two sets of data, showing their working clearly. A
minority of candidates incorrectly rearranged the equation algebraically to calculate 1 / k or
inadvertently substituted the wrong values.

(ii) Some candidates calculated the percentage difference between their two values of k, and then
tested it against a specified numerical percentage uncertainty as a criterion, commonly using 10%,
15% or 20%. Some candidates referred to the percentage uncertainty calculated for A and this
was also credited. Some candidates omitted a criterion, or gave general statements such as ‘this
is valid because the values are close to each other’, which could not be accepted. Occasionally
candidates gave a contradictory statement such as ‘yes, my results do not support this relationship
as my % difference is less than 10%’ or an incorrect statement such as ‘my results support this
relationship as my % difference is greater than 10%’.

(f) Many candidates recognised that two sets of data were insufficient to draw a valid conclusion and
stated an improvement of taking more readings and plotting a graph. Many problems stated by
candidates correctly related to the measurements taken with a valid reason.

Various reasons for difficulty in measuring the angle were given, such as having to hold the
protractor in mid-air or with a shaking hand, or there was parallax error, or there was no vertical
reference, or the string was too thick to read the protractor. Also commonly seen were reasons
relating to the difficulty in measuring the length of the pendulum, e.g. it was difficult to locate the
centre of the bob, or the cork was in the way of the ruler. To gain credit, the quantity that was
difficult to measure must be specified along with the difficulty. Just ‘parallax error’ or ‘difficult to
measure to the centre of the bob’ on its own does not gain credit without linking to the angle or
length L quantity respectively.

Credit is not given for suggested improvements that could be carried out in the original experiment,
such as ‘repeat measurements’, ‘do more readings to get an average value’, ‘ensure reading is
taken perpendicularly’ etc. Unrealistic solutions were also not given credit, e.g. ‘robotic arm’ or
‘mechanical hand’. Problems that were irrelevant or that could have been removed if the candidate
had taken greater care were not given credit. Vague or generic answers such as ‘too few readings’
(without stating a consequence), ‘faulty apparatus’, ‘unstable stands’, ‘use fiducial marker’ (without
stating where to place it), ‘digital protractor’ or ‘use an assistant’ were also not given credit.

The limitations and improvements given by some candidates occasionally related to the experiment
in Question 1 and these could not be given credit.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/34
Advanced Practical Skills 2

Key messages

• Candidates are encouraged to record all their raw data (in the table and elsewhere), show this clearly
and not just state the final mean value.

General comments

In Question 1, candidates are usually expected to collect data and plot a graph using quantities derived from
the data. The experiment will sometimes be electrical in nature, as was the case on this paper. If
candidates have difficulty assembling the circuit or suspect that the circuit is not behaving in the way
anticipated, they should be encouraged to seek assistance from the Supervisor. The Supervisor can then
check that the circuit is connected correctly, change it if necessary, and check that any meters in the circuit
are functioning properly. Any help given to a candidate should be noted on the Supervisor’s Report.
Supervisors are reminded that help should not be given with the recording of results, graphical work or
analysis.

Many candidates record experimental values that are unrealistic. For example, when asked to record the
period of a mechanical oscillator with a period of approximately 2.0 s, some candidates record instead the
time for 10 oscillations, forgetting to divide their value by 10. Others record a value of less than 0.01 s,
having misread the display on the stop-watch. It is good practice to first reflect on what the approximate
value of an experimental measurement is likely to be and then check this estimate against the actual
measured value. In this way, candidates should gain a ‘feel’ for likely experimental values.

Measuring and calculating mass is a further example. In Question 2 on this paper, candidates were asked
to calculate the total mass of a bottle containing 500 cm3 of water, 200 g of slotted masses and a 100 g mass
hanger. Just holding the bottle and feeling its weight should give candidates a rough idea of what the value
is. Candidates recording values less than 1.0 g or several hundred kilograms could have then identified that
an error had been made.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

the Examiners can take this into consideration when marking. No ‘extra’ equipment should be available to
the candidates. In some cases this may disadvantage candidates.

The general standard of the work done by the candidates was good, with many excellent scripts. Candidates
did not seem to be short of time and both questions were attempted by almost all the candidates. They
demonstrated good skills in the generation and handling of data but can improve by giving more thought to
the analysis and evaluation of experiments.

Comments on specific questions

Question 1

(a) Most candidates recorded a value for VS that was in the range 2.00–4.00 V. Some candidates
omitted a unit.

(b) Most candidates correctly recorded a value of V that was less than their value of VS. Fewer
candidates repeated their measurements to find an average value.

Few candidates made the best use of the possible range of values of n. The minimum possible
value for n was zero (i.e. no capacitors connected to the component holders) and the maximum
value for n was 10 (combining all the available capacitors in parallel). To be awarded credit for the
range, candidates needed to have a minimum value for n of 0 or 1, and a maximum value for n of 7
or more.

Most candidates labelled their table of results correctly by including a quantity and a unit (where
appropriate) for each column heading. The quantity and the unit should be separated by a solidus
(/) or with the units in brackets e.g. V / V or V (V).

Most candidates recorded all their raw values of V to the nearest 0.01 V (or all to the nearest
0.001 V). A small number of candidates added a spurious zero to all their values to give the false
impression of greater precision in their measurements.

Raw readings of a quantity should always be recorded to the same precision (the precision of the
instrument being used – in this case a digital voltmeter), and not necessarily to the same number of
significant figures.

Most candidates calculated and recorded their values of 1 / V correctly, i.e. to the same number of
significant figures as (or one more than) the number of significant figures of the raw values of V. A
few candidates rounded their final answers incorrectly.

(d) (i) Candidates were required to plot a graph of 1 / V on the y-axis against n on the x-axis. Most gained
credit for drawing appropriate axes, with labels and sensible scales. The values of n being integer
values made the choice of a scale on the x-axis, and the correct plotting of the points, relatively
straightforward.

Some candidates chose extremely awkward scales, making the correct plotting of points much
more difficult. Some chose the highest and lowest values in their tables as the lowest and highest
points on their graph scales and then calculated intermediate values. Although this appears to
make good use of the graph grid, it invariably makes it very difficult to plot all the points correctly.
Credit cannot be awarded for the scale, and these candidates often lost further credit later for
incorrect read-offs when calculating the gradient or the y-intercept of the line. A few candidates
chose non-linear scales, or scales which meant that one or more points were off the graph grid.

Many candidates gained credit for plotting their tabulated readings correctly. If a point seems
anomalous, candidates should repeat the measurement to check that an error in recording the
values has not been made. If such a point is ignored in drawing the line of best fit, the anomalous
point should be labelled clearly, e.g. by circling the point.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Most candidates plotted their points on the graph paper carefully. Others needed to draw the
plotted points so that the diameters of the points were equal to, or less than, half a small square.
Some candidates plotted points as dots or crosses that were too small to see clearly or were
hidden by the line of best fit (a small but clear pencil cross, or a point with a circle, is
recommended). Some candidates can improve by plotting the points more accurately i.e. to within
half a small square.

The majority of candidates were able to collect a set of data that was awarded credit for quality.

(ii) Many candidates were able to draw a straight line which was a good fit to the points plotted, with a
reasonable distribution of points above and below the line. Others tended to join the first and last
points on the graph, regardless of the distribution of the other points, or draw a line which could
clearly be improved by rotation. A few candidates drew a double line or a line with a ‘kink’ in it
(perhaps by using a small ruler).

(iii) Many candidates used a suitably large triangle to calculate the gradient, gaining credit for correct
read-offs and substitution into ∆y / ∆x. Others needed to check that the read-offs used were within
half a small square of the line drawn, or show clearly the substitution into ∆y / ∆x (not ∆x / ∆y), or
check that the triangle for calculating the gradient was large enough (the hypotenuse should be
greater than half the length of the line drawn) in order to gain credit.

It is important that candidates show their working, making it clear which points they have chosen
for the read-offs e.g. by drawing the triangle on the graph.

Several candidates were able to read the value of the intercept directly from the graph as their
scale on the x-axis started at zero. Others correctly substituted a read-off into y = mx + c to
determine the y-intercept. A point from the table can be used only if the point lies on the line of
best fit.

(e) Most candidates recognised that a was equal to the value of the gradient and b was equal to the
value of the intercept.

The majority of the candidates recorded correct units for a and b (both V–1). Others omitted the
units for a and b.

Question 2

(a) Most candidates calculated the value of M correctly.

(b) (i) Most candidates were able to record a value for the period T of the oscillations in the range
0.50 s ⩽ T ⩽ 1.00 s, although some misread the stop-watch. Fewer candidates repeated their
measurement of the time for multiple oscillations. Measuring the time for one oscillation several
times is not good practice. As a general rule, the time for at least 5 complete oscillations should be
repeated two or three times, and a mean value calculated. All working should be shown, including
the value of n, where n is the number of oscillations.

Several candidates forgot to divide their average value of nT by n to find T.

(ii) Candidates were asked to estimate the percentage uncertainty in their value of T. Most were
familiar with the equation for calculating percentage uncertainties, but many underestimated the
absolute uncertainty in the value of nT. This is dependent not only on the precision of the stop-
watch, but also on the possible errors in judging the beginning and end of an oscillation. A realistic
estimate for the absolute uncertainty in the measurement of the raw value(s) of time is 0.2–0.5 s.

(c) (i) Most candidates recorded a second value of M successfully.

(ii) Almost all candidates recorded a second value of T, and the majority obtained a value for T which
was greater than the value recorded in (b)(i).

(d) (i) Most candidates were able to calculate the two values for k correctly. Some candidates were not
awarded credit because they calculated 1 / k or recorded values on the answer line to only 1
significant figure.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(ii) Candidates were asked to explain whether their results supported a suggested relationship, i.e.
after allowing for the uncertainties in the measurements, could the two values of k be regarded as
equal. To do this, candidates need to test the hypothesis against a specified numerical percentage
uncertainty, either taken from (b)(ii) or estimated themselves. Where candidates state a
percentage uncertainty value themselves, it is a good idea to try to justify this value in some way,
particularly if a very large percentage uncertainty is suggested.

Most candidates were able to calculate the percentage difference between the two values of k, and
then compare this difference to an estimated overall uncertainty for the experiment (e.g. 20%).

Some candidates gave answers such as ‘the difference between the two k values is very
large/quite small’ which is insufficient – a numerical percentage comparison is needed.

(e) (i) The majority of candidates were able to record value(s) for D to the nearest mm and in the
appropriate range. Some only recorded their raw values for D to the nearest cm, or recorded their
value for D on the answer line in cm rather than m.

(ii) Most candidates were able to calculate the value of A successfully.

(iii) Most candidates calculated ρ successfully, though some candidates used the first value of k or an
average value rather than the second value of k.

(f) Many candidates recognised that two sets of data were insufficient to draw a valid conclusion,
though some confused conclusions with results. In this experiment, the main difficulties included
problems getting the bottle to oscillate vertically (the bottle would tilt to one side when oscillating
and sometimes hit the side of the bucket) and judging the exact moment the bottle starts or
completes an oscillation. Also the measurement of M excluded the mass of the bottle itself.

Some candidates simply described measurements that were difficult to make without explaining
why they were difficult e.g. ‘it was difficult to measure D’ or ‘difficult to measure T’. A reason for the
difficulty is also needed to gain credit, e.g. ‘it was difficult to measure D accurately because the
plastic was flexible, deforming when held/the cross-section of the bottle was not a perfect circle/D
was not constant along the length of the bottle’.

Valid improvements included taking more readings for different values of M and then plotting a
suitable graph to test the suggested relationship. Some candidates suggested calculating further
values for k and then calculating an average value, implying that k is constant. They should
instead state that the values of k should be compared with each other to see whether k being
constant is a valid conclusion.

• using an electronic balance to measure M, or a measuring cylinder to measure the water

volume
• fix the masses with tape to the bottom of the bottle or use an alternative to the slotted masses
and mass hanger e.g. lead shot, sand or a single large mass
• record a video of the experiment, then playback using the video recorder’s clock to measure
the period of the oscillations
• measure the diameter D in three different directions and calculating an average value/use
vernier calipers.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/35
Advanced Practical Skills 1

Key messages

• Candidates are encouraged to record all their raw data (in the table and elsewhere), show this clearly
and not just state the final mean value.

General comments

Any help given to a candidate should be noted on the Supervisor’s Report. Supervisors are reminded that
help should not be given with the recording of results, graphical work or analysis.

Comments on specific questions

Question 1

(a) Many candidates stated value z to the nearest 0.1 cm with units and set up the apparatus correctly
so that y was less than 35.0 cm. Some candidates omitted units, or gave readings to the nearest
cm.

(b) Many candidates were able to collect six sets of values of x and y without assistance from the
Supervisor, and showed a correct trend in their values. Some candidates needed help setting up

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

the apparatus. A minority of candidates took 7–8 sets of results and then did not plot all their
observations on the graph grid. Few candidates took the time to repeat their readings. This may
have helped to identify anomalous results and improve data quality. If time is limited, candidates
should be encouraged to look out for possible outliers which do not fit a general trend and repeat
these readings to double-check.

Many candidates chose too small a range over which to conduct the experiment and so the values
ended up too close together. It was common for all values of x to be positive, but candidates
needed to include negative values as well to make full use of the apparatus (i.e. x values to the left
of the 50.0 cm mark). It is expected that candidates consider the whole length provided.

Many candidates were awarded credit for the column heading, giving both the quantity and correct
unit for each heading with both separated by a solidus or with the unit in brackets. Some
candidates omitted either the unit or the separating mark for one of the columns.

Many candidates correctly recorded their raw values for x to the nearest 0.1 cm. Some candidates
stated their measurement to the nearest cm e.g. 20 cm without considering that they can read to
the nearest mm using the ruler provided. A few candidates stated measurements with too many
trailing zeros, e.g. 20.00 cm.

Overall the table work was done well by candidates. Even those who sought the help of the
Supervisor in setting up the apparatus usually gained credit for the table.

Some points were drawn as dots with a diameter greater than half a small square. Many points
were incorrectly plotted so that they were greater than half a square from the correct position. If a
point seems anomalous, candidates should be encouraged to check the plotting and to repeat the
measurement if necessary. If such a point is ignored in assessing the straight line of best fit, the
anomalous point should be labelled clearly (e.g. by circling the point) with no more than one point
being circled.

There is no credit specifically for identifying anomalous points, so candidates should be reminded
that they do not need to identify an anomalous point if they do not think they have one.

Candidates should be encouraged to draw the line according to the positions of the plotted points,
and not to force the line through the origin.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(d) Most candidates recognised that A was equal to the gradient and B was equal to the intercept.
Stronger candidates recorded a value with consistent units for B (m) and no units for A. Weaker
candidates stated incorrect units e.g. B (m–1) or A (m) whilst many others omitted to give units for
B.

(e) Many candidates were able to calculate p using one or both equations provided.

(f) Some candidates were able to draw line W with the same gradient and a lower value of y-intercept.
Some candidates drew lines with different gradients and a few candidates used the same gradient
but a higher value of y-intercept.

Question 2

(a) (i) Many candidates stated a value for T in the appropriate range with a unit, and showed evidence of
repeated timing of five or more oscillations. Some candidates did not repeat sets of readings,
measured only one oscillation, or determined frequency instead of period. There were some cases
of omission of units or very small times e.g. 0.12 s suggesting misreading of the stop-watch.

(ii) Many candidates correctly calculated f. Some candidates incorrectly rounded this value. A
minority of candidates having incorrectly calculated f as T in (i) then inadvertently calculated T
here.

(iii) Many candidates correctly justified the significant figures they had given for the value of f, with
reference to the number of significant figures used in raw time, nT or T. Many candidates gave
reference to just ‘raw readings’ or ‘values in calculation’ without stating what these values were.

(b) (i) Most candidates measured a value of n.

(ii) Most candidates were able to measure a value of the current. Some candidates had difficulty in
taking a reading but still gained credit for writing what their meter reading stated, e.g. 0.0 µA or
0.1 µA. A few candidates left the space for their answers blank and so could not be awarded credit.
Any difficulties with the experiment must be reported by the Supervisor on the Supervisor’s Report
so that the Examiners can take this into account.

(iii) Most candidates are familiar with the equation for calculating percentage uncertainty. Some
candidates made too small an estimate of the absolute uncertainty in the value of I given that it
was a fluctuating reading. Some candidates repeated their readings and correctly gave the
uncertainty in I as half the range, while other candidates did not halve the range.

(ii) Most candidates recorded second n and I values for the tube with fewer turns and recorded a
value less than their first value.

(d) (i) Many candidates were able to calculate k for the two sets of data, showing their working clearly. A
minority of candidates incorrectly rearranged the equation algebraically to calculate 1 / k or
inadvertently substituted the wrong values.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

statement such as ‘this is valid because the values are close to each other’ which was not credited
as there was no justification for the conclusion.

(e) This experiment provided many limitations and improvements to write about, ranging from the
measurements to the practical difficulties of setting up and getting a set of readings. Many
candidates recognised that two sets of data were insufficient to draw a valid conclusion and stated
an improvement of taking more readings and plotting a graph. To gain credit for limitations
concerning measurements, the quantity that was difficult to measure must be referred to along with
the difficulty. For example, ‘it was difficult to measure n as it is not a whole number’ or ‘difficult to
measure the current because it was so small’. Solutions such as ‘use a centre zero analogue
meter’ and ‘use more turns’ gained credit. Credit was awarded for difficulty in the setup e.g. ‘tube
fell over’, ‘magnet hit the tube’, together with stated methods of improvement such as ‘clamp tube
to table’ and ‘use a wider tube’.

Credit is not given for suggested improvements that could be carried out in the original experiment,
such as ‘repeat measurements’, ‘do more readings to get an average value’, ‘ensure reading is
taken perpendicularly’ etc. Unrealistic solutions were also not given credit, e.g. ‘robotic arm’ or
‘mechanical hand’. Problems that were irrelevant or that could have been removed if the candidate
had taken greater care were not given credit. Vague or generic answers such as ‘too few readings’
(without stating a consequence), ‘faulty apparatus’, ‘unstable stand’, ‘use fiducial marker’ (without
stating where to place it), ‘attach spring to rod’ (without specifying how to do this) or ‘use an
assistant’ were also not given credit.

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Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/41
A Level Structured Questions

Key messages

• It is important that candidates use technical language accurately. In this paper, the words that
candidates often confused were atom and molecule, nuclide and nucleus, and force and field.
Candidates are not able to obtain full credit if they use an inappropriate word that makes the response
technically incorrect.

• In defining quantities, candidates need to take care to ensure that the definition they give is
dimensionally correct. This often requires use of the phrase ‘per unit’ where the quantity being defined is
the ratio between two other quantities, or ‘product’ where the quantity being defined is two other
quantities being multiplied together.

• Candidates need to be careful that they do not give more than one answer to a question. If multiple
answers are provided that are contradictory, the candidate cannot be awarded credit for a correct
answer.

• Candidates need to take care to ensure that they read the question properly, understand what is being
asked and give responses that answer the question that is asked. There were several questions on this
paper where a significant number of candidates gave answers to questions that were not asked, but
have been asked in recent past papers. Candidates should be advised not to rely heavily on memorising
previous mark schemes.

General comments

In general terms, candidates scored well on traditional questions involving ‘bookwork’ and not as well on
questions requiring application of knowledge. Working was nearly always shown, which is encouraging, and
the use of English was very good in general and better than in previous series. Candidates should be
encouraged to provide explanations when asked to ‘explain your working’ as some of the credit for the
answer may depend on the explanation.

Comments on specific questions

Question 1

(a) This statement of Newton’s law of gravitation was well-known. A significant number of candidates
did not mention point masses.

(b) (i) Only the strongest candidates were able to answer this question correctly. Many candidates
incorrectly thought that the planets were in equilibrium, either because the speed was constant, the
forces on each planet and the star were equal and opposite, or because the gravitational and
centripetal forces were equal. It was also quite common for a candidate to state that the planets
were not in equilibrium because the radii, masses or forces were different.

(ii) 1. This was generally answered correctly by most candidates.

2. Few candidates gained full credit here by demonstrating that they had used the data for more than
one planet. Most candidates used data for only the first planet in the list. Some candidates did not
look at the column headings carefully and squared the data for T2, hence leading to an incorrect
answer for the mass.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Question 2

(a) (i) Some responses stated that a was directly proportional to y and a was in the opposite direction to
y, but without explaining how the graph showed this. Some omitted the phrase ‘through the origin’.

(ii) This calculation was completed well. There were a few power-of-ten errors caused by not
converting the displacement from mm into m. Some of the least able candidates started with the
incorrect formula a = ωx, which could not be awarded credit.

(b) (i) Candidates who had encountered this idea before found it easy to gain full credit. The
displacement in simple harmonic motion has both positive and negative values. If a position is
being stated, it is important to be precise with terminology: ‘maximum displacement/amplitude’
could not be awarded credit as there was no direction specified.

(ii) Common errors were to use an acceleration of 4.5 m s–2 as this was the end of the line on the
graph, or 5.0 m s–2 as this was the maximum value on the acceleration axis.

Question 3

(a) (i) The weakest candidates often gave answers in terms of the first law of thermodynamics (internal
energy is the work done plus heat supplied). Weak candidates who avoided doing this tended to
answer in terms of the sum of kinetic energy and potential energy of the system, but without
specifying that they meant the energies of the molecules.

(ii) Candidates were either quite familiar with this idea and knew how to relate the change in internal
energy to the change in thermodynamic temperature, or they were not sure at all and again made
reference to the first law of thermodynamics.

(b) A simple approach was to use pV = NkT and substitute. Most candidates chose to use pV = nRT
to find n and then used N = nNA to find N. Some candidates made power-of-ten errors in
converting the volume or thought they had reached the final answer when they had found n, the
number of moles.

Question 4

(a) This question was well answered, and a variety of correct answers were given.

(b) It was quite common for weaker candidates to give long-winded and unnecessary descriptions
either of how an ultrasound transducer works or general descriptions of the medical diagnoses
possible using ultrasound. Candidates should be reminded to read the question carefully and
focus their answers on the specific question being asked.

Common mistakes included:

• not mentioning that the ultrasound is sent into the body as pulses,
• not stating that the time delay between and the relative amplitudes of the transmitted and
reflected pulses gives information about the depth and nature of the boundary between the
body’s structures, not the structures themselves,
• not adequately explaining the purpose of the coupling gel.

Question 5

(a) (i) This was well answered. Candidates need to be specific when referring to the direction of rotation,
e.g. ‘the same direction as the rotation of the Earth’. Clockwise/anticlockwise could not be
awarded credit as these depend on where the observer is.

(ii) This question asked for the position of the satellite. Some candidates gave ‘stationary’ but this is a
description of the velocity relative to the Earth rather than the position.

(iii) Candidates either needed to recall a frequency here or use their knowledge of the electromagnetic
spectrum. Some weaker candidates calculated the frequency associated with a time period of
1 day, i.e. confused the communication frequency with the orbital frequency.

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(b) (i) Many candidates arrived at a correct signal power having used 195 = 10 lg(3000 / P). Weaker
candidates often found a very large signal power having incorrectly used 195 = 10 lg(P / 3000).

(ii) Explaining the need for different frequencies for the up-link and down-link was less well done. A
common misconception amongst weaker candidates was that the signal’s frequency rather than its
power became attenuated during propagation. Stronger candidates grasped the concept that the
attenuated up-link signal would be swamped by the amplified down-link signal if the frequencies
were the same, but weaker candidates’ responses often had a lack of clarity over which signal was
being swamped. Unclear phrases such ‘swamping between’ and ‘swamping with’ were quite
common.

Question 6

(a) The majority of candidates knew this definition.

(b) Correct answers were common. Some candidates made the task harder than it needed to be by
first calculating V using V = E × d and then using Q = V × 4πε0d. Some candidates made arithmetic
mistakes or forgot to convert centimetres to metres. Weaker candidates who had incorrectly
explained field strength tended to use Q = E × 4πε0r to calculate a value for Q.

(c) Many candidates did not give answers in terms of Q. Answers of 4.5 × 10–7 without a unit were
quite common. The weakest candidates tended to give an answer of 6.0 × 10–7, not having
subtracted Q.

Question 7

(a) Candidates found it difficult to give a definition that applies to this specific situation. They often
understood the definition of capacitance, but the question was asking for the definition as
specifically applied to the parallel plate capacitor, so more detail was needed. There was some
confusion between a parallel plate capacitor and capacitors connected in parallel.

(b) (i) Most candidates answered this correctly.

(ii) Many candidates found it difficult to understand how the circuit functions and could not gain credit.
A common incorrect answer was to divide the answer in (b)(i) by 2 rather than 50.

(iii) Many candidates realised that they needed to divide their previous answer by 120.

(c) If a second capacitor of equal capacitance is placed in series with the first, then the capacitance is
halved. Therefore, the current is halved. Many candidates said that the two quantities decreased,
but it was possible to be more specific than this. If values are given in the question, then
candidates should be encouraged to make full use of them.

Question 8

(a) The most common creditworthy description of negative feedback was of the form ‘negative
feedback is when some of the output voltage is fed back to the inverting input of the amplifier’.
Weaker candidates would tend not to state which input the voltage was fed back into.

Some of the weakest candidates tended to write down everything they could remember about op-
amps and quite often contradicted themselves. Some weaker candidates also tended to get things
backwards, e.g. ‘negative feedback increases the gain and reduces the bandwidth’. Some
candidates were not clear about the difference between open-loop and closed-loop gain and gave
‘reduced open-loop gain’ as an incorrect answer.

(b) (i) Candidates generally calculated the gain correctly, although a significant number worked out the
gain (correctly) as –12 and then wrote 12 on the answer line. This is an inverting amplifier circuit
and the gain should be negative.

(ii) This was usually answered correctly. A common mistake was to forget that the power supply to the
op-amp was 6 V and that the output cannot be greater than this value.

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9702 Physics June 2018
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(iii) Many candidates stated that one of the resistors should be swapped for a light-dependent resistor
(LDR) but did not state which resistor should be replaced.

Question 9

(a) Candidates generally realised that there was a force on the wire and usually worked it out as being
upwards. Many candidates did not read the question correctly and gave the force on the wire
rather than the force on the magnets.

(b) (i) This calculation was straightforward, and the majority could recall that F = BIL sin θ. A few weaker
candidates overlooked that the flux density was in mT and their final answer was incorrect by a
factor of 10–3.

(ii) Most candidates were correct. A common incorrect answer was 1.85 × 10–3 N as candidates
divided by sin 60° but forgot to multiply by sin 60° as well.

(c) A few candidates did not draw an alternating waveform. Whatever waveform candidates drew, it
was common for weaker candidates to draw less than two cycles, and to omit to put any values on
the axes. Some of the stronger candidates could have improved their answers by taking more care
when sketching their waveforms and ensuring that the waveform does not have different positive
and negative amplitudes or a changing time period.

Question 10

(a) Most candidates gained credit for this definition.

(b) This question proved to be difficult. Many candidates could state Lenz’s law but found it difficult to
assemble a reasoned argument. Some candidates gave an answer formed around the idea that
the ring jumps upwards because ‘the e.m.f. in the ring opposes the e.m.f. in the coil’. Some of the
stronger candidates did understand the notion of two opposing magnetic fields resulting in an
upward force on the ring, but were unable to come up with a convincing argument in support of
this. The strongest candidates wrote well-reasoned explanations and were awarded full credit.

Question 11

(a) (i) Most candidates were awarded credit, although the word energy was sometimes missed out.

(ii) Candidates found this question difficult. The question was asking for evidence and the answers of
many candidates were not focused on evidence.

(b) Some weak candidates answered in terms of the electrons in the photoelectric effect, and used
phrases such as ‘work function’ and ‘threshold frequency’ that were not relevant to the question.
Some candidates did not understand the one-to-one relationship between a photon and an
electron, and instead constructed arguments based on the absorbed photons making the material
warmer and this making the electrons more energetic. It would be helpful to emphasise further the
idea of the one-to-one interaction to these candidates. Other weaker candidates tended to state
that electrons absorbed energy directly from the incident light without using photons in the
explanation, and had electrons ‘jumping over’ the forbidden band without a sense of which direction
they were jumping in.

As with previous explanations, strong candidates could provide a clear explanation.

Question 12

(a) (i) Most candidates understood the question. Some candidates gave answers to 1 significant figure
when the data were given to 2 significant figures.

(ii) The effect of passing through more than one material was not well understood, and some
candidates incorrectly added, subtracted or occasionally divided the exponential decay terms.

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(b) (i) The weakest candidates thought that contrast had something to do with image clarity or sharpness.
Whilst most candidates knew it had something to do with image brightness and darkness, many
found it difficult to express this. Only a few stated it was the difference in the degree of blackening
between structures.

(ii) Weaker candidates tended to ignore the values they had just calculated, and instead referred to the
absorption coefficients rather than the relative differences in transmitted intensity between soft
tissue and a combination of bone and soft tissue.

Question 13

(a) Candidates found it difficult to give a complete answer. Radioactivity is a nuclear process. Some
answers contained descriptions of fission or did not mention that particles/radiation was emitted.
The key words ‘unstable’ and ‘spontaneous’ were often missing.

(b) (i) Some weaker candidates incorrectly calculated the activity as the gradient of a line drawn from the
origin to the curve at t = 14 minutes. The easiest way to answer the question was to draw a
tangent at 14 minutes and then to find the activity as the gradient of the tangent. Another approach
was to find the half-life and work out the activity from it.

(ii) Most candidates gained credit for this question.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/42
A Level Structured Questions

Key messages

General comments

The question paper contained questions of a variety of levels of difficulty, enabling candidates at different
levels of ability to show what they know. Candidates who knew the ‘bookwork’, read the questions carefully,
took care over their use of technical language and answered the questions asked were able to score well.

Some candidates relied heavily on answers that had been memorised but were not relevant to the question
being asked in Questions 1(a)(i), 1(a)(ii), 3(a), 7(a)(i), 9(a) and 12(a).

There was no evidence that candidates who were properly prepared for the examination had insufficient time
in which to complete the paper.

Comments on specific questions

Question 1

(a) (i) Many candidates answered this question as if answering a question about the meaning of a
gravitational field or about Newton’s law of gravitation. Of those who did recognise that the
question addressed the meaning of the lines of gravitational force, only a small number were able
to express that the lines represented the direction of the force on a mass placed in the field.

(ii) Few candidates recognised that they were asked to give an explanation in terms of lines of
gravitational force. Many gave an equation for the gravitational field strength due to a point mass
and answered in terms of the variables in this equation, but such responses did not answer the
question asked. Of candidates who did discuss lines of force, most were able to state that the lines
of force are radial. Explanations of why radial lines can be approximated as parallel lines close to

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the surface of a planet with large radius were uncommon. More candidates were able to explain
the implications of parallel field lines for constant field strength.

(b) There were many ways of answering this question. The expected starting point, whichever
approach was chosen, was to realise that the threshold for escape is when the kinetic energy of the
rock exceeds the potential energy required to reach infinity. From there, calculation of a relevant
quantity was expected, finally leading to a comparison between the threshold value and the actual
value to lead to the conclusion that the rock escapes into space. The most common valid method
used was to calculate the escape velocity of the Moon to be 2.4 km s–1 and to compare this with the
actual velocity of the rock of 2.8 km s–1. An approach that calculated and compared kinetic and
potential energies was also commonly seen.

A common error was to calculate the orbital velocity of the Moon and equate forces (gravitational
and centripetal forces) rather than energy.

Question 2

(a) Most candidates were able to link the absence of potential energy with the assumption of the
kinetic theory that the forces between molecules in an ideal gas are negligible. Some candidates
contradicted themselves by listing more than one assumption. Others muddled their answers by
talking about the forces within the molecules or the forces between atoms, neither of which could
be awarded credit.

(b) (i) Most candidates were able to give a correct description of the meaning of <c2>. Common incorrect
answers were ‘root mean square speed’ and ‘mean speed squared’ of molecules. A few gave an
answer in terms of the gas rather than its molecules.

(ii) This question was generally well-answered. Some candidates did not establish that T represents
absolute temperature and some referred to a change in temperature rather than absolute
temperature.

(c) (i) 1. This question was well-answered by many candidates. The strongest candidates started with the
most immediately relevant equation pV = NkT, but most determined the number of moles first. A
significant number of candidates calculated the number of moles but then stopped.

2. Many candidates realised that the 2900 J input of thermal energy, in a constant volume change,
represented the increase in internal energy of the gas (and therefore the total increase in kinetic
energy of the particles of the gas). Often, these candidates then gave 2900 J as the average
increase in kinetic energy per molecule without pausing to consider the plausibility of this answer.

(ii) Candidates were expected to equate their answer to (c)(i) part 2 with (3/2)kT to calculate a
temperature increase and then add this to the initial temperature. Candidates starting from 2900 J
arrived at an implausible final temperature but were awarded credit if all of the working was clear.

Question 3

(a) Candidates at this level need to be clear and accurate in their use of terminology and to avoid
colloquial language such as ‘overcoming forces’. Credit was available for establishing why there is
a need for an input of energy. It was not possible to gain full credit without reference to particles in
some form – firstly by discussing the need to break bonds, or weaken forces, between molecules
and secondly by establishing that during melting the potential energy of molecules is increased.
Credit was also awarded for explaining that, because the volume of the substance does not
change, the required input of energy cannot be through work being done on the system and so it
must be provided by thermal energy.

A significant minority of candidates misread the question and gave an explanation of why the
temperature does not change during melting.

(b) (i) Most candidates were awarded credit for giving the correct equation relating thermal energy input
to temperature change, and many were also able to show how this equation applied to the loss of
energy from the water and from the can in cooling from 38 °C to 23 °C to give the required value.
Common errors were to add the two masses together before applying the equation or to use
absolute temperatures rather than temperature changes.

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Principal Examiner Report for Teachers

(ii) It was less common to see candidates realise that the loss of energy in (b)(i) was used partly to
raise the temperature of the ice, both before and after melting, as well as to provide the latent heat
needed to melt the ice. Partial credit was available to candidates who took account of the rise in
temperature in one of the two stages (either from –18 °C to 0 °C using the specific heat capacity of
ice, or from 0 °C to 23 °C using the specific heat capacity of water).

Question 4

(a) The defining properties of simple harmonic motion were well-known by many candidates.

(b) (i) 1. Many candidates were able to identify the damping as light damping, and to explain this in terms of
either continuing oscillation or gradual decrease in amplitude. Common mistakes were to conflate
displacement with amplitude or to conflate exponential with gradual. Candidates who gave
‘exponential decrease in amplitude’ as the justification for light damping appeared to not appreciate
that the decrease in amplitude is exponential in all three forms of damping.

2. Many candidates were awarded credit for discussing either the nature of the resistive forces acting
on the mass or the loss of energy from the system. A common misunderstanding was that the
springs cause the damping and that energy stored in the springs is lost from the system.

(ii) 1. This question was answered fully and well by very many candidates. Common mistakes were to
neglect the conversion of the mass of the trolley to SI units or to forget the square root at the end of
the calculation. The most common fundamental error was to start from a = –ωx (and hence to
deduce ω = 2k/m) rather than a = –ω2x.

2. Many candidates successfully deduced the period of the oscillations from their value of ω. A
smaller number realised that time t1 represents 1.5 periods.

Question 5

(a) (i) The basic meaning of bandwidth as a range of frequencies was generally well-known by many.

(ii) It is essential, with questions such as this, that candidates appreciate the comparative nature of
what is being asked. Statements such as ‘good quality’ and ‘limited number of stations’ are
meaningless unless referenced to a standard, and in the absence of any reference point are
equally true for all bandwidths. For example, the number of stations that can be fitted into a
frequency range is ‘limited’ for all bandwidths. The difference for waves of greater bandwidth is
that the number is more limited. Responses needed to be comparative in nature.

Some candidates discussed quality of sound rather than quality of signal and some asserted that
‘more information’ could be carried without any reference to time rate. A common error with the
disadvantage was to relate fewer stations to area on the ground rather than to a region of the
frequency spectrum.

(b) (i) This part was universally well-answered by candidates who clearly understood frequency
modulation.

(ii) Partial credit was available to candidates who showed some understanding of the concept of
frequency deviation, but who used 5 V rather than 3 V as the voltage variation. Some candidates
showed a lack of understanding of this topic by using either the raw frequency deviation figure or
the frequency of the signal wave and using one of those as the basis of the frequency modulation.

(iii) Candidates found this question difficult. Many candidates did not calculate the period. Most of the
candidates who worked out the period did not appreciate that the time between maximum and
minimum frequency is half the period.

Question 6

(a) Many candidates were able to identify the defining equation of capacitance as the ratio of charge to
potential. It is worth emphasising to candidates that definitions in terms of units, when a question is

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asking for the definition of a quantity, cannot be awarded credit. The use of equations requires
definition of the terms in the equation.

(b) This question asked for circuit diagrams showing how three capacitors can be connected, so credit
could be awarded only when the components drawn were clearly capacitors and three in number.
Answers showing resistors or combinations of six capacitors were common from weaker
candidates. Most candidates who knew the topic were able to answer successfully, provided it was
made clear in some way between which points the capacitance had the required value.

(c) (i) Calculation of the combined capacitance of the two capacitors in series was done well by many
candidates. Common errors were to combine them in parallel rather than series, to forget to take
the reciprocal at the end, or to get as far as 6/5 but to neglect to carry out the final calculation.

(ii) 1. Most candidates were awarded at least partial credit, and many deduced the 9.6 µC figure. Having
arrived at the ‘correct’ answer, some candidates did not appreciate that the total charge for
capacitors connected in series is the same as the charge on each capacitor, and attempted to
apply some sort of ratio to their 9.6 µC figure.

2. Many candidates were awarded full credit. Some candidates who started off with E = ½CV2 then
incorrectly used 8 V as the p.d. across the 3 µF capacitor.

Question 7

(a) (i) The concept of feedback in general was well-known by most. Very few candidates attempted to
explain the ‘negative’ aspect of the question, and those who did often answered in terms that were
specific to the operational amplifier rather than addressing the concept of negative feedback in
principle.

(ii) It was important for candidates to appreciate the comparative nature of what was being asked. For
example, it was common to see responses suggesting that negative feedback will result in ‘no
saturation’. It is possible for an amplifier to become saturated even with negative feedback, and the
point about negative feedback is that it makes saturation less likely (or occur over a narrower range
of input voltages).

(b) (i) The formula for the gain of a non-inverting amplifier was well-known and successfully applied by
many candidates. A common error was the use of the inverting amplifier formula instead. Some
candidates incorrectly gave their gain with units of V.

(ii) Use of the gain calculated in (b)(i) to calculate the output voltages in this part was often done well.
The most common mistakes were incorrect signs and a lack of understanding that, for output
voltages above 9.0 V, the amplifier would become saturated.

(iii) A small number of candidates answered this question successfully. Of the candidates who did
identify that a thermistor was required, only a small number accurately identified where in the circuit
it needed to be placed.

Question 8

(a) Credit could be awarded based on the required points being evident in a diagram. However, it was
rare to see diagrams that added to the quality of an answer. Many candidates put time and effort
into producing neat and tidy diagrams that were unable to score credit on account of being
inadequately labelled, so it is important to encourage candidates to label their diagrams. It was
more common for the relative orientation of the two fields, and the velocity of the particle relative to
the fields, to be being credited on the basis of the point being explicitly made in the narrative.

Many candidates gave full and detailed explanations of how a particle can pass through the region
of the fields undeviated, but did not go on to make it sufficiently clear that this only happens for
particles with a particular velocity.

(b) This question was well-answered, with a large number of candidates able to score full credit. Many
other candidates were able to show that the mass was 3.3 × 10–26 kg but then stopped short of
showing the division by 1.66 × 10–27 to reach 20 u. A small number of candidates used the mass of

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a proton rather than the unified atomic mass unit, and some did not demonstrate awareness of the
need to convert the diameter to the radius.

(c) This part was well-answered by many candidates. Common errors were to draw semicircles with
smaller rather than larger radius and to take insufficient care over the drawing of the semicircle.
Such responses showed paths that either did not pass through the slit, or were not semicircular, or
remained on the original path for too long.

Question 9

(a) This question gave candidates the opportunity to receive credit for some basic ‘bookwork’, and
most candidates were successful. Some candidates answered a completely different question, and
instead gave a description of Faraday’s law of electromagnetic induction. Some gave a formula but
did not define the symbols used in it. Others conflated flux and flux density, and some gave
responses that contradicted themselves by giving two definitions, one of which was incorrect.

(b) This part was well-answered, with many candidates receiving full credit. It was a ‘show that’
question, which meant that candidates did need to show the working clearly for the calculation of
the cross-sectional area in order to be able to access full credit.

(c) Many candidates were awarded at least partial credit. Most realised the times between which the
induced e.m.f. will be zero, and many realised that the non-zero parts of the graph are stepped.
The stronger candidates were able to show the steps at the correct magnitudes, appreciating that
the second change, providing for the same change of magnetic flux linkage over half the time, will
result in an induced e.m.f. of twice the magnitude of the first. Fewer candidates realised that both
sloping parts of the B-t graph had the same sign of gradient, so the two steps should each show an
e.m.f. of the same polarity.

Question 10

(a) Many candidates were able to describe the photoelectric effect as the emission of electrons when
electromagnetic radiation is incident on a surface. In some cases, although electromagnetic
radiation was mentioned, it was not clear enough that it is incident on a surface. A few candidates
were confused and talked about emission of photons from the surface.

(b) (i) The identity of a photon as a packet of energy was generally well-known. The idea that this energy
is quantised energy of electromagnetic radiation was less well understood. A significant minority of
candidates thought that electromagnetic waves ‘carried’ the photons rather than the photons being
the electromagnetic energy in packets. The most common error from candidates who did have the
right idea was to miss out the crucial reference to energy.

(ii) This question was well-answered by many candidates. The relationship between photon energy
and wavelength was generally well-known, and most candidates who knew the relationship were
able to carry out the calculation correctly. Irrelevant or incorrect starting equations (such as
E = mc2 or E = hλ) could not be awarded credit.

(iii) Most candidates who calculated a photon energy in (b)(ii) were able to make a correct deduction
about emission from the two metals that was consistent with their photon energy.

Question 11

(a) There was a wide variety of performance in this question. As ever, it is important that candidates
answer the question asked. There were several detailed explanations of magnetic resonance
imaging that were irrelevant to the question. It is crucial that candidates take care with their
wording and use technical terminology accurately.

For this question, the fundamental principle that candidates had to explain in order to be awarded
credit is that CT scans divide up the object being scanned into slices. Each slice is scanned in
turn, many times, in order to produce a 2-dimensional image. Finally, this process is repeated for
multiple slices and the 2-dimensional images are combined together to build up a 3-dimensional
image of the whole structure. Candidates who omitted discussion of ‘slices’ or ‘sections’ of the

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body were not able to score credit. Some answers were confused (e.g. talking about the images
being ‘sliced’) and could not be awarded credit.

Other common pitfalls were to omit reference to X-rays as being the radiation used for scanning
and to confuse when the image is 2-dimensional and when it is 3-dimensional. Candidates who
simply try to memorise and reproduce previous mark schemes, without developing any real
understanding of what they are trying to describe, often make these mistakes.

(b) (i) In both parts of (b), a common mistake was to give responses that merely restated the question. In
describing the need to compile many 2-dimensional images together, many candidates simply
stated that this was ‘not possible without fast computers with large memories’. The idea that this
compilation required large numbers of calculations was often missing.

(ii) Many responses restated that more exposure to X-rays was needed, without explaining this by
reference to the many images taken in CT scans.

Question 12

(a) A significant minority of candidates answered a question asking for the definition of decay constant
rather than the meaning of radioactive decay. Of those who did address the question that was
asked, crucial parts of the answer were sometimes missing. Some answers did not contain
reference to ‘emission’ or ‘unstable’. Some described the particles emitted as themselves being
radioactive. Candidates who knew their basic ‘bookwork’ were able to be awarded full credit.

(b) (i) The labelling of the lines on the graph was generally done well, and many candidates were able to
score full credit. There were a few instances of candidates contradicting themselves by labelling
the same line with two letters.

(ii) Most candidates knew how to calculate decay constant from half-life. Some candidates rounded
their answer to 2 significant figures, even though it was obtained entirely from 3 significant figure
data.

(c) Some candidates were able to deduce that the consequence of nuclide F having a much larger
decay constant that nuclide E is that it has a much shorter half-life (than E). The comparison here
was essential in order to be able to answer the question properly. Only a small number of
candidates were able to go on to explain the consequence of this comparison, that nuclei of nuclide
F decay almost as soon as they are formed.

Many candidates asserted that nuclei of F decay ‘faster’ than those of nuclide E. The answers of a
large number of candidates suggested that they did not realise that nuclei of F do not exist until a
nucleus of E has decayed.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/43
A Level Structured Questions

Key messages

General comments

Comments on specific questions

Question 1

(a) This statement of Newton’s law of gravitation was well-known. A significant number of candidates
did not mention point masses.

(ii) 1. This was generally answered correctly by most candidates.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Question 2

(ii) Common errors were to use an acceleration of 4.5 m s–2 as this was the end of the line on the
graph, or 5.0 m s–2 as this was the maximum value on the acceleration axis.

Question 3

Question 4

(a) This question was well answered, and a variety of correct answers were given.

Common mistakes included:

Question 5

(ii) This question asked for the position of the satellite. Some candidates gave ‘stationary’ but this is a
description of the velocity relative to the Earth rather than the position.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(b) (i) Many candidates arrived at a correct signal power having used 195 = 10 lg(3000 / P). Weaker
candidates often found a very large signal power having incorrectly used 195 = 10 lg(P / 3000).

Question 6

(a) The majority of candidates knew this definition.

Question 7

(b) (i) Most candidates answered this correctly.

(ii) Many candidates found it difficult to understand how the circuit functions and could not gain credit.
A common incorrect answer was to divide the answer in (b)(i) by 2 rather than 50.

(iii) Many candidates realised that they needed to divide their previous answer by 120.

Question 8

(ii) This was usually answered correctly. A common mistake was to forget that the power supply to the
op-amp was 6 V and that the output cannot be greater than this value.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

(iii) Many candidates stated that one of the resistors should be swapped for a light-dependent resistor
(LDR) but did not state which resistor should be replaced.

Question 9

(ii) Most candidates were correct. A common incorrect answer was 1.85 × 10–3 N as candidates
divided by sin 60° but forgot to multiply by sin 60° as well.

Question 10

(a) Most candidates gained credit for this definition.

Question 11

(a) (i) Most candidates were awarded credit, although the word energy was sometimes missed out.

(ii) Candidates found this question difficult. The question was asking for evidence and the answers of
many candidates were not focused on evidence.

As with previous explanations, strong candidates could provide a clear explanation.

Question 12

(a) (i) Most candidates understood the question. Some candidates gave answers to 1 significant figure
when the data were given to 2 significant figures.

(ii) The effect of passing through more than one material was not well understood, and some
candidates incorrectly added, subtracted or occasionally divided the exponential decay terms.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Question 13

(ii) Most candidates gained credit for this question.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/51
Planning, Analysis and Evaluation

Key messages

• Candidates should be encouraged to read the questions carefully before answering them to understand
what is required. Planning a few key points before commencing Question 1 is useful.

• In Question 1, candidates’ responses should include detailed explanations of experimental procedures

such as how to control variables, how to take measurements and how to analyse the data.

• Graphical work should be carefully attempted and checked. Candidates should take care when reading
information from the graph.

• The numerical answers towards the end of Question 2 require candidates to show all their working
particularly when determining uncertainties. A full understanding of significant figures and the treatment
of uncertainties is required.

• The practical skills required for this paper should be developed and practised with a ‘hands on’
approach.

General comments

In Question 1, the methods that candidates described often did not contain enough detail and did not
mention the appropriate measurements. It is advisable that candidates should think carefully about the
experiment following the points given on the question paper and to imagine how they would perform the
experiment in the laboratory. There were many unworkable set-ups shown in diagrams and a few
candidates chose the wrong independent and dependent variables. A large number of candidates did well
on the analysis section. Many candidates did not provide enough additional detail. For example, a
statement such as ‘the experiment was repeated the results averaged’ is too generic and does not contain
enough detail. It is essential for candidates to have experienced practical work in preparation for answering
this paper.

In Question 2, the initial algebraic task was difficult and this led to problems in later parts of the question for
some candidates. Candidates should be familiar with completing a results table for quantities and their
uncertainty, and with finding the gradient and y-intercept of a graph. For a significant number of candidates,
credit was not awarded because the points were not plotted correctly, the line of best fit or worst acceptable
line was not drawn correctly or coordinates were wrongly read off. Some candidates did not realise that
there was a false origin and so the y-intercept could not be read directly from the y-axis.

To be successful, candidates should be advised that mathematical working requires a clear statement of the
equation used with correct substitution of numbers, and the answer calculated with a unit including the
correct power of ten. At all stages, candidates must keep in mind the unit of each answer. Candidates
should set out their working in a logical and readable manner.

There was a significant minority of candidates who omitted the last part of the paper. This could be due to
the difficulty of the equation in Question 2 which was required for the analysis in the final parts of the paper.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Comments on specific questions

Question 1

Most candidates correctly identified the independent and dependent variables. Candidates should be
encouraged then to consider the control of variables and to explicitly state the quantities that need to be kept
constant to make the experiment a fair test. Many candidates stated that the distance r needed to remain
constant, although some candidates identified r as the independent variable. Some candidates use the
incorrect term ‘control’ rather than the correct term ‘constant’.

Credit is available for the method of data collection. Candidates were awarded credit for a clearly labelled
diagram. Diagrams should be drawn of the set-up of the experiment and should include all the equipment
that is to be used. In this question, candidates needed to show labelled spheres with sphere T clearly
attached to a support, e.g. a stand and clamp or hook from the (labelled) ceiling, and sphere S placed on a
top-pan balance. There should also be clear indications of distance(s) that are to be measured. There were
a significant number of diagrams that were unworkable such as sphere T incorrectly suspended, sphere S
being clamped, sphere S not in contact with the balance, or both spheres placed horizontally.

To obtain the value of F, readings from the top-pan balance are required. Many candidates did not indicate
there was a mass/force/reading difference to be taken. Some candidates mentioned to tare the balance
after placing sphere S on it. Credit for additional detail was available for converting the change in mass
reading to force. The statement F = mg could only be awarded credit if m (change in mass) and g
(acceleration of free fall) were defined. Candidates should be encouraged to define symbols in any equation
that is used.

To obtain the value of the e.m.f. V of the power source, a voltmeter is required placed in parallel with the
power source. This could be awarded based on a correct electrical circuit diagram. Very few candidates
were awarded credit as the majority of candidates either drew the connected sphere as part of a complete
circuit (in effect measuring the p.d. across sphere T) or put the voltmeter in series.

Most candidates described a measurement of distance r between the spheres with a ruler. Stronger
candidates were often awarded credit on the basis of a diagram showing a rule placed adjacent to distance r
in the diagram with r clearly indicated. A mark for additional detail was available for explaining how the
distance r can be obtained from the measured readings. A further mark could then be obtained for the
measurement of the diameter (hence radius) of the spheres, although this had to be relevant to the
measurement of r. Some candidates described very well the checking of the spheres to ensure that they
were the same diameter and then measuring from the top of one to the top of the other. Others described
measuring the distance between the spheres and adding the radii.

Credit was available for the analysis of data. The majority of candidates selected the correct axes for the
graph, but a small proportion of candidates incorrectly suggested plotting r on one of the axes. Candidates
must explicitly state the quantities to be plotted on each axis either in the text or on drawn axes. A mark was
available for explaining how the graph would confirm the suggested relationship. Candidates needed to use
the words ‘relationship is valid if’, the word ‘straight’ to describe the line and also state ‘passing through the
origin’.

The additional detail section has a maximum of six marks that could be awarded. Candidates should be
encouraged to write their plans including appropriate detail; often candidates’ answers suggested that they
did not have sufficient practical experience. Vague responses were not credited. It is essential that
candidates’ answers are relevant to the experiment in the question rather than general ‘textbook’ rules for
working in the laboratory.

Credit was available for a safety precaution. A clearly reasoned precaution relevant to the experiment was
required, including a reason why the safety precaution is selected. For example, insulating gloves are worn
to avoid electric shock. Other good safety precautions included the use of shrouded leads or the avoidance
of bare connections owing to the high voltages used.

Candidates must be specific when repeating readings and averaging. The relevant quantities need to be
stated. In this experiment, candidates needed to state that for each value of V, the experiment would be
repeated and the average F determined.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Other creditworthy additional detail included realising that an insulator is required between the charged
spheres and the balance/stand and clamp, discharging the spheres by earthing, giving a method to ensure
the charge on S is constant and preventing draughts from moving sphere T.

Question 2

(a) Candidates who were mathematically confident were able to work through the algebra and achieve
credit. A significant number of candidates arrived at wrong and sometimes complicated
expressions for both the gradient and y-intercept. A few candidates omitted the minus sign in the
y-intercept.

(b) Completing the table was straightforward. Some candidates did not consider the appropriate
number of significant figures. The first four readings in the s column were to two significant figures
while the last two readings were to three significant figures. The calculated values of the 1 / s2
column could be given to two or three significant figures for the first four readings and three or four
significant figures for the last two readings. Most candidates found the uncertainties correctly.

(c) (i) The majority of the stronger candidates were awarded full credit. The points and error bars were
straightforward to plot. A significant number of candidates drew large ‘blobs’ for the plotted points
which could not be given credit. Candidates need to take greater care over the accuracy of the
error bars and ensure that the error bar is symmetrical.

(ii) The drawing of the straight lines has continued to improve. Candidates who are successful appear
to be using a sharp pencil and a transparent 30 cm ruler which covers all of the points. For this
particular set of data, the line of best fit should not have passed through the lowest point.

The worst acceptable line was drawn well in general with a noticeable number of strong candidates
drawing a line which passed through all error bars. Candidates should clearly indicate the lines
drawn. Where a dashed line is used to represent the worst acceptable line, the dashed parts of the
line should cross the error bars.

(iii) Most candidates clearly demonstrated the points that they used to calculate the gradient. Some
candidates misread coordinates or did not use a sufficiently large triangle. A small number of
candidates chose points that did not lie on the lines. Candidates should be encouraged to read
carefully the quantities from the axes and to pay attention to powers of ten and units. When
determining the uncertainty in the gradient, candidates need to show their working including the
coordinates that they have used from the worst acceptable line.

(iv) The majority of the candidates who were awarded full credit set out their working clearly. Stronger
candidates often substituted a data point from the gradient calculation into y = mx + c. When
determining the y-intercept of the worst acceptable line, candidates need to show clearly the
substitution into y = mx + c of a point from the worst acceptable line and the gradient of the worst
acceptable line from (c)(iii).

(d) (i) Many candidates found this part difficult. It was essential that the gradient and y-intercept were
used. Candidates who used the gradient and y-intercept correctly and gave their answers to an
appropriate number of significant figures were given credit. The determination of D was generally
achieved by those candidates who used the correct formulae in (a). The determination of N
required correct logical working with numbers substituted for each quantity. In addition, the correct
power of ten and units were also required. Some stronger candidates correctly used a combination
of the y-intercept and gradient to determine N. Candidates should be encouraged to consider both
the powers of ten and the units used on the plotted graph.

(ii) Many candidates found this part difficult and a significant minority of candidates omitted it.
Candidates who fully understood the treatment of uncertainties and, in particular, how the theory is
applied to square root and squared terms were able to gain credit. Stronger candidates clearly
presented their working in a readable, logical order. Some candidates attempted to find the
percentage uncertainty by maximum/minimum methods. Again, stronger candidates who
demonstrated clearly their method gained credit.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/52
Planning, Analysis and Evaluation

Key messages

• Candidates should be encouraged to read the questions carefully before answering them to understand
what is required. Planning a few key points before commencing Question 1 is useful.

• In Question 1, candidates’ responses should include detailed explanations of experimental procedures

such as how to control variables, how to take measurements and how to analyse the data.

• Graphical work should be carefully attempted and checked. Candidates should take care when reading
information from the graph.

• The practical skills required for this paper should be developed and practised with a ‘hands on’
approach.

General comments

In Question 1, the methods that candidates described often did not contain enough detail and did not
mention the appropriate measurements. It is advisable that candidates should think carefully about the
experiment following the points given on the question paper and to imagine how they would perform the
experiment in the laboratory. A few candidates chose the wrong independent and dependent variable.
Many candidates did well on the analysis section although a significant number of candidates stated that the
straight line would pass through the origin. Many candidates did not provide enough additional detail. It is
essential for candidates to have experienced practical work in preparation for answering this paper.

In Question 2, the initial algebraic task was difficult and this led to problems in later parts of the question for
some candidates. Candidates should be familiar with completing a results table for quantities and their
uncertainty and with drawing a graph to find the gradient. For a significant number of candidates, credit was
not awarded because the points were not plotted correctly, the line of best fit or worst acceptable line was
not drawn correctly or there were wrong readings of coordinates for the gradient.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Comments on specific questions

Question 1

Candidates should be advised to read the question carefully before starting their answer. Most candidates
correctly identified the independent and dependent variables. Candidates should be encouraged then to
consider the control of variables and which quantities need to be kept constant. Some candidates use the
incorrect term ‘control’ rather than the correct term ‘constant’.

Candidates also needed to give the method of changing the wavelength and of measuring the quantities h
and λ. Appropriate measuring instruments and techniques must be stated. Stronger candidates clearly
indicated the use of filters when using a white light source or suggested using lasers of different colours to
change the wavelength of the incident light.

To obtain the value of the wavelength λ of the incident light, a large number of candidates suggested
carrying out an experiment using two slits or a diffraction grating. Having stated an appropriate experiment,
candidates could then gain credit for stating an equation showing how the wavelength λ of the incident light
could be determined. Candidates should be encouraged to clearly define the symbols that are used.
Candidates who used filters could gain credit for suggesting recording the value from the filter, although it
would have been better to provide a more detailed method.

Most candidates correctly suggested measurement of distance h with a ruler. Some candidates were
awarded credit on the basis of a clearly labelled diagram which included the metre rule adjacent to the
distance h which was clearly indicated. Some candidates were awarded credit for explaining how the
distance h was found accurately by measuring the distance to the top and bottom of the maxima and
determining the mean value of h.

Credit was available for the analysis of data. The majority of candidates selected the correct axes for the
graph, usually stating that h was plotted on the y-axis and λ on the x-axis. Candidates must explicitly state
the quantities to be plotted on each axis, either in the text or on drawn axes; simply writing the equation of a
straight line underneath the given expression was not sufficient to gain credit. Candidates also needed to
explain how to find the values of the constants d and B using the graph. The constants must be the subject
of the expression used. It was expected that d would be expressed in terms of the gradient and B in terms of
the y-intercept. Candidates who reversed the axes by plotting λ on the y-axis and h on the x-axis could still
gain full credit, although the determination of B was more difficult. There was an additional detail mark for
explaining how the graph would confirm the suggested relationship. Candidates are expected to state the
words ‘relationship is valid if’ and the word ‘straight’ to describe the line. Some candidates incorrectly stated
that the line passed through the origin and could not be awarded credit since there had to be a y-intercept.

Credit was available for a safety precaution which required a clearly reasoned precaution relevant to the
experiment. A reason must be given as to why the safety precaution is selected. In this experiment, many
candidates suggested the use of lasers or intense light sources, and could receive credit for a safety
precaution to prevent damage to the eyes from the light. Candidates need to give sufficient detail; ‘wear
goggles’ on its own was not accepted since it was not clear whether the goggles were protecting the eyes
from intense light or moving objects.

A further mark for additional detail was awarded to candidates who suggested the use of an intense light
source. Candidates needed to explicitly state this requirement. Candidates who decided to use a laser
could have been awarded credit here for explaining why a laser was being used. The mark could also be
awarded for the use of a collimated light source.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Other creditworthy additional detail included explaining how the zero order maximum was obtained and thus
the given order was measured, performing the experiment in a dark room and cleaning the surface of the
CD.

Question 2

(a) Candidates who were mathematically confident were able to work through the algebra and achieve
credit for determining an expression for the gradient. A significant number of candidates arrived at
wrong and sometimes complicated expressions. A few candidates were confused by the minus
sign. A common error was to omit the ‘1’.

(b) Completing the table was straightforward. Some candidates did not consider the appropriate
number of significant figures. Since R was given to two significant figures, the calculated readings
of the nR column could be given to two or three significant figures.

Most candidates found the uncertainties correctly. A number of candidates incorrectly determined
the absolute uncertainty in nR, often writing incorrectly that the uncertainty for n = 2 was 1 or 1.0
instead of 0.9 or 0.94.

(ii) The drawing of the straight lines has continued to improve. Candidates who are successful appear
to be using a sharp pencil and a transparent 30 cm ruler which covers all of the points. For this set
of data, the line of best fit should not have passed through both the highest and lowest points.

(d) (i) Many candidates found this part difficult. It was essential that the gradient was used. Candidates
who used the gradient and gave their answers to an appropriate number of significant figures were
given credit. The determination of C was generally achieved by those candidates who had a
correct expression in (a). Credit was also available for the correct power of ten and correct units
for C. Candidates should be encouraged to consider both the powers of ten and the units used on
the plotted graph.

(ii) Many candidates who fully understood the treatment of uncertainties realised that the percentage
uncertainty in C was equal to the percentage uncertainty in the gradient. Stronger candidates
clearly presented their working in a readable, logical order. Some candidates attempted to find the
percentage uncertainty by determining the maximum or minimum value of C, and candidates who
clearly demonstrated this method gained credit.

(e) Many candidates found this part difficult and a significant minority of candidates omitted it. Again,
stronger candidates clearly presented their working in a readable, logical order. Common mistakes
were power-of-ten errors or not correctly interpreting the 90% value for V / E.

Stronger candidates who realised that the fractional uncertainty in K was the same as the fractional
uncertainty in C often gained credit. Some candidates attempted to find the absolute uncertainty
by maximum/minimum methods. Again, stronger candidates who demonstrated clearly their
method gained credit.

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

PHYSICS

Paper 9702/53
Planning, Analysis and Evaluation

Key messages

• Candidates should be encouraged to read the questions carefully before answering them to understand
what is required. Planning a few key points before commencing Question 1 is useful.

• In Question 1, candidates’ responses should include detailed explanations of experimental procedures

such as how to control variables, how to take measurements and how to analyse the data.

• Graphical work should be carefully attempted and checked. Candidates should take care when reading
information from the graph.

• The practical skills required for this paper should be developed and practised with a ‘hands on’
approach.

General comments

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Comments on specific questions

Question 1

© 2018
Cambridge International Advanced Subsidiary and Advanced Level
9702 Physics June 2018
Principal Examiner Report for Teachers

Question 2

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Cambridge A-Level Physics 2018 Report

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Cambridge A-Level Physics 2018 Report

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Cambridge International Advanced Subsidiary and Advanced Level

9702 Physics June 2018

Comments on specific questions

Comments on specific questions

Comments on specific questions

Comments on specific questions

Comments on specific questions

(a) Most answers were correct.

(a) The majority of answers were fully correct.

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Comments on specific questions

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(h) (i) The calculation of k was correct in nearly all cases.

Comments on specific questions

Comments on specific questions

(a) Most candidates calculated the value of M correctly.

Several candidates forgot to divide their average value of nT by n to find T.

(c) (i) Most candidates recorded a second value of M successfully.

(ii) Most candidates were able to calculate the value of A successfully.

Other good answers included:

• using an electronic balance to measure M, or a measuring cylinder to measure the water

Comments on specific questions

(b) (i) Most candidates measured a value of n.

Comments on specific questions

(ii) 1. This was generally answered correctly by most candidates.

Common mistakes included:

(a) The majority of candidates knew this definition.

(b) (i) Most candidates answered this correctly.

(a) Most candidates gained credit for this definition.

As with previous explanations, strong candidates could provide a clear explanation.

(ii) Most candidates gained credit for this question.

Comments on specific questions

Comments on specific questions

(ii) 1. This was generally answered correctly by most candidates.

Common mistakes included:

(a) The majority of candidates knew this definition.

(b) (i) Most candidates answered this correctly.

(a) Most candidates gained credit for this definition.

As with previous explanations, strong candidates could provide a clear explanation.

(ii) Most candidates gained credit for this question.

• In Question 1, candidates’ responses should include detailed explanations of experimental procedures

Comments on specific questions

• In Question 1, candidates’ responses should include detailed explanations of experimental procedures

Comments on specific questions

• In Question 1, candidates’ responses should include detailed explanations of experimental procedures

Comments on specific questions

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