ZNOTES.

ORG

UPDATED TO 2023-2025 SYLLABUS

CAIE IGCSE
PHYSICS
SUMMARIZED NOTES ON THE THEORY SYLLABUS
Prepared for Shivam for personal use only.
CAIE IGCSE PHYSICS

Iron nails and steel paper clips can be magnetised by

1. Electricity and Magnetism hanging them from a magnet.
Each nail or clip magnetises the next in a chain, with
unlike poles attracting each other.
1.1. Simple Phenomena of Magnetism Removing an iron chain by pulling the top nail causes it
to collapse because iron shows temporary magnetism.
<b>Magnetic Materials</b> Steel chains do not collapse when removed because they
Ferromagnetic materials like iron can be made into have permanent magnetism.
magnets. Soft materials (e.g. iron) are easily magnetised but lose
Magnetic materials are naturally attracted to magnets magnetism quickly.
even when not magnetized.. Hard materials (e.g. steel) are harder to magnetise but
remain magnetised longer.
<b>Magnetic Poles</b>

Magnetic poles attract magnetic materials and are found

near the ends of magnets.
Poles always come in pairs: north and south.
Every magnet has a North Pole ($N$) and a South Pole
($S$).
The North Pole of a magnet points towards the Earth's
geographic North Pole.

<b>Law of Magnetic Poles</b>

Similar poles ($N-N$ or $S-S$) repel each other.

Opposite poles ($N-S$) attract each other.
The attraction or repulsion decreases as poles move <b>Magnetic and Non-magnetic Materials</b>
farther apart.
Magnetic materials (iron, steel, nickel, cobalt) are
attracted to magnets and can be magnetised.
Non-magnetic materials (e.g., aluminium, wood) are not
attracted to magnets and cannot be magnetised.

<b>Magnetic Fields</b>

<b>Induced Magnetism</b>

Magnetic materials can become magnetized when near a

magnet.

<b>Magnetisation of Iron and Steel</b>

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A magnetic field is the region around a magnet where Current Increase: Higher current in the coil results in
magnetic forces act. stronger magnetism.
Field strength is higher where magnetic field lines are More Turns: Increasing the number of turns in coils
closer together and lower where they are further apart. around the core increases magnet strength.
Magnetic fields are shown using lines of force, showing Closer Poles: Moving the magnetic poles closer together
the direction from North to South poles. increases electromagnet strength.
The density of these lines indicates field strength: closer
lines represent stronger magnetic fields.

<b>Electromagnets</b>
1.2. Electrical quantities
They are formed from a coil of wire through which an Electric Charge
electrical current passes.
Like/same charges (+ and + or – and – ) repel, while
Magnetism is temporary and can be switched on and off,
unlike charges (+ and –) attract.
unlike permanent magnets.
They contain a core of soft iron that only becomes Force Between Charges
magnetised when current flows through the coil.
The force between electric charges decreases as their
<b>Factors Affecting Electromagnet Strength</b> separation increases.
Positive charges repel other positive charges and attract
negative charges.
Negative charges repel other negative charges and
attract positive charges.

Charges, Atoms, and Electrons

Atoms consist of a central nucleus with protons (positive)

and electrons (negative) orbiting around it.
Protons and electrons have equal but opposite charges,
making atoms electrically neutral overall.

Production of Charges

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Charges are produced by friction, which transfers Electric Current is defined as charge passing a point per
electrons between materials. unit time, symbolized as
Electrons move between materials during rubbing; ($I = \frac{Q}{t}$).
protons remain in the nuclei and do not move. Unit of current is the ampere ($A)$, with one milliampere
($mA$) equal to one-thousandth of an ampere and is
Units of charge measured by an ammeter.
Unit of charge is the coulomb ($C$), defined as the
Charge is measured in coulombs ($C$) and defined in
terms of the ampere ($A$) charge passing a point when a steady current of 1
The charge on an electron is ($e = -1.6 \times 10^{-19}$) ampere flows for 1 second ($1C = 1As$).
$C$. Charge Calculation
Electrons, Insulators, and Conductors $Q = I \times t$
where $Q$ is charge, $I$ is current, and $t$ is time in
Insulators: Electrons are firmly bound to atoms; rubbing seconds.
can charge them statically. Conventional Current
Conductors: Electrons can move freely; they require
insulation to hold a charge. Conventional current flows from positive to negative
terminals of a battery, opposite to electron flow.
| Type | Description | Examples | |----|----|----| | Circuit diagrams show conventional current direction
Insulators | Electrons are firmly bound to atoms; rubbing with arrows, while electrons move in the opposite
can charge them statically. | Plastics (polythene, cellulose direction.
acetate), Perspex, nylon | | Conductors | Electrons can
move freely; require insulation to hold a charge. | Metals, Direct and Alternating Current
carbon | | Direct Current (d.c.) | Alternating Current (a.c.) | |----|----|
| Electrons flow continuously in one direction. | Electrons
Electric Fields
regularly change their direction of flow. | | Provided by
When charges are near each other, they experience a batteries | Produced by generators. |
force known as the electric force. Frequency of Alternating Current
Electric field is a region where a charge feels a force due
to nearby charges. Frequency refers to the number of complete cycles per
second.
Uniform electric field exists between oppositely charged
parallel metal plates, shown by evenly spaced lines It is measured in Hertz ($Hz$), where 1 $Hz$ equals one
perpendicular to the plates. cycle per second.
The direction of the electric field is indicated by arrows,
representing the force acting on a small positive test
charge (pointing away from positive charges and towards
negative charges).

1.3. Voltage, Resistance and Power

The Ampere and the Coulomb (units of current and Electromotive Force $(e.m.f.)$
charge)

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Chemical actions inside a battery produce electron They can change current in a circuit (rheostat mode) or
excess at the negative terminal and shortage at the act as a potential divider by dividing voltage across
positive terminal components as desired.
Battery maintains electron flow (electric current) in a
connected circuit as long as chemical actions last.
The battery does work when moving the charge around
the circuit.
Electromotive force ($e.m.f.$) is the electrical work done
by a source in moving unit charge around a complete
circuit.
Electromotive force is measured in volts $(V)$.
Potential Difference
Resistance depends on the length, cross-sectional area,
Electric current transfers energy from a battery to circuit and material of the wire
components and surroundings. Resistance increases with length but decreases with a
Potential difference ($p.d.$) is the work done by unit larger cross-sectional area
charge passing through a component Formula: ($R \propto \frac{l}{A}$)
$P.d.$ is measured in volts.
Voltage is sometimes used instead of $p.d.$ $I–V$ graphs and Ohm’s Law
1 volt = 1 joule per coulomb 1 $V$ = 1 $\frac{J}{C}$ )
Metals and some alloys give $I–V$ graphs that are
Formula: $V = \frac{W}{Q}$ or $W = Q \times V$
straight lines through the origin, showing that $I$ is
Resistance directly proportional to $V$ or that $I \propto V$.
Doubling $V$ doubles $I$.
Electrons move more easily through some conductors Such conductors obey Ohm’s law: $V = IR$
when $p.d.$ is applied. Ohmic or linear conductors are the conductors where
Resistance is the opposition of a conductor to current. resistance does not change with $V$.
Good conductors have low resistance while poor
conductors have high resistance
Ohm (Ω) is the unit of resistance.
Formula: $R = \frac{V}{I}$

Variable Resistors

Semiconductor Diode

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Diode has small resistance when connected one way and An increase in temperature generally increases the
very large resistance when $p.d.$ is reversed. resistance of metals.
It conducts electricity in one direction only, and it is a Thermistors' resistance is different and decreases with
non-ohmic conductor. rising temperature.
It is a non-ohmic conductor

Filament Lamp
Light-dependent Resistor (LDR)
Non-ohmic conductor at high temperatures
$I–V$ graph curve flattens as $V$ and $I$ increase, Resistance of some semiconductors decreases with
showing increasing resistance with increasing current increased light intensity.
and increasing temperature. Light-dependent resistors (LDRs) use this property to
function.
$I–V$ graph for an LDR is similar to that of a thermistor
LDR is also a non-ohmic conductor.

Thermistor

Power in Electric Circuits

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Power defined as work done or energy transferred per In a series circuit, there is a single path for the current to
time taken: $P = \frac{W}{t}$ flow.
$ P$ is power in watts ($W$), $W$ is work done in joules The current remains the same throughout:
($J$), $t$ is time in seconds ($s$) Current ($I$) is consistent at every point in the series
For a steady current $(I)$ in a device with a potential circuit.
difference ($V$) across it, the work done has a formula $ The reading on an ammeter will be identical no matter
W = I \times t \times V $ where it is placed in the circuit.
Substituting work done with the power $P = IV$
multiplied by time in seconds ($t$), the energy Current in a Parallel Circuit
transferred is: $E = Pt = IVt$ In a parallel circuit, components are connected side by
Example side, providing alternative paths for current flow.
The total current is the sum of the currents through
Lamp with 240 $V$ supply and 0.25 $A$ current each branch
Power = $P = IV$ = 240 $V$ $\times$ 0.25 $A$ = 60 $W$ If the total current from the source is ($I_0$), and the
60 $J$ of energy transferred to the lamp each second current through each branch is $I_1, I_2$ and $I_3$ then
$I_0 = I_1 + I_2 + I_3$
Voltage in terms of power and current

Volt can be defined as a watt per ampere: $V = \frac{P}

{I}$
If all energy transferred to thermal energy in a resistor of
resistance $R$:
$P = V \times I$ = $IR$ $\times$ $I$ = $I^2R$
Doubling the current produces four times the thermal
energy per second $P = I^2R$
Larger unit for energy: kilowatt-hour ($kWh)$
1 $kWh$ = 1000 $\frac{J}{s}$ $\times$ 3600 $s$ =
3600000 $J$ = 3.6 $MJ$
The cost of electricity in houses is calculated by using
$kWh$ where each $kWh$ has a fixed price and is Potential Difference $(p.d.)$ in Series and Parallel
multiplied by the units you consume. Circuits

1.4. Electric Circuits In a series circuit, the total potential difference across
the components is the sum of the individual potential
Electrical component symbols differences: $V_0 = V_1 + V_2 + V_3$
In a parallel circuit, the potential difference across each
component is the same as the potential difference across
one branch: $V_{\text{across each branch}} = V_0 $

Cells, Batteries, and Electromotive Force ($e.m.f.$)

Cells in series increase the total $e.m.f.$ of the battery.

For example, if two 1.5 V cells are connected in series
then the $e.m.f.$= 1.5 $V$ + 1.5 $V$ = 3.0 $V$
Resistors in Series
Current in a Series Circuit

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In a series circuit, the total resistance $( R_0 )$ is the sum In a thermistor, resistance decreases with increasing
of the individual resistances: $R_0 = R_1 + R_2 + R_3$ temperature.
Given resistors $R_1, R_2,$ and $R_3$ the total voltage When it’s used in a potential divider circuit:
($V$) across them is: $V = I \times R$ As temperature rises, the thermistor's resistance
decreases.
Worked Example This lowers the combined resistance of the two
resistors, increasing the current if the supply voltage
For a 4.5 V battery across resistors of 3 $\Omega$, 4
$\Omega$ and 5 $\Omega$ in series: remains constant.
Combined resistance: $R_0 = R_1 + R_2 + R_3$= 3 The potential difference across the fixed resistor
$\Omega$ + 4 $\Omega$ + 5 $\Omega $= 12 $\Omega$ increases relative to that across the thermistor.
Current ($I $): $I$ = $\frac{V}{R}$ = $\frac{4.5 \text{V}}{12 A variable resistor can also act as a potential divider by
adjusting the position of the contact, changing the
\Omega}$ = 0.375 $A$
$p.d.$ across $4$ $\Omega$ resistor: $V_2$ = $I \times output potential difference.
R_2$= 0.375 $A$ $\times$ 4 $\Omega$ = $1.5 $ $V$

Resistors in Parallel

The combined resistance $( R_0 )$ of resistors in parallel

is given by: $\frac{1}{R_0}$ = $\frac{1}{R_1}$ + $\frac{1}
{R_2}$ + $\frac{1}{R_3}$…
Two resistors $R_1$ and $R_2$ have resistance of
$\frac{1}{R_0}$ = $\frac{1}{R_1}$ + $\frac{1}{R_2}$=
$R_0$ = $\frac{R_1 \times R_2}{R_1 + R_2}$

Properties of Parallel Circuits

1. The current from the source is greater than the
current in each branch. Potential Divider
2. The combined resistance of parallel resistors is less For two resistors $R_1$ and $R_2$ in series with a supply
than that of any individual resistor. voltage $( V )$:
The total current $( I )$ is given by: $I = \frac{V}{R_1 +
1.5. Applications of electric circuits R_2}$
Increase in Resistance of a Conductor Light-Dependent Resistor (LDR)
In metals, current is carried by free electrons. As the
temperature of the metal increases:
The atoms vibrate more, making it harder for electrons
to move.
This results in an increase in resistance.
From Ohm's Law $V = IR$ , if resistance ($R$) increases while
maintaining a constant current$( I )$, the potential
difference ($V$) across the conductor also increases.
Variable Potential Divider

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An LDR’s resistance decreases with increasing light A relay allows a small current to control a larger current
intensity. needed to operate an appliance.
In a circuit, as light intensity increases: In a switching circuit:
The LDR’s resistance decreases, allowing more If the switching circuit output is high, a small current
current to flow. flows through the relay, closing the mains switch.
This increase in current can light a lamp or cause This isolates the low voltage circuit from the high
other actions. voltage mains supply.

Light-Emitting Diode (LED)

An LED emits light when forward-biased (cathode

connected to the negative terminal):
Reverse bias (anode connected to the negative
terminal) does not emit light and can damage the
LED if the reverse voltage exceeds 5 $V$.
A suitable resistor $R$ (e.g. 300 Ω on a 5 $V$ supply)
is needed to limit the current.

Semiconductor Diode
A diode allows current to pass in only one direction:
Thermistor Forward-biased: current flows when the anode is
connected to the positive terminal and the cathode
A thermistor's resistance decreases significantly with to the negative terminal.
temperature increase. Reverse-biased: the diode does not conduct and has
In a series circuit with a thermistor: high resistance.
As temperature rises, its resistance drops, decreasing
the potential difference across it.
This causes an increase in voltage across a series
resistor, which can trigger a relay or alarm.

1.6. Electrical safety

Dangers of Electricity
Relays

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Damaged Insulation: Exposes wires, increasing shock Live and Neutral Wires: Both supply electricity and the
and fire risk. neutral is earthed.
Overheated Cables: Can lead to fire. Earth Wire: Provides safety by connecting metal cases to
Damp Conditions: Increase shock severity due to earth.
reduced resistance.
Excess Current: From overloaded plugs, extension leads,
and multiple sockets.
Electric Shock: Current flows from an electric circuit
through a person's body to earth.
Dry Skin: Resistance ~10,000 Ω and current around
24 mA (it is safe).
Wet Skin: Resistance ~1,000 Ω and current ~240 mA
(can be deadly).
Larger currents are more dangerous.
Longer exposure increases risk.

Reducing Risk Switches and Fuses

Turn off power before repairs. Switches and fuses are in the live wire to prevent shocks.
Use earth pin and cord grips. Fuse breaks the circuit if the current exceeds safe levels.
Keep appliances dry and away from water. Circuit Breakers
Avoid trailing cables and damage, especially with cutting
tools. Electromagnetism breaks the circuit when current
exceeds a preset level.
First Aid for Electric Shock Advantages: Faster operation and can be reset.
Switch off the power if the person is still in contact with Earthing
the equipment.
Call for medical assistance. Prevents shock by providing a path for fault currents.
Earth pin connects appliance metal cases to earth,
Causes of fires preventing them from becoming live.
Flammable materials near hot appliances or wiring. Double Insulation
Overheated wiring produces excessive current and can
lead to fire. Appliances with two layers of insulation don’t need an
Preventive Measures: earth wire.
Match fuse rating to appliance.
Do not overload sockets or use too many adapters. 1.7. Electromagnetic induction
Use thick wires for high-power appliances.

House Circuits Process of generating electricity from a changing

magnetic field.

Electromagnetic Induction Experiments

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Straight Wire and U-shaped Magnet Inserting magnet into coil (solenoid) induces current in
Wire held still between magnet pole leads to no induced one direction.
current. A solenoid is a coil of wire wound in a helical shape
Moving wire vertically (up or down) between poles that generates a magnetic field when an electric
induces current because of changing magnetic flux current passes through it.
(cutting magnetic field lines) Removing magnet from solenoid induces current in the
Upward movement: current flows in one direction. opposite direction.
Downward movement: current flows in the opposite No current is induced when magnet is stationary inside
direction. solenoid.
Deflection on meter is temporary and occurs only while Current direction reverses with the direction of magnet
wire is moving. movement.
This also works if the solenoid is moved instead of the
magnet.

Factors Affecting Induced $e.m.f.$

Faster movement of magnet or coil increases induced

Bar Magnet and Coil (solenoid) e.m.f.
More turns in the coil increase the induced e.m.f.
Stronger magnets increase the induced e.m.f.
$e.m.f.$ is directly proportional to the rate at which the
conductor cuts through magnetic field lines.

Direction of Induced $e.m.f.$ (Lenz’s Law)

Induced $e.m.f.$ always opposes the change causing it.
If a magnet approaches a coil, the induced current
generates a magnetic field that opposes the motion.
If a magnet is withdrawn, the coil’s induced current
generates a field that attracts the magnet.
Magnetic Fields

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Variation of Magnetic Field Strength
Straight Wire:
When current flows through a vertical wire, iron Magnetic field strength decreases with distance from the
filings around it form circles. wire.
Meaning that around a straight wire, there are Field lines spread out as distance increases.
circular magnetic field lines. Increasing current strengthens the magnetic field and
Field direction changes with current direction lines become closer together.
(upwards or downwards through the wire) Reversing current direction reverses the direction of the
Use right-hand grip rule: direction of thumb (upwards magnetic field.
or downwards) indicates magnetic field direction by
the remaining fingers (clockwise or anti-clockwise). 1.8. Applications of electromagnetic
effects
Relay
A relay is a switch that operates using an electromagnet.
It allows one circuit to control another
When current flows through the coil, it magnetizes the
soft iron core.
The magnetized core attracts the L-shaped iron
armature.
Solenoid The armature rocks on its pivot and closes contacts in
A long cylindrical coil produces a magnetic field another circuit.
similar to a bar magnet.
End A behaves like the north pole, and end B behaves
like the south pole.
Right-hand grip rule: grip solenoid in current
direction, thumb points to the north pole.
Magnetic field inside the solenoid is stronger and
denser compared to outside.

Components
Coil: Creates the magnetic field.
Soft Iron Core: Magnetized by the coil, attracts the
armature.
L-shaped Iron Armature: Moves to close or open
contacts.
Contacts: Switches the second circuit on or off.

Reed Switch

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A reed switch uses magnetic fields to control a circuit. A device that produces sound by ringing is an electric bell
Operated by current flowing through a coil, which Pressing the bell push completes the circuit.
magnetizes reeds of magnetic material. Current flows through electromagnet coils, magnetizing
Current flows: Reeds become magnetized, attract each them.
other, and close the circuit. Electromagnet attracts a soft iron bar (armature),
Current stops: Reeds lose magnetization, separate, and causing the hammer to hit the gong.
open the circuit. The circuit breaks at contact screw point
Electromagnet loses magnetism, armature returns to its
original position.
The springy metal strip reconnects the circuit, and the
cycle repeats as long as the bell push is pressed.

Loudspeaker
It converts electrical signals into sound waves.
Varying currents pass through a coil placed in a magnetic
field.
Magnetic fields interact, causing the coil to vibrate.
A paper cone attached to the coil moves with it. 1.9. Motors and generators
Vibrations create sound waves in the surrounding air.
Components Simple $d.c.$ Electric Motor
Coil: Receives electrical signals and vibrates.
Magnet: Provides the magnetic field for interaction.
Paper Cone: Moves with the coil to produce sound.

Electric Bell

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Components Fleming’s Left Hand Rule is used for the $d.c.$

Rectangular coil: Fixed up on an axle that can rotate. motor
C-shaped magnet: Provides the magnetic field.
Split-ring commutator: A copper ring split into two
halves, connected to the ends of the coil. It rotates
with the coil.
Brushes: Carbon blocks pressed against the commutator
to supply current continuously.

The $a.c$ Generator

Components

Rectangular coil: Positioned between the poles of a C-

shaped magnet.
Slip rings: Connected to the ends of the coil, rotate with
Operation the coil.
When direct current ($d.c.)$ flows through the coil, a Carbon brushes: Press against the slip rings to conduct
force acts on the coil due to the interaction with the current.
magnetic field.
This force creates a turning effect, causing the coil to
rotate.
The split-ring commutator reverses the direction of
current in the coil as it rotates, making sure there is
continuous rotation by maintaining the direction of
force.

Operation

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As the coil rotates in the magnetic field, it cuts through This occurs when current changes in one coil, inducing a
the field lines, inducing an electromotive force ($e.m.f.$) voltage in a neighboring coil.
The $e.m.f.$ varies as the coil moves Magnetic field lines from the primary cut through the
Vertical Position: No $e.m.f.$ as the coil cuts the least secondary coil, inducing voltage.
number of field lines. Induced voltage increases with a soft iron rod or
Horizontal Position: Maximum $e.m.f.$ as the coil cuts complete iron ring core due to increased magnetic field
the most field lines. lines.
The direction of $e.m.f.$ reverses as the coil continues to
rotate, producing alternating current ($a.c.$) in the
circuit.
The frequency of the $a.c$. is determined by the rotation
speed of the coil. For example, a coil rotating twice per
second generates an $a.c$. with a frequency of 2 $Hz.$

Fleming’s Right Hand Rule is used for the

$a.c.$ generator.

Transformer Equation

The alternating voltage applied to the primary induces an

alternating voltage in the secondary.
Relationship given by $\frac{V_p}{V_s}$ = $\frac{N_p}
{N_s}$
$V_p$ and $V_s$ the primary and secondary voltages.
$N_p$ and $N_s$ are the primary and secondary turns.
1.10. Transformers Step-up transformer: More turns are on secondary $(V_s
> V_p)$.
The transformer changes alternating voltage to different Step-down transformer: fewer turns on secondary, ($V_s
values. < V_p$).
Consists of primary and secondary coils on a soft iron
core. Worked Example
Coils can be wound on top of each other or separate
limbs. A transformer steps down the mains supply from 230V
to 10V.
Mutual Induction Turns ratio: $\frac{N_p}{N_s}$ = $\frac{230V}{10V}$ =
$\frac{23}{1}$
If the secondary has 80 turns, the primary has $80$
$\times$ $2$3= 18$2$ turns.

Energy Losses

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If $V$ s stepped up, current $I$ is stepped down The nucleus of an atom consists of protons and
proportionally. neutrons.
Ideal transformer (100% efficient): $I_p V_p$ = $I_s V_s$ Three basic particles in an atom include protons,
$I_p$ and $I_s$ are primary and secondary currents. neutrons, and electrons.
If $V$ is doubled, $I$ is halved. Proton = a hydrogen atom minus an electron charge
$+1$, mass about 2000 times that of an electron.

2. Nuclear Physics Neutron: Uncharged and with a mass almost equal to

that of a proton.
Relative charges: Proton = +1 and neutron = 0 while
2.1. Nuclear model of the atom electron = -1.
Protons and neutrons are located in the nucleus and
Current atomic model are together called nucleons.

Electrons orbit a positively charged nucleus.

Mostly empty space between the orbits and the nucleus.

~~Scattering experiments by Ernest Rutherford~~

α-particles directed at thin gold foil.

Observations of α-particles:

| Particle | Relative Mass | Relative Charge | Location | |----

|----|----|----| | Proton | 1 | +1 | In nucleus | | Neutron | 1 |
0 | In nucleus | | Electron | $\frac{1}{1840}$ | -1 | Outside
nucleus |
In a neutral atom the number of protons equals the
number of electrons.
Atomic number ($Z$): Number of protons in the nucleus
| Observation | Description | Proof of atomic model | |----|- (it also equals the number of electrons).
---|----| | Most α-particles | Passed through the gold foil Mass number ($A)$: Total number of nucleons (protons +
without deflection. | Atom is mostly empty space. | | Some neutrons) in the nucleus.
α-particles | Deflected at small angles. | Presence of a Relationship: Number of neutrons = $A - Z$.
dense, positively charged nucleus which repels the α- Nuclide notation: Atom $X$ is represented as
particles | | Approximately 1 in 8000 α-particles | Deflected ${}^{A}_{Z}X$, where $A$ is the nucleon number and $Z$
back towards the source at large angles. | Nucleus is very is the proton number.
small and dense compared to the rest of the atom. | Relative charge: Product of proton number ($Z$) and the
Rutherford’s nuclear model charge of a proton.
Positive charge and most mass are concentrated in a Relative mass: Total mass of neutrons and protons;
small, dense nucleus. approximately $A$ times the mass of a proton.
Electrons orbit the nucleus at a large distance away.
Nucleus and electrons occupy about one-million-
millionth of the atom’s volume.
The nucleus

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Isotopes
Reactors use controlled chain reactions to produce
Forms of the same element with the same number of energy.
protons but different number of neutrons. Control rods absorb neutrons to regulate the reaction.
Example: Chlorine has isotopes ${}^{35}{17}Cl$ and Graphite moderates neutrons to slow down fission.
${}^{37}{17}Cl$ while Hydrogen has isotopes ${}^{1}
{1}H$, deuterium ${}^{2}{1}H$, and tritium
${}^{3}_{1}H.$

Isotopes have identical chemical properties

but different physical properties.

Nuclides

Radioactive isotopes are called radioisotopes or

radionuclides and have unstable nuclei.

Nuclear Energy

Einstein’s equation: $E = mc^2$, where $E$ is energy,

$m$ is mass, and $c$ is the speed of light. 2.2. Types of Radioactivity
Mass loss in nuclear reactions results in energy release.
Nuclear reactions involve large energy changes Natural Background Radiation
compared to other physical and chemical changes. Radiation sources include:
Nuclear fission Cosmic rays (high-energy particles from the Sun) are
mostly absorbed by the atmosphere, but some reach the
Uranium-235 is an isotope that undergoes fission when
Earth's surface.
struck by neutrons.
Radon gas present in the air.
Fission breaks the nucleus into smaller radioactive
Granite rocks in homes, particularly in Scotland, emit
nuclei, releasing additional neutrons and energy.
radioactive radon gas that can accumulate in poorly
Mass loss is converted into kinetic energy of fission
ventilated areas.
products.
Radioactive potassium-40 is present in food and
Neutrons from fission can trigger further fission
absorbed by our bodies.
reactions.
Various radioisotopes are used in medical procedures.
Nuclear fusion Radiation from nuclear power stations and fallout from
nuclear bomb testing
Nuclear Fusion is the joining of light nuclei to form a
heavy nucleus and releases energy Ionising Effect of Radiation

Nuclear Reactor The ability of radiation to make atoms lose or gain

electrons and become charged.
A charged electroscope discharges when a lighted match
or a radium source is brought near the cap.

$\text{Electroscope Discharge:}$ $\text{Neutral Atom}$

$\rightarrow$ $\text{Positive Ion}$ + $\text{Electron}$

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A lighted match knocks electrons out of air molecules,

creating positive ions.
Radiation causes ionisation by neutralising the charge on
the electroscope.

$\text{Ionisation:}$ $\text{Neutral Atom}$ $+$

$\text{Electron}$ $\rightarrow$ $\text{Negative Ion}$
Geiger–Müller (GM) Tube

The ionising effect of radiation is used to detect

radiation. | Type of Radiation | Mass | Charge | Penetrating Power |
Radiation entering a GM tube creates argon ions and Ionising Power | |----|----|----|----|----| | Alpha (α) | High
electrons, which then cause more ionisation. (Helium nucleus) | +$2$ | Low (stopped by paper) | High | |
Beta (β) | Low (electron) | -$1$ | Moderate (stopped by few
Alpha, Beta, and Gamma Radiation mm of aluminum) | Moderate | | Gamma ($\gamma$) |
Alpha Particles $(\alpha)$ None (electromagnetic wave) | 0 | High (stopped by several
The nucleus with two protons and two neutrons $cm$ of lead) | Low |
Stopped by thick paper; range in air is a few Particle Tracks
centimetres. Cloud chambers reveal the tracks of particles based on
The high ionising power of alpha particles is due to the ionisation they produce.
their increased mass (compared to gamma and beta), Alpha Particles: Straight, thick tracks.
so it's more likely to ionise an atom Beta Particles: Thin, straight or twisted tracks.
Deflected by electric and magnetic fields. Gamma Rays: Eject electrons which then produce
Represented as helium ions with a double positive tracks similar to $\beta$ particles.
charge.
Beta Particles ($\beta$) Electric deflection
fast-moving electron
Stopped by a few millimetres of aluminium; range in The positive alpha particles are heavier and slowly
air is several metres. deflect towards the negative plate.
Lower ionising power than alpha particles. The negative beta particles are lighter and quickly deflect
Deflected by electric and magnetic fields. towards the positive plate.
Streams of high-energy electrons. The neutral electromagnetic gamma radiation remains
Gamma Radiation ($\gamma$) undeflected.
Electromagnetic radiation having high frequency
Most penetrating
Stopped only by many centimetres of lead.
Least ionising power.
Not deflected by electric and magnetic fields.
Electromagnetic radiation.

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An α-particle is a helium nucleus with two protons and

two neutrons.
When an atom undergoes α-decay, its nucleon number
decreases by 4 and its proton number decreases by 2.
Example: When radium ($^{226}{88}\text{Ra}$) emits
and alpha particle, it becomes radon $(^{222}
{86}\text{Rn})$.
The equation for this decay is: $^{226}{88}\text{Ra}
\rightarrow ^{222}{86}\text{Rn} + ^{4}_{2}\text{He}$
Beta Decay (β-decay)

In β-decay, a neutron changes into a proton and an

Magnetic deflection electron.
The proton remains in the nucleus, while the electron is
Alpha particles follow the rule of positive conventional emitted as a β-particle.
current. The nucleon number stays the same, but the proton
Fleming’s left-hand rule is used with the middle finger number increases by 1.
pointing in the direction of alpha particles. Example: Radioactive carbon $^{14}{6}\text{C}$ decays
Beta particles are shown in the direction opposite to the into nitrogen ($^{14}{7}\text{N}$) by β-emission.
middle finger, as they represent electron flow, which is The equation for this decay is: $^{14}{6}\text{C}
the opposite of conventional current. \rightarrow ^{14}{7}\text{N} + ^{0}_{-1}\text{e}$
Gamma radiation is not deflected.
Gamma Emission (γ-emission)
After α- or β-decay, some nuclei are left in an excited or
energetic state.
Rearrangement of protons and neutrons releases energy
in the form of γ-emissions.
γ-emissions are high-energy electromagnetic waves with
no mass or charge.

Nuclear Stability

Stability of a nucleus depends on the number of protons

2.3. Radioactive decay and half-life ($Z$) and neutrons ($N$).
Stable nuclides fall within a specific stability level called
Radioactive Decay the stability line.
Radioactive decay is the emission of an α-particle or a β- For light nuclides, $N = Z$.
particle from an unstable nucleus. For heavier nuclides, $N > Z$.
This changes the nucleus into that of a different element Unstable nuclides decay to move towards the stability
until a stable element is formed. line.
These changes are spontaneous and random Nuclides above the stability line decay by β-emission to
decrease the $\frac{N}{Z}$ ratio.
Alpha Decay (α-decay) Nuclides below the stability line decay by beta emission
(β+) to increase the $\frac{N}{Z}$ ratio.
Nuclei with more than 82 protons usually decay by α-
emission.

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Half-Life
Exposure to small doses of radiation is not damaging,
The half-life of an isotope is the time taken for half the but large doses are harmful to health.
nuclei in a sample to decay. Nuclear radiation's ionising effect damages cells and
It is a measure of the rate at which a radioactive tissues, it can lead to gene mutations.
substance decays. Damage can cause cell death and cancers.
Each isotope has its own special half-life. α-particles are less dangerous unless the source is
It can be from fractions of a second to millions of years. ingested or inhaled.
A decay curve plots the activity of a sample over time, β- and γ-radiation can cause radiation burns, eye
showing the exponential decrease in activity. cataracts, and cancer.
The activity decreases by half in each half-life period Radiation hazard signs warn of the presence of
from the previous half-life period. radioactive material.
Example: If a sample's activity is 80 decays per second, it
will reduce to 40 in one half-life, then to 20 in the next,
and so on.
Radioactive decay is random and unpredictable; the
exact time when a particular nucleus will decay cannot
be determined.
The overall decay rate of a sample follows a predictable
pattern, called its half-life.

Safety Precautions

Minimize exposure time to radiation.

Keep a large distance between the radiation source and
individuals.
Use shielding materials that absorb radiation to protect
people.
In industry, sources are handled with long tongs and
transported in thick lead containers.
2.4. Safety precautions Workers are protected by lead and concrete walls and
wear radiation dose badges.
Dangers of Nuclear Radiation Radiation dose badges track the amount of radiation
exposure over a period, typically one month.
The badge has windows that allow different types of
radiation to expose photographic film, indicating
exposure levels when developed.

3. Space Physics
3.1. The Earth and the solar system
Motion of the Earth

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Waning (where the moon's illumination decreases) phases
The Earth spins on its axis, causing day and night. follow, leading to the last quarter and old crescent
One complete rotation takes 24 hours.
Day is for the half of the Earth facing the Sun and night
for the half facing away.

Rising and setting of the Sun

Earth's rotation causes the Sun to appear to move east

to west daily.
Rises exactly in the east and sets exactly in the west at
equinoxes.
In northern hemisphere summer, the Sun rises north of
east and sets north of west.
In winter, rises and sets south of these points.

The seasons Orbital speed

Caused by Earth's motion around the Sun (365 days) and Average orbital speed: $u$ = $\frac{2\pi r}{T}$
tilt of its axis. $r$ is the average radius of the orbit.
$T$ is the orbital period (time for one orbit)
Motion of the Moon
The Moon travels in a circular path around the Earth
Moon is a satellite of Earth, orbiting approximately every Distance traveled in one orbit is the circumference of
month the circle, $2\pi{r}$
Average distance from Earth is about 400,000 $km$. Time taken for one orbit is $T$
Revolves on its axis, always showing the same side to Speed is distance divided by time, so orbital speed is
Earth $\frac{2\pi r}{T}$
Reflects sunlight, has no atmosphere, weaker
The Solar System
gravitational field (one-sixth of Earth)
It contains:
Phases of the Moon
The sun as a star
Moon's appearance changes during its monthly orbit Eight planets in elliptical orbits (slightly oval orbits)
New Moon: Moon between Sun and Earth, unlit side Dwarf planets and asteroids orbiting the Sun
faces Earth Moons orbiting many planets
Crescent appears and increases until the first quarter Smaller bodies like comets and natural satellites
(half of the Moon visible)
Inner Planets
Full Moon: Moon opposite Earth from the Sun, fully
visible Mercury, Venus, Earth, Mars
Small, similar size
Solid and rocky with layered structures
High density
Formed close to the Sun where it was too hot for gases
to condense, allowing only metals and silicates to form
solid bodies
In the early Solar System, the Sun's heat caused lighter
gases to evaporate, leaving only heavy elements like iron
and silicon to form solid planets.

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Outer Planets Elliptical Orbits
Jupiter, Saturn, Uranus, Neptune Planets, dwarf planets, and comets orbit the Sun in an
Much larger and colder ellipse
Mainly consist of gases, low density Sun is at one focus of the ellipse, not the center
Many moons and rings of icy materials Comets have highly elliptical orbits, while planets' orbits
Formed in cooler regions where gases could condense, are more circular
capturing even the lightest elements
In the outer regions of the Solar System, lower Origin of the Solar System
temperatures allowed gases like hydrogen and helium to Formed from gravitational attraction pulling together
remain in solid or liquid forms, leading to the formation clouds of hydrogen gas and dust (nebulae)
of gas giants with thick atmospheres. Solar System formed about 4500 million years ago
Planets formed from the disc of matter left over from the
nebula that formed the Sun
Inner planets formed from materials with high melting
temperatures like metals and silicates
Outer planets formed from light molecules that existed
in solid icy forms, growing large enough to capture
hydrogen

Asteroids

Pieces of rock of various sizes, mostly between Mars and

Jupiter
Orbit around the Sun
Similar density to inner planets
Burn up in Earth's atmosphere as meteors

Comets Travel Times

Dust embedded in ice made from water and methane Distance from the Sun to Earth: approximately 150
Orbits the Sun in highly elliptical paths million $km$ ($1.5 × 10^8 $ $km$)
Develop a bright long tail when approaching the Sun due Speed of light: 300,000 kilometers per second ($km/s$)
to radiation pressure
Using the formula for time:
$\text{Time} = \frac{\text{Distance}}{\text{Speed}}$
Substitute the values:
$\text{Time} = \frac{1.5 \times 10^{8}}{300,000}$
Calculate the time:
$\text{Time} \approx \frac{1.5 \times 10^{8}}{300,000} \text{
seconds}$ $\approx 500 \text{ seconds}$
Convert the time from seconds to minutes:
$\text{Time} \approx \frac{500}{60}$ $\approx 8.33 \text{
minutes}$

It takes light from the sun around 8 minutes to reach the

Earth.

3.2. The sun

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Medium-sized star composed mainly of hydrogen and Interstellar clouds of dust and gas collapse under
helium. gravitational attraction.
Emits energy in the infrared, visible, and ultraviolet A protostar forms as mass increases and core
regions of the electromagnetic spectrum. temperature rises.
Hydrogen fuses into helium when the core is hot
Source of Energy enough, resulting in a star.
Energy from nuclear reactions in the core. Star Types
Hydrogen undergoes nuclear fusion to form helium,
releasing energy. Large mass: Blue or white stars.
Energy from the core heats outer layers, causing them to Smaller mass: Yellow or red dwarfs (e.g., the Sun).
glow and emit radiation.
Life Cycle of Stars
Stable Phase
Forces of gravity inward balance with thermal pressure
outward.
Stable phase lasts up to 10 billion years.
Hydrogen converts to helium in the core.

Red Giant/Red Supergiant

As hydrogen depletes, the star becomes unstable.
Core collapses; outer layers expand and cool.
Star turns into a red giant (or red supergiant if massive).
Helium fuses into carbon in the core.
Nuclear Reactions in Stars Low Mass Stars
Stars like the Sun are powered by nuclear fusion.
Core conditions End Stage
Core collapses into a white dwarf after all helium is
Hot and dense enough for hydrogen to fuse into helium. used.
Fusion process releases energy, maintaining high core Outer layers expelled, forming a planetary nebula.
temperatures. White dwarf cools into a black dwarf over about a
Some core energy moves to outer layers, which emit billion years.
electromagnetic radiation.
High Mass Stars (more than 8 times the Sun’s mass)
Light-years

Distance light travels in a vacuum in one year.

1 light-year = $9.5 × 10¹²$ $km$ = $9.5 × 10¹⁵$ $m$
Galaxies
Large collections of stars, gas, and dust.

3.3. Origin and life cycle of stars

Formation

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End Stage Occurs when a source of waves (e.g., sound or light)

Use hydrogen rapidly, with a shorter stable phase moves relative to an observer.
(about 100 million years). Approaching Source: Waves are compressed, resulting in
After helium fusion, core collapses into a red a higher frequency and pitch (blue shift for light).
supergiant. Receding Source: Waves are stretched, resulting in a
Fusion of carbon into heavier elements occurs until lower frequency and pitch (red shift for light).
iron forms.
Supernova explosion releases energy and heavy
elements into space.
Neutron Star: Dense core, may act as a pulsar.
Black Hole: Extremely dense core with gravitational
field so strong that even light cannot escape;
identified by X-ray radiation from nearby material.

Speed of Recession

The speed at which distant galaxies are moving away can

be calculated from the amount of redshift observed.
Some of the most distant galaxies are receding at speeds
up to one-third the speed of light.
The observed redshift supports the idea that the
Universe is expanding, which is consistent with the Big
3.4. The universe Bang theory.
Big Bang Theory
Milky Way
Initial State: Proposes that the Universe began from an
Approximately 100,000 light-years in diameter.
extremely hot and dense state around 14 billion years
Contains around 800 billion or more stars.
ago.
A spiral galaxy with a central bulge and spiral arms.
Expansion: The Universe has been expanding ever since
Redshift the Big Bang.

The phenomenon where light from distant galaxies shifts Microwave Background Radiation
towards the red end of the spectrum (longer
This radiation is a remnant from the Big Bang and fills
wavelength).
the entire Universe.
Light emitted from stars in distant galaxies appears
The radiation has been redshifted into the microwave
redder compared to light from closer galaxies.
region due to the expansion of the Universe.
Doppler Effect Provides strong evidence for the Big Bang theory and
insights into the early Universe.

Age of the Universe

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Hubble’s Law: The relationship between the speed of

recession ($v$) and the distance ($d$) of galaxies is given
by: $v = H_0 \times d$
Hubble Constant ($H_0$): $H_0 = \frac{v}{d}$
$H_0$ measures the rate of the Universe's expansion. A
higher value indicates a faster rate of expansion.
$H_0$ is estimated to be approximately $2.2 \times
10^{-18} \text{ s}^{-1}$
Age Estimation: The age of the Universe is
approximately: $\text{Age of the Universe} \approx
\frac{1}{H_0}$
Detailed Calculation

$\text{Age of the Universe} \approx \frac{1}{2.2 \times

10^{-18} \text{ s}^{-1}} \approx 4.5 \times 10^{17} \text{
s}$
$\text{Age of the Universe} \approx \frac{4.5 \times
10^{17} \text{ s}}{3.2 \times 10^7 \text{ s/year}} \approx
1.4 \times 10^{10} \text{ years} \approx 14 \text{ billion
years}$

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CAIE IGCSE
Physics

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