Chapter 10 Electricity at work

Answers

Think about electricity

What actually moves when an electric current Electric current is due to the movement of
flows? electric charges (electrons) through a
conductor in a circuit.
What is the difference between current and Current is a measure of the amount of electric
voltage? charge moving past a given point in a circuit
every second, whereas voltage is a measure
of the amount of electrical energy gained or
lost by electric charge as it moves through
the circuit.
Why don’t all the lights in your house go out The lights in your house are connected in
when one light globe or fluorescent tube parallel. When a bulb blows, there is still a
breaks? closed pathway between each of the other
bulbs and the power supply, so they remain
lit.
If a car battery is rechargeable, why doesn’t it After a few years, lead sulfate builds up on
last forever? the electrodes and becomes so hard that
recharging cannot take place.
How many electronic components can be Millions of components can be fitted onto a
fitted onto a silicon chip? chip no larger than the fingernail on your
little finger.
What is an LED and how is it different from An LED is a light-emitting diode, which
a torch or light globe with a filament? allows electric current to flow in only one
direction. Light globes with filaments allow
current to flow in either direction and use
much more energy, most of which is wasted
as heat.
What’s the connection between electricity A magnetic field is produced by an electric
and magnetism? current, and an electric current can be
produced by the relative movement between
a magnetic field and a coil of wire.
How is AC different from DC? AC stands for alternating current, which is
the type of current that is provided by power
points that you plug into. It changes direction
about 100 times every second. DC stands for
direct current, which is the type of current
provided by a battery. Direct current flows in
only one direction.

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Chapter 10 Electricity at work Answers

Why do you need a safety switch as well as Fuses or circuit breakers open circuits before
fuses or circuit breakers in your meter box? they overheat but are too slow to react to
dangerously high current. Safety switches
can turn off the power much more quickly.
Why is it dangerous to use electrical Tap water contains charged particles (ions)
appliances in the wet or near water? due to substances dissolved in it. It is
therefore a good conductor of electric
current. If water comes into contact with the
live sections of the appliance while you are
using it, you may be electrocuted. This is
because charge can flow from the appliance
through the water and through you into the
Earth.

Your quest
Electricity — it’s everywhere!
1–6. Group activities – student responses will vary.
What’s the connection?
7. A magnetic field is a region in which a magnetic force is experienced by a magnet. It can
be represented by a map of lines showing the size and direction of a magnetic force. You
know when it is present because it makes a compass, magnet or iron filings move.
8. An electric field is a region in which an electric force exists. It can be represented by a
map of lines showing the size and direction of the electric force. Its existence can be
demonstrated by placing very light objects, as tiny pieces of paper or a balloon nearby. As
long as the objects are light enough, an electric force causes them to move.
9. Examples of evidence include the movement of a compass needle near a wire carrying
electric current, the production of an electric current by a generator containing magnets,
the movement produced by an electric motor and the way that electromagnets work.

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Chapter 10 Electricity at work Answers

10.2 Electricity in transit

1. In a lightning bolt, the transfer of electrical energy occurs very quickly; in an electrical
circuit, the electrical energy transfer occurs more slowly.
2. All electric circuits consist of a power supply to provide the electrical energy, a load in
which electrical energy is converted into other useful forms of energy, and a conducting
path that allows electric charge to flow around the circuit.
3. Electric current is a measure of the amount of electric charge passing a particular point in
an electric circuit every second; voltage is a measure of the amount of electrical energy
gained or lost by electric charge as it moves through the circuit.
4. A switch in an electric circuit allows you to have control over whether the conducting
path is complete and current can flow.
5. Copper has a low resistance and so it is a good conductor of electricity. It is also
relatively cheap and ductile (able to be drawn into a wire).
6. (a)

(b)

7. Voltage is also known as potential difference because it measures the change in the
potential (stored) energy of charge as it moves between one place and another.
8. In a television remote control, the chemical potential energy of the batteries inside is
converted into electrical energy.
9. Answers may vary. Examples include:
(a) light bulb
(b) speaker
(c) heating coil; element in an electric kettle

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Chapter 10 Electricity at work Answers

(d) relay; motor.

10.

11. Student activity.

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Chapter 10 Electricity at work Answers

10.3 A light in the dark

1.

2. (a) Battery
(b) Light globe
(c) Spring and metal strip
3. When the switch is closed, electric current flows through the circuit. Electrical energy is
released in the globe, which makes it glow brightly.
4.

5. 3 × 1.5 = 4.5 volts

6. In the speaker, electrical energy is released as sound. In the motor, energy is released as
energy of movement when the tape in the cassette moves.
7. Student activity.
8. Answers will vary due to individual student research and responses.

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Chapter 10 Electricity at work Answers

10.4 Series and parallel circuits

1. In a series circuit, the globes are connected so that the same electric current flows
through both globes, one after the other. In a parallel circuit, the electric current splits so
that some of the current flows through one of the globes and the rest of the current flows
through the other.
2.
a. When light globes are connected in series, the same electric current flows through
each globe. The globes share the voltage of the power supply.
a. When the globes are connected in parallel, the electric current splits to be shared
by the globes. Each globe uses the same voltage.

3.

4.

(Although the power supply is shown here as a battery, it would in fact be a mains AC
supply.)
5.
a. B and C remain lit. Nothing goes out because the path through B and C is
unaffected.
b. Only A remains lit. C goes out because the path through B and C is blocked while
the path through A remains unaffected.
c. Only A remains lit. B goes out because the path through B and C is blocked while
the path through A remains unaffected.

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6.

7. (a) Half of the current from the battery flows through each globe.
(b) It is a parallel circuit. The current splits.
(c) The difference is that each globe is connected directly to the battery. In the circuit in
part B of Investigation 10.5, the top globe is connected directly to the other globe.
The effect on the electric current is the same.
8. Student responses will vary but the circuits should have a cell, two switches and a bell all
in parallel. Correct symbols for the switch and cell are expected, but any sensible symbol
for bell is acceptable.
9. Student responses will vary but the circuits should have a cell, two switches and a bell all
in series. Correct symbols for the switch and cell are expected, but any sensible symbol
for bell is acceptable. An example is shown.

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Chapter 10 Electricity at work Answers

10.5 Scale and measurement: Made to measure

1. (a) Current
(b) Voltage or potential difference
2. An ammeter should always be connected in series in an electrical circuit. The positive
terminal of the ammeter should always be connected so that it is closer to the positive
terminal of the power supply than the negative terminal of the power supply.
3. A voltmeter should be connected in parallel with the part of the circuit across which the
voltage is being measured.
4. Answers may include:

 intermittent parallax error caused by not reading the scale directly from the front

 estimation errors

 random fluctuations in the voltage

 unstable/poor contact at connections.

5. Random errors can be reduced by repeating measurements numerous times and
calculating an average.
6. Answers may include:

 parallax errors caused by consistently reading the scale of an ammeter or voltmeter

from one side instead of directly in front

 an incorrect zero reading when there is no current or voltage

 uneven scales on the voltmeter.

7. Multiple scales allow voltmeters and ammeters to be read to a higher degree of precision
over different value ranges.
8. 350 mA
9. The ammeter has been connected with its negative terminal closest to the positive
terminal of the power supply; the voltmeter has been connected in series instead of
parallel.
10. A voltmeter should be connected in parallel with the part of the circuit across which the
voltage is being measured. In the case of the circuit in Investigation 10.6, the potential
drop across the bulb was being measured; therefore, the voltmeter was connected in
parallel with it.
11. The 500 mA scale should be used first as it is the highest value scale. If the smaller scale
were chosen and the connected circuit yielded a current that exceeded 50 mA, it is
possible the ammeter could be damaged.

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12. (a) The ammeter has been drawn obliquely, so the scale cannot be read front on, which
is necessary if there is not to be a parallax error in the measurement taken.
(b) 325 mA
13. If the parallax error is continuous throughout the trial because the scale is consistently
being read from the side rather than the front, then it is a systematic error, because all of
the readings in the trial will be similarly affected by being too high or too low. If,
however, the scale is read from the side rather than the front only once or twice during
the experiment, and the other readings were made correctly, then it could be classified as
a random error.
14. Random errors are caused by such things as random fluctuations in the quantity that is
measured or by poor connections; using a digital measuring instrument will not change
this.
15. No matter how much care you take with your measurements in order to reduce
systematic, there will always be errors because of limitations of your equipment’s degree
of precision as well as random errors that cannot be controlled.
16. 3.05 A (±0.05 A)
17. Answers will vary due to individual student research and responses.

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10.6 Electricity in a packet

1. A battery is made up of two or more cells. The ‘batteries’ that go in torches are actually
cells.
2. A chemical reaction that releases charge and allows current to flow
3. Carbon and zinc
4. They contain a different electrolyte.
5. Lead sulfate builds up on the electrodes and becomes hard. This prevents recharging.
6. Examples include clocks, torches, MP3 players, computers, calculators and mobile
phones.
7. The electrolyte in car batteries is sulfuric acid.
8. They produce a very steady voltage compared with other dry cells.
9. The battery recharges only while the engine is running. The lead sulfate will build up on
the electrodes more quickly.
10. Answers will vary due to individual student research and responses.

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10.7 Driving on batteries

1. Answers may include:

 They will reduce the demand for oil.

 They do not release exhaust gases.

 They are very quiet.

2. Answers may include:

 They can travel only short distances before needing recharging.

 They do not accelerate quickly and can usually reach speeds of only 100 km/h.

 They are more expensive than petrol-driven cars.

 Batteries are expensive and need replacing after a few years.

 Power stations would need to supply more energy for recharging batteries.
3. A hybrid car uses a petrol engine as well as a bank of rechargeable batteries.
4. Cars are now lighter and better designed, needing less energy to run. Lighter batteries are
being developed, and petrol is becoming more expensive.
5. The large number of heavy batteries needed to supply energy makes the car too heavy to
move along the road faster.
6. The petrol engine can recharge the batteries while the car is moving, provide extra
energy to make the car travel faster and reduce the number of batteries needed.
7. (a) Answers may include:

 The exhaust fumes of hybrid cars still contribute to air pollution.

 Hybrid cars still require petrol.

 The rechargeable batteries are expensive to replace.

 The batteries are very heavy, which decreases the ability of the car to accelerate
as rapidly.
(b) Answers will vary but could include measurements of range, emissions, cost to
travel certain ranges, acceleration and top speed.
8–11. Answers will vary due to individual student research and responses.

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10.8 A question of resistance

1. 15 V
2. 0.4 A or 400 mA.
3. R = V/I
= 12 V/0.8 A
= 15 Ω
4. The resistance does not change, and the graph of V versus I yields a straight line.
5. Insulators have a very large electrical resistance.

6. The electric current is proportional to the voltage drop across it (I  V).

7. The voltage and the current will change if the resistance changes.
8. Electric current decreases.
9. V = RI
= 100 Ω × 0.250 A
= 25 V
10. (a) When I = 200 mA, V = 1.5 V.
V
R=
I
1.5V
=
0.200A
= 7.5Ω

(b) No. The resistance is not constant. When the voltage is 3.0 V, the current is 360 mA
and the resistance is:
V
R=
I
3.0V
=
0.360A
= 8.3Ω

(c) The electric current would double (400 mA).

11–13. Answers will vary due to individual student research and responses.

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10.9 Electronics—it’s a small world

1. Circuits containing electronic components
2. It can store information, process it and control other electric circuits.
3.
Componen Circuit symbol Function
t
capacitor To store electric charge for a short time

diode To allow electric current to flow in only one

direction
LED To transform electrical energy into light energy

transistor To change the size or direction of electric current

as a result of very small changes in voltage across
them

4. Today’s components are smaller, lighter and less fragile. They are also cheaper.
5–7. Answers will vary due to individual student research and responses.

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10.10 Magnetic effects of electricity

1. A permanent magnet retains its magnetic field after being removed, whereas a temporary
magnet does not.
2. A magnetic field is a region of space in which a magnetic force exists.
3. An electromagnet consists of a coil of wire wound around an iron core. A solenoid is
simply the coil of wire.
4. Electromagnets can be switched on or off.
5. The permanent magnet attracts and repels the voice coil, making it move rapidly
backwards and forwards.
6. As the voice coil vibrates, so does the cone. The cone in turn makes the surrounding air
vibrate.
7. Answers will vary. Devices may include: computer hard drives; speakers; microphones;
doorbells; appliances with motors such washing machines and fridges; alternators in cars;
and maglev trains.
8. A compass is a small magnet.
9. The Magnetic North Pole of the Earth acts as the south pole of a magnet. We know this
because the north pole of other magnets, including compasses, point towards it.
10. Sound energy is transformed into kinetic energy of the diaphragm, which is then
transformed into electrical energy.
11. Student activity.

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10.11 Motoring along

1. Motors convert electrical energy into kinetic energy. Speakers use the magnetic field
produced by an electric current to convert electrical energy into sound energy.
2. AC current changes direction. DC current flows in one direction only.
3.
Moving parts Non-moving parts
armature field magnets
shaft brushes
commutator
rotor coil

4. Answers will vary due to individual student research and responses.

5. The AC current in the rotor coil reverses itself so it keeps turning in the same direction.

6. Student responses will vary but should include explanations of the function of the
following parts (typical key points provided).

 Field magnets surround the coil and provide the magnetic field to interact/push on
the coil to make it spin

 Rotor coils carry current to make a magnetic field to interact with the magnetic
field of the magnet

 Axle/shaft coil spins around this.

 Commutator allows the armature to keep rotating in the same direction

 Brushes brush against the commutator to allow current to flow without the need
for fixed wires

7. Answers will vary due to individual student research and responses.

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Chapter 10 Electricity at work Answers

10.12 Generating electricity

1. He moved a coil of wire through a magnetic field.
2. Electric current produced by an AC generator changes direction after every half turn.
3. By increasing one or more of:
• the number of turns of wire in the coil
• the strength of the magnet
• the speed of the relative movement between the coil and the magnetic field.
4. (a) It has field coils, rotor coils, a shaft, an armature and brushes.
(b) The brushes make contact with slip rings instead of a commutator.
(c) The electric motor uses an electric current to turn a shaft. The generator uses a
turning shaft to generate an electric current.
5. The speaker can act like a small generator to produce a small changing electric current.
When you speak in front of it, the vibrating cone moves the voice coil. The relative
movement between the coil and the permanent magnet inside causes current to flow. This
current is changing in direction and can be used to drive another speaker.
6. Student activity.
7. Answers will vary due to individual student research and responses.

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Chapter 10 Electricity at work Answers

10.13 Electricity at home

1. The turbines turn, causing coils of wires to rotate inside huge electromagnets.
2. (a) Steam
(b) Steam
(c) Falling water
3. AC stands for alternating current; DC stands for direct current.
4. The transformers reduce the voltage from 240 volts to the required 9 volts or 12 volts.
5. The appliance should be replaced or taken to a qualified repairer.
6. Water is a good conductor of electric current. If an appliance makes contact with water
and a person touches it, he or she could be electrocuted.
7. Fuses and circuit breakers can open the main circuit and stop the flow of current when
the current becomes too large.
8. A circuit breaker has a special switch that opens automatically if the current is too large.
A fuse melts when the current is too large.
9. (a) Chemical energy, heat and kinetic energy are converted into electrical energy in a
coal-fired power station.
(b) Gravitational potential energy and kinetic energy are converted into electrical energy
in a hydroelectric power station.
10. Both methods of generation involve the movement of coils of wire in a magnetic field.
11. (a) Chemical energy stored in coal
(b) Gravitational potential energy
12. The third socket is connected to the ground and is there because metal appliances have
plugs with three pins. The third pin needs to be connected to the ground in case of a fault
in the appliance.
13. Answers will vary but should include heat to the surroundings when burning coal,
movement of turbines and coils, transmission from the power station to homes and the
production of light.
14–16. Answers will vary due to individual student research and responses.

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10.14 Concept maps and flowcharts

1.

2. Student responses will vary. An example is shown.

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3. Student responses will vary. An example is shown.

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4.

a. Student responses will vary. An example is shown.

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b. Student responses will vary. An example is shown.

5.

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Looking back
1.
Word Description
Static electricity The build-up of charge on an object
Electron Particle in an atom with a negative charge
Proton Positively charged particle in the nucleus of an atom
Current The flow of electrons around a closed circuit
Voltage The energy supplied to move electrons around a closed
circuit
Conductor A material that allows current or heat to flow through it
Closed circuit A path that has no breaks in it
Insulator A material that does not allow current or heat to flow through
it easily

2. (a) Parallel
(b) Parallel
(c) Series
(d) Parallel
3. Answers may vary but should have the ammeter in series.

4. (a) Conduction is the transfer of heat through a substance as a result of neighbouring

vibrating particles colliding and transferring kinetic energy.
(b) Convection is the process by which heat is transferred as a result of the actual
movement of more energetic particles from one region to a region where the
particles are less energetic.
(c) Radiation is the transfer of heat in the form of electromagnetic radiation.
5. (a) Convection
(b) Radiation
(c) Conduction
(d) Radiation and convection

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Chapter 10 Electricity at work Answers

(e) Conduction
6. Aluminium foil acts as a reflector of radiant heat. Cooked meat may be kept warm once
it is removed from the oven as the heat is reflected by the foil back into the meat. Plastic
would allow radiant heat to be transmitted to the cooler air and lost from the meat much
faster than if foil were used.
7. These solids contain lots of air spaces, and air — being a gas and having large distances
between its particles — is a poor conductor of heat.
8. (a) When the air temperature is higher than your skin temperature, the body loses heat
by producing perspiration. The energy needed to change the liquid water into water
vapour comes from your skin, so the evaporation of the perspiration reduces the skin
temperature. By causing the dilation of blood vessels near the skin surface, heat
from the blood is transferred to the skin. The cooling of the skin allows the blood to
be cooled as the heat is removed from the blood. This allows the core temperature of
the body to be maintained.
(b) Light, loose-fitting clothing allows air to circulate next to the skin so that the
processes of evaporation of perspiration and radiation can occur to remove heat.
(c) The blood vessels become larger so that an increased volume of blood can transfer
heat to the skin to be transferred to the air by radiation.
9. C and D as they are closed circuits.
10. (a) Conduction: the transfer of heat through collisions between particles
(b) Convection: the transfer of heat through the flow of particles
(c) Radiation: a method of heat transfer that does not require particles to transfer heat
from one place to another
(d) Current: the flow of electrons through a region
(e) Insulation: materials which do not allow heat to flow easily
(f) Energy: the capacity to perform work
(g) Metal: a material which conducts heat and electricity; typically malleable and
ductile
(h) Charge: a property of matter that affects how the matter behaves in an electric or
magnetic field, a positive charge is due to a loss of electrons, a negative charge is due to
an excess of electrons.
(i) Vibration: repeated, fast back-and-forth movements
(j) Voltage: the amount of energy that is pushing electrons around a circuit, per
coulomb of charge that flows between two points
(k) Reflection: bouncing off the surface of a substance
(l) Circuit: a path which allows electricity to flow
11. Voltage
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12. (a) B and C

(b) D and E
13. (a) 4 volts
(b) 12 volts
(c) 6 volts
14. (a) 200 mA
(b) 300 mA
(c) 500 mA
15. D and E
16. D and E
17. By placing a wire between the terminals of the battery
18.
(a) The electric current through globe C is 0.200 A.
(b) The resistance of globe C while the current is flowing is 20 Ω.
V
R=
I
4V
=
0.200A
= 20Ω

19.
Quantity Unit Abbreviation
Voltage Volt V
Electric current Ampere or amp A
Resistance Ohm Ω

20.

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21. (a) 310 mA

(b) 3.1 A
22. (a) Examples include:

 fluctuations in the current

 loose connections.
(b) Examples include:

 not reading the scale from directly in front (parallax error)

 not calibrating the ammeter so it still gives a reading even when there is no
current flowing

 the scale not being printed unevenly (usually on cheap devices)

 the ammeter being tilted.

23. A battery is made up of two or more cells connected in series.
24. A conductor that obeys Ohm’s Law (i.e. an ohmic resistor) produces the same value of
resistance for the full range of voltage and current values. When these values are plotted
on a graph, a straight line of constant gradient results. A non-ohmic conductor produces
different values of resistance for some current ranges. The graph produced by plotting
the voltage and current values for these conductors produces a curve in at least part of the
range.
25. (a) The electric current decreases.
(b) The voltage across the resistor increases.
(c) The voltage across the lamp decreases.
26. (a) Capacitor
(b) Transistor
(c) Light-emitting diode (LED)
(d) Silicon diode
27. Answers may include: kitchen appliances, such as ovens, stoves and kettles; all
communication devices, such as computers; cleaning devices; and gardening and
household maintenance and building devices.
28. (a) Resistor
(b) Silicon diode
(c) Light-emitting diode (LED)
(d) Capacitor

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(e) Transistor
29. Answers may vary. Examples are given.

Device Sensor Response

Ovens, stoves Heat The heat sensor regulates the cooking temperature.
Automatic car Light The light sensors detect changes in daylight and
headlights, interior turn the lights off or on as required.
displays
Security lighting Light Lights turn on when the sun goes down and off
when there is enough light.
Air conditioners Heat The heat sensor regulates the room temperature.

30. Answers may vary. Examples are given.

 Computers: can perform many functions at once

 Washing machines: can perform many functions

 Refrigerators: keep different parts of the appliance at difference temperatures

 TV remote controls: can perform many functions

31.

The magnetic field is strongest at point A.

32. The flowchart should show the following sequence of events:
(a) An electric current changes direction flows rapidly though the voice coil.
(b) The voice coil produces a magnetic field that also rapidly changes direction.
(c) The voice coil is alternatively attracted and repelled by the permanent magnet.

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Chapter 10 Electricity at work Answers

(d) The voice coil pushes the cone of the speaker backwards and forwards.
(e) The vibrating coil causes the air nearby to vibrate, producing sound.
33. (a) Labels clockwise from top right: field magnet, shaft, brush, armature, rotor coil
(b)
Part Purpose
Field magnets Act as permanent magnets
Armature Turns when the rotor coil is pushed or pulled by the field magnets
Rotor coil Produces a magnetic field that interacts with the one produced by
the field magnets
Shaft Turns with the motor, turning the device to which the motor is
attached
Brushes Lightly touch the commutator so that current can flow through the
rotor coil
Commutator Allows the current in the rotor coil to change direction so that the
armature rotates in the same direction rather than backwards and
forwards

34. (a) An electromagnet is a temporary magnet. It loses its magnetism when there is no
electric current flowing.
(b) The coil of wire
(c) The iron core increases the strength of the magnetic field while the current is
flowing.
35. (a) Roll the thin card into a tube that will fit the bar magnet inside, and then wind the
wire around the tube (all in one direction). Connect the ends of the wire together,
and then move the magnet rapidly in and out of one end of the tube.
(b) Galvanometer

36.  Increase the number of turns of wire in the rotor coil.

 Increase the electric current in the field coils (or the strength of the field magnet).

 Increase the speed of the relative motion between the rotor coils and the field coils or
field magnet.
37. The turbines are turned by steam and cause the coils in the generator to rotate.

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