Electric motors (HT only) - AQA GCSE Physics Exam Questions

Practise 20 exam-style questions on Electric motors (HT only) for AQA GCSE Physics. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.

20 questions
1–6 marks each
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20 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

In a d.c. motor, the current reaches the spinning coil through two carbon brushes that press against the commutator.

Explain why brushes are needed.

Model answer

The brushes press against the rotating commutator, so they keep an electrical contact with it as it turns. This lets current flow into and out of the coil continuously without the supply wires becoming twisted round the axle.

Explanation

The coil is spinning but the supply is fixed, so a sliding contact is needed. Carbon is used because it conducts and is slippery, so it causes little friction and wear.

Mark scheme

1 mark
the brushes keep an electrical contact with the commutator while it rotates
1 mark
so current can flow into the coil without the connecting wires getting twisted
allow the coil can turn freely while still connected to the supply

Question 2

2 marks

Describe the main energy transfer in an electric motor, and state where energy is wasted.

Model answer

The current transfers energy electrically to the motor, which increases the kinetic energy of the coil and whatever it is driving. Some energy is wasted, heating the coil because of its resistance and heating the bearings and brushes because of friction.

Explanation

A motor transfers energy from the electrical supply to a kinetic energy store. No motor is 100% efficient: resistance and friction transfer some energy to the thermal energy store of the motor and its surroundings.

Mark scheme

1 mark
energy is transferred electrically (by the current) to the kinetic energy (store) of the coil / whatever it turns
allow electrical energy to kinetic energy
1 mark
some energy is wasted as thermal energy (heating) because of the resistance of the coil / friction
allow sound

Question 3

3 marks

The coil in a real electric motor has hundreds of turns of wire wound on an iron core.

Explain why this makes the motor more powerful than one with a single loop of wire and no core.

Model answer

Each turn of wire in the coil experiences a force, so with hundreds of turns the forces add together, giving a much larger total force and turning effect. The iron core becomes magnetised in the field and makes the magnetic field through the coil stronger, which increases the force on every wire even more.

Explanation

The force on a single wire is F = B I l. More turns multiply this force, and the iron core increases B. Both make the turning effect bigger, so the motor can turn a bigger load.

Mark scheme

1 mark
each turn experiences a force, so with many turns the forces add up
1 mark
so the total force and turning effect on the coil are much larger
1 mark
the iron core becomes magnetised and increases the magnetic flux density through the coil
allow the iron core makes the magnetic field stronger

Question 4

4 marks

A student makes a simple motor from a coil of enamelled (insulated) wire resting on two metal supports connected to a battery, with a magnet underneath. At one end of the coil, she scrapes the insulation off the whole of the wire. At the other end, she scrapes the insulation off only one half of the wire, so that the bare half touches the support for only half of each turn.

Explain why this coil keeps turning in the same direction.

Model answer

For half of each turn, the bare half of the wire touches the support, so there is a current in the coil and the forces on its sides give a turning effect in one direction. For the other half of the turn, the insulated half touches the support, so the circuit is broken: there is no current and no force, so nothing pushes the coil back the other way. The coil's momentum carries it round through this half turn until the bare wire touches again and the next push begins.

Explanation

This simple motor does the job of a commutator in a different way: instead of reversing the current every half turn, it switches the current off for the half turn when the forces would act the wrong way.

Mark scheme

1 mark
for half of each turn the bare wire touches the support, so there is a current in the coil
1 mark
the forces on the coil then give a turning effect in one direction
1 mark
for the other half turn the insulated side touches the support, so there is no current and no force (no turning effect in the opposite direction)
1 mark
the coil's momentum carries it round until the current flows again
allow inertia

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