Electric Motors
Build confidence in Electric Motors for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: a simple d.c. motor uses the motor effect to make a current-carrying coil rotate, with a split-ring commutator reversing the current each half turn.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
An electric motor uses the motor effect to produce continuous rotation. This is a Triple-only and Higher-only topic.
A coil of wire carrying a current sits in a magnetic field. The forces on the two sides of the coil act in opposite directions, because the current flows in opposite directions on each side. This produces a turning effect that rotates the coil.
A split-ring commutator is essential. Without it the coil would turn half a turn and then stop, because the forces would then oppose the rotation. The commutator reverses the current direction every half turn, so the forces always act to keep the coil rotating in the same direction.
Revision notes
How the motor turns
A current-carrying coil is placed in a magnetic field.
The current flows in opposite directions on the two sides of the coil, so the forces on them act in opposite directions. This produces a turning effect that rotates the coil.
The split-ring commutator
The commutator reverses the direction of the current in the coil every half turn.
Without it, the coil would rotate half a turn and then the forces would oppose further rotation, so it would stop. The commutator ensures the forces always act to keep it turning the same way.
Increasing the turning effect
Increase the current, increase the magnetic flux density, or increase the number of turns on the coil.
Each produces a larger force on the sides of the coil and therefore a greater turning effect.
Key points
- A motor uses the motor effect to rotate a coil.
- Current flows in opposite directions on the two sides.
- The forces therefore act in opposite directions.
- A split-ring commutator reverses the current each half turn.
- Without it the coil would stop after half a turn.
- More current or turns increases the turning effect.
Worked examples
Example 1
Explain why the coil in an electric motor rotates. [3 marks]
Model answer
The current flows in opposite directions on the two sides of the coil (1 mark). The forces on the two sides therefore act in opposite directions (1 mark), producing a turning effect that rotates the coil (1 mark).
Example 2
Explain the purpose of the split-ring commutator. [3 marks]
Model answer
It reverses the direction of the current in the coil every half turn (1 mark). Without it the forces would oppose the rotation after half a turn and the coil would stop (1 mark). The commutator ensures the forces always act to keep the coil rotating in the same direction (1 mark).
Example 3
State two ways to increase the speed of an electric motor. [2 marks]
Model answer
Increase the current flowing through the coil (1 mark). Increase the number of turns on the coil, or use a stronger magnetic field (1 mark).
Common mistakes
Saying the commutator supplies the current.
Its function is to reverse the current direction every half turn.
Forgetting why the coil would stop without it.
The forces would oppose rotation after half a turn.
Saying both sides of the coil feel forces in the same direction.
They are opposite, which is what creates the turning effect.
Confusing a motor with a generator.
A motor uses current to produce motion; a generator uses motion to produce current.
Exam tips
- Explain rotation through opposite forces on the two sides.
- Give the full three-step commutator explanation.
- List several ways to increase the turning effect.
- Remember this is Triple-only AND Higher-only.
Key terms
- Electric motor
- A device using the motor effect to produce rotation.
- Split-ring commutator
- A device reversing the current every half turn.
- Turning effect
- The rotational effect produced by the forces on the coil.
- Coil
- The loop of wire carrying the current in the motor.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.