An engineer uses an electric motor in a piece of industrial machinery. The motor contains a rectangular coil of wire placed between two permanent magnets. Explain how the motor produces continuous rotation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric motor converts electrical energy into kinetic energy. Inside the motor, a coil of wire is connected to a power supply and sits between the poles of two permanent magnets.
Model answer (5 marks)
Current flows through the coil, creating a magnetic field around it.
The coil’s magnetic field interacts with the magnetic field of the permanent magnets.
The two sides of the coil that are perpendicular to the magnetic field experience forces in opposite directions.
These opposing forces cause the coil to rotate.
A split‑ring commutator reverses the current every half turn, keeping the torque in the same direction and allowing continuous rotation.
The coil’s magnetic field interacts with the magnetic field of the permanent magnets.
The two sides of the coil that are perpendicular to the magnetic field experience forces in opposite directions.
These opposing forces cause the coil to rotate.
A split‑ring commutator reverses the current every half turn, keeping the torque in the same direction and allowing continuous rotation.
Examiner tips
- Use the word ‘current’ and ‘magnetic field’ early; mention the Lorentz force. Show the sequence: current → field → interaction → force → rotation → commutation. Remember to state that the commutator reverses the current every 180°.
Common mistakes
- Failing to mention the commutator or the reversal of current. Confusing the direction of the magnetic field with the direction of the force. Omitting the interaction between the coil’s field and the permanent magnets’ field.
Mark scheme (5 marks)
- Current flows through the coil, producing a magnetic field around the wire
- The coil's magnetic field interacts with the magnetic field of the permanent magnets
- The two sides of the coil that are perpendicular to the magnetic field experience forces in opposite directions
- The opposing forces on the two sides of the coil cause it to rotate
- The direction of the current is reversed every half turn (e.g. using a split-ring commutator) so that the coil continues to rotate in the same direction
Key terms in this question
electric motor · permanent magnet
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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