A student sets up a simple electric motor using a rectangular coil of wire in a magnetic field. The motor stops rotating when the plane of the coil is perpendicular to the magnetic field. Explain why the coil begins to rotate when a current flows, and why it would continue to rotate rather than stop in a real motor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A rectangular coil of wire is connected to a direct current (d.c.) supply and placed between the poles of a permanent magnet.
Model answer (4 marks)
A current‑carrying conductor in a magnetic field experiences a force (the motor effect). In a rectangular coil the two opposite sides are in opposite directions, so the forces on them are opposite and produce a turning effect – a torque – that causes the coil to rotate.
When the coil rotates a half turn, the current direction in the two sides is reversed by the split‑ring commutator. This keeps the torque acting in the same direction, so the coil continues to rotate instead of stopping.
When the coil rotates a half turn, the current direction in the two sides is reversed by the split‑ring commutator. This keeps the torque acting in the same direction, so the coil continues to rotate instead of stopping.
Examiner tips
- Show the motor effect and torque on the coil; mention opposite forces on opposite sides.
- Explain the role of the commutator in maintaining torque direction.
- Use the exact terms: ‘motor effect’, ‘torque’, ‘split‑ring commutator’.
- Keep the answer concise – 4 marks only.
Mark scheme (4 marks)
- A current-carrying conductor in a magnetic field experiences a force (motor effect)
- The forces on opposite sides of the coil act in opposite directions, producing a turning effect / torque / causing the coil to rotate
- A split-ring commutator reverses the direction of the current every half turn
- This keeps the turning force (torque) acting in the same direction so the coil continues to rotate
Key terms in this question
Related
- All Eduqas GCSE Physics revision notes →
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- Decode the mark scheme abbreviations →
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