Explain why a current-carrying coil inside a d.c. electric motor continues to rotate in the same direction rather than reversing back and forth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A simple d.c. electric motor consists of a rectangular coil of wire placed between the poles of a permanent magnet. The coil is connected to a d.c. power supply via a split-ring commutator.
Model answer (4 marks)
A current‑carrying coil in a d.c. motor experiences a force because the magnetic field of the permanent magnet interacts with the magnetic field produced by the current in the coil. The forces on opposite sides of the coil are in opposite directions, giving a torque that turns the coil. The split‑ring commutator reverses the connections between the coil and the power supply every half turn. By reversing the current direction at each half turn, the forces on each side of the coil always act in the same rotational direction, so the coil continues to spin the same way.
Examiner tips
- Use the term ‘torque’ and ‘force’ to show understanding of the motor effect.
- Explain the role of the commutator in keeping the torque direction constant.
- Show the sequence: force → torque → commutator reversal → same torque direction.
- Keep the answer concise – 4 points only, no extra detail.
Common mistakes
- Forgetting to mention the commutator’s role in reversing the current.
- Describing the coil as rotating back and forth instead of continuously.
- Using vague terms like ‘magnetic influence’ without specifying force and torque.
Mark scheme (4 marks)
- The coil experiences a force due to the interaction between the magnetic field of the magnet and the magnetic field produced by the current in the coil.
- Forces on opposite sides of the coil act in opposite directions, producing a turning effect / torque on the coil.
- Every half turn, the split-ring commutator swaps / reverses the connections between the coil and the power supply.
- Reversing the current direction every half turn keeps the forces on each side of the coil acting in the same rotational direction, so the coil continues to spin the same way.
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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