# 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.

> OCR GCSE Physics A: Gateway Science (J249) — P3.3 Magnetism and the motor effect · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. 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.
2. Forces on opposite sides of the coil act in opposite directions, producing a turning effect / torque on the coil.
3. Every half turn, the split-ring commutator swaps / reverses the connections between the coil and the power supply.
4. 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

- [current-carrying coil](https://www.gradenine.co.uk/glossary/current-carrying-coil)

## Related

- [Revision notes for OCR GCSE Physics A: Gateway Science (J249)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-current-carrying-coil-inside-248e8324) · Published by Druglandscape Ltd.