# A security door in a museum uses an electric motor to open and close automatically. The motor contains a coil of wire placed between two permanent magnets. Explain how the motor produces rotation when a current flows through the coil.

> OCR A-Level Physics B: Advancing Physics (H557) — 4.1 Charge and current · Explain · 5 marks

> An electric motor in the museum's security door contains a rectangular coil of wire mounted on an axle and positioned between the poles of two permanent magnets. When a current flows through the coil, it rotates continuously to drive the door mechanism.

## Mark scheme (5 marks)

1. A current-carrying wire in a magnetic field experiences a force
2. The two sides of the coil that are perpendicular to the magnetic field experience forces in opposite directions
3. These opposite forces cause the coil to rotate
4. Fleming's left-hand rule can be used to determine the direction of the force on each side of the coil
5. The strength of the force (and hence the rotation) depends on the magnetic flux density and the magnitude of the current

## Key terms

- [electric motor](https://www.gradenine.co.uk/glossary/electric-motor)

## Related

- [Revision notes for OCR A-Level Physics B: Advancing Physics (H557)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-security-door-in-a-museum-0013ddd1) · Published by Druglandscape Ltd.