# A hospital uses a device that converts sound into electrical signals to monitor a patient's heartbeat sounds. Inside the device, a coil of wire is free to move around a fixed permanent magnet. Explain how this device produces an electrical current when sound waves reach it.

> OCR A-Level Physics A (H556) — 4.3 Electrical circuits · Explain · 5 marks

> A microphone is placed against a patient's chest to detect heartbeat sounds. The microphone contains a fixed permanent magnet surrounded by a coil of wire that is attached to a diaphragm.

## Mark scheme (5 marks)

1. Sound waves cause pressure variations that make the diaphragm (and attached coil) vibrate / move back and forth
2. The moving coil cuts through the magnetic field of the permanent magnet / there is relative motion between the coil and the magnetic field
3. A potential difference is induced across the ends of the coil (generator effect)
4. Because the coil is part of a complete circuit, a current flows
5. The current produced is alternating / changes direction because the coil moves back and forth / the direction of motion repeatedly reverses

## Related

- [Revision notes for OCR A-Level Physics A (H556)](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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