# A smoke detector contains a small amount of a radioactive substance that emits alpha particles. The alpha particles ionise the air inside the detector, allowing a small electric current to flow. When smoke enters the detector, the current drops and the alarm sounds. Explain why alpha particles are suitable for use inside the smoke detector, and why this type of detector does not pose a significant radiation risk to people in the building.

> WJEC A-Level Physics (Wales) — 6.1 Radioactivity · Explain · 5 marks

> Smoke detectors used in homes and offices contain a radioactive source. The source has a long half-life and emits alpha particles continuously.

## Mark scheme (5 marks)

1. Alpha particles are strongly ionising / cause strong ionisation of air
2. This allows a current to flow through the ionised air between the detector plates
3. Alpha particles have a very short range in air (a few centimetres)
4. Alpha particles cannot penetrate the plastic casing of the detector / are stopped before reaching people
5. Alpha particles cannot penetrate the skin / dead skin cells absorb alpha radiation, so even if some escaped they would not reach living tissue

## Key terms

- [alpha particles](https://www.gradenine.co.uk/glossary/alpha-particles)
- [ionise](https://www.gradenine.co.uk/glossary/ionise)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](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-smoke-detector-contains-a-small-41684f38) · Published by Druglandscape Ltd.