# A bicycle pump contains a fixed mass of air. A cyclist places their thumb firmly over the nozzle so that no air can escape, then pushes the handle inward, compressing the air into a smaller volume. The compressed air feels noticeably warmer. Explain, in terms of molecules, why the pressure of the gas increases when the volume decreases, and why the temperature of the gas rises during this compression.

> WJEC A-Level Physics (Wales) — 4.1 Thermal physics · Explain · 5 marks

## Mark scheme (5 marks)

1. The molecules collide with the walls of the container, producing pressure.
2. When volume decreases, the molecules are closer together / in a smaller space, so collisions with the walls are more frequent.
3. More frequent collisions with the walls means a greater force on the walls, so pressure increases.
4. During compression, work is done on the gas / the handle does work on the molecules.
5. This increases the kinetic energy of the molecules, so their average speed increases and temperature rises.

## Key terms

- [pressure](https://www.gradenine.co.uk/glossary/pressure)
- [volume](https://www.gradenine.co.uk/glossary/volume)
- [temperature](https://www.gradenine.co.uk/glossary/temperature)
- [molecules](https://www.gradenine.co.uk/glossary/molecules)

## 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-bicycle-pump-contains-a-fixed-23c146f9) · Published by Druglandscape Ltd.