# A fixed quantity of gas is trapped inside a cylinder fitted with a movable piston. The gas is compressed at constant temperature so that its volume is reduced. Explain what happens to the pressure of the gas as its volume decreases.

> Eduqas A-Level Physics — 2.3 Capacitance (A-Level only) · Explain · 5 marks

> A fixed quantity of gas is trapped inside a cylinder fitted with a movable piston. The piston is pushed inward slowly so that the temperature of the gas remains constant throughout.

## Mark scheme (5 marks)

1. The number of gas particles remains constant (fixed quantity of gas)
2. Reducing the volume means the particles are confined to a smaller space
3. Particles collide with the walls of the container more frequently
4. Each collision exerts a perpendicular force on the walls, so more collisions per second means a greater total force on the walls
5. Therefore the pressure increases, consistent with the pressure-volume relationship (pressure is inversely proportional to volume at constant temperature)

## Key terms

- [pressure](https://www.gradenine.co.uk/glossary/pressure)
- [volume](https://www.gradenine.co.uk/glossary/volume)
- [fixed quantity of gas](https://www.gradenine.co.uk/glossary/fixed-quantity-of-gas)

## Related

- [Revision notes for Eduqas A-Level Physics](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-fixed-quantity-of-gas-is-b00b57ab) · Published by Druglandscape Ltd.