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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The air molecules collide with the walls of the pump, producing pressure.
When the cyclist pushes the handle inward the volume of the pump decreases, so the molecules are confined to a smaller space and collide with the walls more frequently.
More frequent collisions give a greater force on the walls, so the pressure rises.
During this compression work is done on the gas – the handle does work on the molecules.
This work increases the kinetic energy of the molecules, raising their average speed and therefore the temperature of the gas.
When the cyclist pushes the handle inward the volume of the pump decreases, so the molecules are confined to a smaller space and collide with the walls more frequently.
More frequent collisions give a greater force on the walls, so the pressure rises.
During this compression work is done on the gas – the handle does work on the molecules.
This work increases the kinetic energy of the molecules, raising their average speed and therefore the temperature of the gas.
Examiner tips
- Use the term ‘collisions with the walls’ to link to pressure. Show the chain: smaller volume → more collisions → higher pressure. Mention that work done on the gas raises kinetic energy → temperature rise.
- Include the word ‘molecules’ to satisfy the ‘in terms of molecules’ requirement.
Mark scheme (5 marks)
- The molecules collide with the walls of the container, producing pressure.
- When volume decreases, the molecules are closer together / in a smaller space, so collisions with the walls are more frequent.
- More frequent collisions with the walls means a greater force on the walls, so pressure increases.
- During compression, work is done on the gas / the handle does work on the molecules.
- This increases the kinetic energy of the molecules, so their average speed increases and temperature rises.
Key terms in this question
pressure · volume · temperature · molecules
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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