A bicycle pump contains a fixed mass of air. When the nozzle of the pump is blocked and the handle is pushed in, the volume of the air decreases at constant temperature. Explain why the pressure of the air inside the pump increases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist blocks the nozzle of a bicycle pump and pushes the handle inward, compressing the trapped air to a smaller volume. The temperature of the air remains constant throughout.
Model answer (4 marks)
The trapped air molecules move in all directions and collide with the pump walls.
1. When the volume is reduced the molecules are forced closer together.
2. They therefore travel a shorter distance between collisions with the walls.
3. Each collision changes the molecule’s momentum, exerting a force on the wall.
4. Because collisions occur more frequently, the force per unit area – i.e. the pressure – increases.
1. When the volume is reduced the molecules are forced closer together.
2. They therefore travel a shorter distance between collisions with the walls.
3. Each collision changes the molecule’s momentum, exerting a force on the wall.
4. Because collisions occur more frequently, the force per unit area – i.e. the pressure – increases.
Examiner tips
- Use the sequence: volume ↓ → density ↑ → mean free path ↓ → collision frequency ↑ → pressure ↑
- Mention constant temperature to justify ideal gas behaviour
- Keep answer concise, use correct terminology
Common mistakes
- Confusing pressure increase with temperature rise
- Saying pressure rises because molecules move faster – they actually move at the same speed
Mark scheme (4 marks)
- The gas molecules collide with the walls of the pump (more frequently / more often)
- Because the molecules are closer together / the volume is smaller, so they travel a shorter distance between collisions with the walls
- Each collision exerts a force on the wall (because the molecule changes momentum / direction on rebounding)
- Greater frequency of collisions means a greater force per unit area / greater pressure on the walls
Key terms in this question
pressure · volume · constant temperature
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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