A cyclist pumps air into a bicycle tyre that has a fixed volume. After pumping, the tyre feels noticeably warmer. Explain why the pressure inside the tyre increases as air is pumped in, and why the tyre becomes warmer during this process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist uses a hand pump to inflate a bicycle tyre. The tyre's volume does not change as it is inflated. After pumping, the cyclist notices that the tyre feels warm to the touch.
Model answer (4 marks)
More air particles are added to the same fixed volume, so the number density of gas molecules increases.
The increased number of molecules means collisions with the tyre walls become more frequent.
Pressure is the perpendicular force per unit area; more frequent collisions raise the force on the walls, so the pressure rises.
Work is done by the cyclist’s hand pump to compress the gas into the tyre; this work is transferred as kinetic energy to the molecules, raising their temperature and making the tyre feel warm.
The increased number of molecules means collisions with the tyre walls become more frequent.
Pressure is the perpendicular force per unit area; more frequent collisions raise the force on the walls, so the pressure rises.
Work is done by the cyclist’s hand pump to compress the gas into the tyre; this work is transferred as kinetic energy to the molecules, raising their temperature and making the tyre feel warm.
Examiner tips
- Use the chain: add particles → higher density → more collisions → higher pressure; then explain work → kinetic energy → temperature rise.
- Mention the tyre’s volume is fixed, so compression is not changing volume but adding particles.
- Show the link between work done and temperature increase (first law of thermodynamics).
Common mistakes
- Failing to state that the volume is fixed and pressure rises because of added particles, not compression.
- Confusing the increase in pressure with a decrease in volume; or not linking work to temperature rise.
Mark scheme (4 marks)
- More air (gas) particles are introduced into the same volume
- More frequent collisions of particles with the container walls
- Pressure is the perpendicular force per unit area, so more frequent collisions increase the pressure
- Work is done on the gas as it is compressed into the tyre, transferring energy to the particles, increasing their kinetic energy and raising the temperature of the gas
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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