A bicycle tyre is inflated by pumping air into it. The volume of the tyre remains constant. Explain why the pressure inside the tyre increases as more air is pumped in.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
More air molecules are added to the tyre while its volume stays constant. The molecules move randomly and collide with the tyre walls. With more molecules, collisions occur more frequently per unit time. The increased collision frequency raises the total force on the walls per unit area, so the pressure inside the tyre increases.
Examiner tips
- Use the word "more" to show increase in molecules and collisions
- Explain that pressure is force per unit area and link to collision frequency
- Keep answer concise and use correct terminology
Common mistakes
- Failing to mention the fixed volume or the role of collisions
- Using vague phrases like "more gas" without explaining the mechanism
- Mixing up pressure with force or volume
Mark scheme (4 marks)
- More (air) particles/molecules are present in the same (fixed) volume
- The particles are in constant random motion and collide with the walls of the tyre
- More collisions occur per unit time / the frequency of collisions with the walls increases
- Greater total force acts on the walls per unit area, so pressure increases
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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