A bicycle tyre is pumped up so that it contains more air, but the volume of the tyre stays the same and the temperature does not change. Explain why the pressure inside the tyre increases when more air is added.

Eduqas GCSE Physics — 2.2 The kinetic theory and gas pressure · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Adding more air introduces more gas molecules into the tyre. With a fixed volume and constant temperature, the average speed of the molecules remains the same, but the number of molecules that collide with the tyre walls per second increases. Pressure is the perpendicular force per unit area exerted by these collisions. Therefore, with more collisions per second and the same area, the force per unit area rises, so the pressure inside the tyre increases.

Examiner tips

  • Use the kinetic theory definition of pressure (force/area from collisions).
  • Mention that temperature is constant, so average speed stays the same.
  • Explain that more molecules = more collisions per second.
  • Show the logical chain: more molecules → more collisions → higher force per area → higher pressure.

Common mistakes

  • Failing to state that temperature is constant, leading to incorrect reasoning about speed.
  • Confusing pressure with force or area separately, rather than force per unit area.
  • Using vague terms like "more air" without linking to more molecules and collisions.

Mark scheme (4 marks)

  1. Adding more air means there are more gas particles/molecules inside the tyre
  2. The particles collide with the walls of the tyre more frequently / there are more collisions per second with the tyre walls
  3. Pressure is the perpendicular force per unit area acting on the surfaces of the tyre as a result of these collisions
  4. Because the volume is constant, the increased number of collisions results in a greater force per unit area, so pressure increases

Key terms in this question

pressure · volume · temperature

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