A bicycle tyre is pumped up and then left in a cool garage overnight. A cyclist notices that the tyre feels less firm in the morning compared to just after pumping. Using kinetic theory, explain why the pressure inside the tyre decreases as the temperature of the air inside falls.

WJEC GCSE Physics (Wales) — 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)

As the tyre cools, the average kinetic energy of the air molecules decreases.
The molecules therefore collide with the tyre walls with less force per collision.
They also collide with the walls less frequently because their speed is lower.
The reduced force and reduced collision frequency mean a lower force per unit area on the tyre walls, i.e. a lower pressure.

Examiner tips

  • Use the term "average kinetic energy" and link it to temperature. Explain both the force per collision and the collision frequency. Show the chain from kinetic energy → collision force/frequency → pressure. Keep the answer concise and use the exact wording from the mark scheme.

Common mistakes

  • Confusing temperature with pressure; stating that pressure falls because the tyre expands. Using vague phrases like "the air gets less energetic" without linking to collisions. Omitting the point about collision frequency or force per collision.

Mark scheme (4 marks)

  1. As temperature falls, the average kinetic energy of the gas molecules decreases
  2. The molecules hit the walls of the tyre with less force (per collision)
  3. The molecules collide with the walls less frequently
  4. Less force per unit area on the tyre walls means lower pressure (linking reduced collisions/force to lower pressure)

Key terms in this question

kinetic theory · pressure · temperature

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