Explain what happens to the pressure of a fixed mass of gas when it is heated at constant volume, in terms of the behaviour of the gas particles.

Eduqas A-Level Physics — 4.1 Thermal physics · Explain · 5 marks · View as Markdown

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

A sealed metal canister contains a fixed mass of gas. The canister is placed in an oven and its temperature rises.

Model answer (5 marks)

Heating the gas increases its temperature, giving the particles more kinetic energy.
The particles move faster, so their average speed rises.
They collide with the walls more frequently.
Each collision exerts a greater force on the walls because the particles hit harder.
Therefore the overall force on the walls increases, so the pressure rises.

Examiner tips

  • Use the word ‘explain’ to describe the sequence of events, not just state facts.
  • Show the link between temperature, speed, collision frequency and force to justify the pressure change.

Common mistakes

  • Confusing volume change with pressure change; the volume is constant.
  • Failing to mention that the force per collision increases, not just the frequency.

Mark scheme (5 marks)

  1. Heating the gas increases the temperature / gives the particles more energy
  2. The particles move faster / have greater average speed
  3. Particles collide with the walls more frequently / more often
  4. Each collision exerts a greater force on the walls / particles hit the walls harder
  5. Therefore the pressure increases / greater overall force on the walls increases pressure

Key terms in this question

pressure · particles

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