Explain, using kinetic theory, what happens to the particles in a gas when the gas is heated at constant volume, and why this causes the pressure of the gas to increase.

Eduqas A-Level Physics — 3.3 Kinetic theory · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Heating increases the internal energy of the gas, so the particles gain kinetic energy and move faster.
The increased speed means collisions with the walls occur more frequently.
Each collision is harder, exerting a larger force on the walls.
The combination of more frequent and harder collisions raises the total force on the walls, so the pressure increases.

Examiner tips

  • Use the phrase "gain kinetic energy" to show understanding of heating. Show the chain: energy → speed → collision frequency & force → pressure. Mention constant volume to emphasise no change in volume. Use the word "pressure" explicitly in the final sentence.

Common mistakes

  • Failing to state that the gas is at constant volume. Confusing internal energy with potential energy. Omitting the link between collision frequency and pressure.

Mark scheme (5 marks)

  1. Heating gives the particles more energy (internal/kinetic energy)
  2. The particles move faster / have greater speed
  3. Particles collide with the walls of the container more frequently
  4. Each collision exerts a greater force on the walls (particles hit harder)
  5. More frequent and harder collisions increase the overall force on the walls, so pressure increases

Key terms in this question

kinetic theory · pressure · particles

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