Describe what happens to the particles in a fixed mass of gas when the gas is heated at constant volume, and explain how this causes the pressure of the gas to increase.

Eduqas A-Level Physics — 3.4 Thermal physics · Describe and 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 the gas increases its temperature, giving the particles more kinetic energy. The particles therefore move faster and have a greater average speed. Because the volume is fixed, the faster particles collide with the walls of the container more frequently. Each collision exerts a greater force on the wall, as the particles hit the walls harder. The combination of more frequent and harder collisions results in a greater force per unit area on the walls, so the pressure of the gas increases.

Examiner tips

  • Use the terms "temperature", "kinetic energy", "average speed", "collisions", "force per unit area" – these are the key phrases examiners look for.
  • Show the causal chain: heating → higher temperature → faster particles → more frequent/harder collisions → higher pressure.
  • Remember to mention that volume is constant, so the only way pressure changes is through collision frequency/force.

Common mistakes

  • Failing to state that the volume is constant, leading to confusion about expansion.
  • Using vague language such as "particles move more" without linking to speed or collision frequency.
  • Omitting the explanation that pressure is force per unit area, so increased force and frequency raise pressure.

Mark scheme (5 marks)

  1. Heating the gas increases the temperature of the gas / gives the particles more energy
  2. The particles move faster / have greater average speed
  3. Particles collide with the walls of the container more frequently
  4. Each collision with the wall exerts a greater force on the wall / particles hit the walls harder
  5. More frequent and/or harder collisions result in greater force per unit area on the walls, so pressure increases

Key terms in this question

pressure · particles

Related

More Thermal physics questions

▶ Try answering this question with AI marking (free) →