A sealed container holds a fixed amount of gas. The temperature of the gas is increased. Explain, using kinetic theory, why the pressure of the gas increases when its temperature is increased.

WJEC A-Level Physics (Wales) — 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)

1. Gas particles are in constant random motion.
2. Increasing temperature raises the average kinetic energy of the particles.
3. Higher kinetic energy causes particles to collide with the walls more frequently.
4. Each collision exerts a greater force on the wall because the particles hit harder (greater momentum transfer).
5. Greater force on the same area of the wall results in higher pressure.

Examiner tips

  • Use the sequence: motion → kinetic energy → collision frequency → force per collision → pressure. Include the word "force" and "momentum" to show understanding of the kinetic theory explanation.

Common mistakes

  • Confusing temperature with pressure; not explaining the kinetic energy link. Failing to mention that pressure is force per unit area or that collisions are more frequent and harder.

Mark scheme (5 marks)

  1. Gas consists of particles that are in constant random motion
  2. Increasing temperature increases the kinetic energy of the particles
  3. Particles collide with the walls of the container more frequently
  4. Each collision exerts a greater force on the walls (because particles hit harder / with greater momentum)
  5. Greater force on the same area of wall means greater pressure

Key terms in this question

kinetic theory · pressure · temperature

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