A fixed quantity of gas is compressed to a smaller volume at constant temperature. Explain, in terms of particles, why the pressure of the gas increases.

Eduqas GCSE Physics — 2.6 Particle physics and using physics to investigate the universe · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The gas particles collide with the walls of the container, giving a perpendicular force on the walls. Pressure is the perpendicular force per unit area on the container walls. When the volume is reduced, the same number of particles occupy a smaller space, so collisions with the walls become more frequent. More frequent collisions produce a greater perpendicular force per unit area, so the pressure increases.

Examiner tips

  • Use the word "collide" and "force" to show particle action
  • Explain that pressure = force/area
  • Show that reducing volume keeps particle number constant but increases collision frequency
  • Link higher collision frequency to higher pressure

Common mistakes

  • Forgetting to mention that temperature is constant, so kinetic energy per particle does not change
  • Confusing volume with pressure or area alone
  • Using vague terms like "more particles" instead of "more collisions"

Mark scheme (4 marks)

  1. The particles collide with the walls of the container, producing a (perpendicular) force on the walls / surfaces
  2. Pressure is the (perpendicular) force per unit area on the container walls
  3. At smaller volume the particles are closer together / the same number of particles occupy a smaller space, so collisions with the walls are more frequent
  4. More frequent collisions mean a greater (perpendicular) force per unit area on the walls, so pressure is higher

Key terms in this question

volume · temperature · particles

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