A nurse presses the plunger of a sealed syringe, reducing the volume of the air trapped inside. Explain why the temperature of the air inside the syringe increases when the plunger is pushed in.

OCR GCSE Physics A: Gateway Science (J249) — P1.1 The particle model · Explain · 4 marks · View as Markdown

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

A sealed syringe contains a fixed amount of air. When the plunger is pushed inwards, the volume of the trapped air decreases.

Model answer (4 marks)

The volume of the air decreases, so the particles collide with the walls more frequently. Work is done on the gas when the plunger is pushed in, transferring energy to the particles. This increases their kinetic energy, and since temperature is a measure of the average kinetic energy, the temperature of the air rises.

Examiner tips

  • Use the word ‘work’ to link force and energy transfer
  • Show the chain: volume ↓ → collisions ↑ → kinetic energy ↑ → temperature ↑
  • Mention the plunger does work on the gas
  • Keep the answer concise and in a logical order

Common mistakes

  • Failing to mention that work is done on the gas
  • Confusing pressure increase with temperature increase
  • Using vague terms like ‘more energy’ without linking to kinetic energy

Mark scheme (4 marks)

  1. The volume of the air decreases, so the particles collide with the walls more frequently
  2. Work is done on the gas / a force is applied to the gas by the plunger
  3. This transfers energy to the particles, increasing their kinetic energy
  4. Temperature is a measure of the average kinetic energy of the particles, so temperature increases

Key terms in this question

temperature · volume

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