A sealed syringe contains a fixed mass of air. When the plunger is pushed in quickly, the temperature of the air inside rises. Explain why the temperature of the air increases when the volume is reduced.

Eduqas A-Level Physics — 3.4 Thermal physics · Explain · 5 marks · View as Markdown

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

A student pushes the plunger of a sealed syringe inward, reducing the volume of the trapped air. The student notices the outside of the syringe feels warm after compression.

Model answer (5 marks)

The plunger pushes the air particles into a smaller volume, so they are compressed.
Work is done on the gas by the plunger, transferring energy to the gas.
This energy increases the internal energy of the air.
The particles therefore move faster, gaining kinetic energy.
A higher average kinetic energy of the particles gives a higher temperature.

Examiner tips

  • Use the word "compress" and "work" to show understanding of the process.
  • Explain the chain: compression → work → internal energy → kinetic energy → temperature.
  • Keep the answer concise and use the exact terms from the mark scheme.

Common mistakes

  • Confusing the temperature change with a change in pressure only.
  • Failing to mention that work is done on the gas rather than energy leaving the gas.
  • Using vague phrases like "the air gets hot" without linking to internal energy.

Mark scheme (5 marks)

  1. The air particles are compressed into a smaller volume
  2. Work is done on the gas / energy is transferred to the gas by the plunger
  3. This energy increases the internal energy of the gas
  4. The particles move faster / have greater kinetic energy
  5. Higher average kinetic energy of particles means a higher temperature

Key terms in this question

temperature

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