Explain why a gas in a thermally insulated container rises in temperature when it is rapidly compressed.

IB DP Physics Standard Level (2023 syllabus) — B.1 Thermal energy transfers · Explain · 4 marks · View as Markdown

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

A bicycle pump is sealed at its outlet so that no air escapes. The handle is pushed in rapidly, compressing the trapped air.

Model answer (4 marks)

Work is done ON the gas by the moving piston/handle during compression.
Because the container is thermally insulated, no thermal energy is transferred to or from the surroundings (Q = 0).
The internal energy of the gas therefore increases (by an amount equal to the work done on it).
Greater internal energy means greater average kinetic energy of the gas molecules, so temperature rises.

Examiner tips

Use the word ‘work’ to show the energy transfer; mention the piston/handle. State that Q = 0 because the container is insulated. Link the increase in internal energy to the work done. Explain that higher internal energy gives higher kinetic energy and therefore a higher temperature.

Common mistakes

Confusing work done by the gas with work done on the gas. Forgetting to state that Q = 0. Saying the temperature rises because the gas is compressed without linking to internal energy. Using vague terms like ‘energy’ instead of ‘internal energy’ or ‘kinetic energy’.

Mark scheme (4 marks)

Work is done ON the gas by the moving piston/handle during compression.
Because the container is thermally insulated, no thermal energy is transferred to or from the surroundings (Q = 0).
The internal energy of the gas therefore increases (by an amount equal to the work done on it).
Greater internal energy means greater average kinetic energy of the gas molecules, so temperature rises.

Key terms in this question

temperature