Explain what happens to the molecules and the internal energy of a fixed mass of gas when it is compressed at constant temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The molecules are forced closer together, so the average distance between them decreases.
The volume of the gas decreases, which means the molecules collide with the walls more frequently.
Because the temperature is held constant, the average kinetic energy of the molecules remains unchanged.
The internal energy of a gas is the sum of the kinetic and potential energies of its molecules.
The potential energy of the molecules increases as they are compressed, so the internal energy of the gas increases.
The volume of the gas decreases, which means the molecules collide with the walls more frequently.
Because the temperature is held constant, the average kinetic energy of the molecules remains unchanged.
The internal energy of a gas is the sum of the kinetic and potential energies of its molecules.
The potential energy of the molecules increases as they are compressed, so the internal energy of the gas increases.
Examiner tips
- Use the word ‘compress’ and mention ‘constant temperature’ to show you understand the conditions.
- Explain that kinetic energy stays the same but potential energy rises – this gives the full 5 marks.
Common mistakes
- Saying the internal energy stays the same – ignores the change in potential energy.
- Confusing volume decrease with temperature increase – the temperature is constant.
Mark scheme (5 marks)
- The molecules are closer together / the average distance between molecules decreases
- The volume decreases so molecules hit the walls more frequently / there are more collisions with the walls per second
- Temperature is constant so the average kinetic energy of the molecules does not change
- The internal energy of a gas is the sum of the kinetic energy and potential energy of the molecules / internal energy depends on both kinetic and potential energy
- The potential energy of the molecules changes (increases) as they are forced closer together, so the internal energy of the gas changes (increases)
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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