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A-Level Physics May/June 2024 Q3(b)(i): With reference to molecular kinetic and potential energies, describe and explain how th…
A-Level Physics · Paper 9702/42 · May/June 2024 · Question 3(b)(i) · [3 marks]
With reference to molecular kinetic and potential energies, describe and explain how the internal energy of the system changes when: (i) a gas is heated at constant volume so that its temperature increases
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The internal energy of a system is the sum of the random kinetic and potential energies of its molecules.
When the gas is heated at a constant volume, the average separation between the molecules does not change. Therefore, there is no change in the molecular potential energy.
The temperature of the gas increases. As the average kinetic energy of the molecules is directly proportional to the thermodynamic temperature, the molecular kinetic energy increases.
Since the kinetic energy of the molecules increases while the potential energy remains unchanged, the total internal energy of the gas increases.
How the marks are awarded
- B1 — The first mark is for correctly identifying that a constant volume implies no change in average molecular separation, and therefore no change in molecular potential energy.
- B1 — The second mark is for linking the increase in temperature directly to an increase in the average kinetic energy of the gas molecules.
- B1 — The final mark is for correctly concluding that because internal energy is the sum of kinetic and potential energies, the overall internal energy must increase.
Common mistakes
- Only mentioning the increase in kinetic energy with temperature, but failing to discuss the potential energy component, which is explicitly required by the question.
- Incorrectly stating that potential energy also increases, perhaps by confusing it with the increased energy of collisions.
- Failing to justify why potential energy is constant by not linking it to the constant volume and constant average separation of molecules.
- Stating that internal energy is just kinetic energy, which is only true for an ideal gas and is an incomplete definition for a general case.
Examiner tip: Always define internal energy as the sum of random molecular kinetic and potential energies, then analyse how each component changes based on the specifics of the process described.
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