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A-Level Physics October/November 2024 Q4(b): Explain how molecular movement causes the pressure exerted by a gas.
A-Level Physics · Paper 9702/42 · October/November 2024 · Question 4(b) · [3 marks]
Explain how molecular movement causes the pressure exerted by a gas.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
Gas molecules are in continuous, random motion. They collide with the walls of the container.
During a collision with the wall, a molecule rebounds, causing its velocity to reverse and therefore its momentum to change.
By Newton's second law, a rate of change of momentum is equal to a force. The wall exerts a force on the molecule to change its momentum. By Newton's third law, the molecule exerts an equal and opposite force on the wall. The pressure exerted by the gas is the total force from all these collisions acting on a unit area of the wall.
How the marks are awarded
- B1 — The first mark is awarded for stating that the gas molecules collide with the walls of the container.
- B1 — The second mark is for correctly identifying that the momentum of a molecule changes as a result of the collision with the wall.
- B1 — The final mark is for linking the change in momentum to a force on the wall (via Newton's laws) and then relating this force over an area to the concept of pressure.
Common mistakes
- Stating that molecules collide with each other, but not explicitly mentioning collisions with the container walls, which are the cause of pressure on the container.
- Mentioning collisions with the walls but failing to link this to a change in momentum.
- Jumping from 'collisions' to 'pressure' without the crucial intermediate steps of explaining the change in momentum and the resulting force on the wall.
- Confusing energy with momentum, for example by stating that kinetic energy changes during the (assumed elastic) collision.
Examiner tip: For 'explain' questions about the kinetic theory of gases, always build your answer as a logical sequence, starting from the microscopic behaviour of molecules and linking it step-by-step to the macroscopic property in question.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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