Edexcel A-Level Physics A2 5.3.129 Absolute Zero and Molecular Kinetic Energy Questions

Practise Edexcel A-Level Physics absolute zero and molecular kinetic energy by linking temperature to molecular motion, momentum and pressure.

Exam points

estimate absolute zero by plotting gas pressure against temperature and extrapolating to zero pressure
explain pressure changes using molecular kinetic energy, momentum and collision frequency

[Maximum number: 6]

According to the Football Association (FA) rules, the football used in a professional match should have a circumference of between 68.0 cm and 70.0 cm . The pressure of the air in the football must be between 60 kPa and 110 kPa above atmospheric pressure.

The pressure inside a football decreases as the temperature of the air inside falls. The volume of the ball remains constant.

Explain why, in terms of the motion of the molecules.

Indicative content:

- As the temperature of the air decreases, the mean kinetic energy of the molecules decreases.
- Therefore the root mean square speed decreases;
equivalently, since $E_k=p^2/(2m)$ , the average momentum of the molecules decreases.
- The change of momentum of a molecule during a collision with the container wall decreases.
- The rate of collisions with the wall also decreases.
- Hence the rate of change of momentum, and therefore the force on the wall, decreases.
- The pressure exerted by the gas decreases because p=F/A.

Award according to the official levels-based structure and reasoning grids shown below.

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Edexcel A-Level Physics A2 5.3.129 Absolute Zero and Molecular Kinetic Energy Questions question 1