AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Changing the temperature of a gas, held at constant volume, changes the…
Full specification wording
Particle motion in gases: Changing the temperature of a gas, held at constant volume, changes the pressure exerted by the gas.
Seal a gas in a rigid container so the volume cannot change, then warm it.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 2 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Seal a gas in a rigid container so the volume cannot change, then warm it. The mean kinetic energy of the molecules rises, so they move faster. Two things follow. Each molecule hits a wall harder, exerting a greater force during each collision, and each molecule reaches the walls more often, so collisions with the walls become more frequent. Pressure is the total force on the walls divided by their area, and both contributions have gone up, so the pressure rises. Cooling reverses both steps and the pressure falls. This is why an aerosol can carries a warning against heating: the volume is fixed, so the pressure inside climbs until the can may burst.
What earns the marks
- Stating that the mean kinetic energy, or mean speed, of the molecules increases.
- Stating that the molecules exert a greater force on the walls during each collision.
- Stating that the frequency of collisions with the walls increases.
- The conclusion that the pressure of the gas increases.
Common mistakes and how to put them right
Saying the molecules themselves expand when heated.
Claiming the pressure rises because there are more molecules, when the container is sealed.
Writing that the gas pushes harder because it is hotter, with no mention of collisions with the walls.
Describing collisions between molecules only, and never collisions with the container walls.
Exam tips
- Four marks usually means four steps: kinetic energy, force per collision, frequency of collisions, pressure. Write them in that order.
- Say collisions with the walls of the container, not just collisions, so the examiner can award the mark.
- State clearly that the volume and the number of molecules are unchanged, so the examiner knows you have not confused this with a volume change.
Try the lesson
The start of the Particle motion in gases: Changing the temperature of a gas, held at constant volume, changes the pressure exerted by the gas. lesson, made from checked teaching notes for this specification point.
Have a go first
A gas is sealed in a rigid container so its volume cannot change. The gas is cooled. What happens to the pressure of the gas?
Listen
Seal a gas in a rigid container so the volume cannot change, then warm it. The mean kinetic energy of the molecules rises, so they move faster. Two things follow. Each molecule hits a wall harder, exerting a greater force during each collision, and each molecule reaches the walls more often, so collisions with the walls become more frequent. Pressure is the total force on the walls divided by their area, and both contributions have gone up, so the pressure rises. Cooling reverses both steps and the pressure falls. This is why an aerosol can carries a warning against heating: the volume is fixed, so the pressure inside climbs until the can may burst. A strong answer earns marks for these points. stating that the mean kinetic energy, or mean speed, of the molecules increases. stating that the molecules exert a greater force on the walls during each collision. stating that the frequency of collisions with the walls increases. the conclusion that the pressure of the gas increases. Exam tip: Four marks usually means four steps: kinetic energy, force per collision, frequency of collisions, pressure. Write them in that order.
Key words
Say what each word means before you turn the card.
constant volume
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collisions with walls
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frequency of collisions
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force per collision
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See every definition
- constant volume
- A gas is held at constant volume when it is sealed in a rigid container so the volume cannot change.
- collisions with walls
- Molecules hitting the walls of the container, which happens more often when the molecules move faster.
- frequency of collisions
- Each molecule reaches the walls more often, so collisions with the walls become more frequent.
- force per collision
- Each molecule hits a wall harder, exerting a greater force during each collision.
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