AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Students should be able to use the particle model to explain how…
Full specification wording
Pressure in gases (physics only): Students should be able to use the particle model to explain how increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure.
Let a fixed mass of gas expand into a bigger container while keeping the temperature the same.
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
Let a fixed mass of gas expand into a bigger container while keeping the temperature the same. The number of molecules has not changed and, because the temperature is constant, neither has their mean speed or mean kinetic energy. What changes is the geometry: the molecules are now further apart and travel further between hits, so they strike the walls less often. Fewer collisions per second, each no harder than before, means a smaller total force, and that force is now shared over a larger wall area. Both effects reduce the force per unit area, so the pressure falls. Compression is the same argument in reverse, which is why a pushed syringe plunger raises the pressure inside.
What earns the marks
- Stating that the molecules are further apart in the larger volume.
- Stating that the frequency of collisions between molecules and the walls decreases.
- Stating that the force is spread over a larger surface area, or that the force per unit area falls.
- Stating that the mean speed, or mean kinetic energy, of the molecules is unchanged because the temperature is constant.
Common mistakes and how to put them right
Saying the molecules slow down as the gas expands, which the mark scheme refuses because the temperature is constant.
Linking the pressure change to a change in kinetic energy, which is not credited in a constant temperature question.
Claiming there are fewer molecules in the larger volume.
Describing collisions between molecules rather than collisions with the container walls.
Exam tips
- Say explicitly that the temperature is constant so the mean kinetic energy does not change; it protects you from the commonest lost mark.
- Keep the answer to collision frequency and area; do not mention speed or energy increasing.
- The same three sentences, reversed, answer a compression question, so learn one argument and turn it round.
Try the lesson
The start of the Pressure in gases (physics only): Students should be able to use the particle model to explain how increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure. lesson, made from checked teaching notes for this specification point.
Have a go first
A fixed mass of gas is held at a constant temperature. The volume of the container is increased. Using the particle model, why does the pressure of the gas decrease?
Listen
Let a fixed mass of gas expand into a bigger container while keeping the temperature the same. The number of molecules has not changed and, because the temperature is constant, neither has their mean speed or mean kinetic energy. What changes is the geometry: the molecules are now further apart and travel further between hits, so they strike the walls less often. Fewer collisions per second, each no harder than before, means a smaller total force, and that force is now shared over a larger wall area. Both effects reduce the force per unit area, so the pressure falls. Compression is the same argument in reverse, which is why a pushed syringe plunger raises the pressure inside. A strong answer earns marks for these points. stating that the molecules are further apart in the larger volume. stating that the frequency of collisions between molecules and the walls decreases. stating that the force is spread over a larger surface area, or that the force per unit area falls. stating that the mean speed, or mean kinetic energy, of the molecules is unchanged because the temperature is constant. Exam tip: Say explicitly that the temperature is constant so the mean kinetic energy does not change; it protects you from the commonest lost mark.
Key words
Say what each word means before you turn the card.
particle model
Try answering aloud or in your head before turning the card.
constant temperature
Try answering aloud or in your head before turning the card.
collision frequency
Try answering aloud or in your head before turning the card.
force per unit area
Try answering aloud or in your head before turning the card.
See every definition
- particle model
- A way of explaining the behaviour of a gas in terms of its molecules, including how far apart they are and how often they strike the container walls.
- constant temperature
- A condition in which the mean speed and mean kinetic energy of the molecules do not change as the gas expands.
- collision frequency
- How often the molecules strike the container walls, which falls when the molecules are further apart and travel further between hits.
- force per unit area
- The total force of the collisions shared over the wall area, which falls when the force is spread over a larger surface area.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Pressure in gases (physics only): Students should be able to use the particle model to explain how increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge