Pressure in Gases
Learn Pressure in Gases for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) physics topic: gas pressure is caused by particles colliding with the container walls, and for a fixed mass at constant temperature pressure and volume are inversely related.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
A gas exerts a net outward pressure at right angles to the walls of its container. This is a Triple-only topic.
The pressure arises from molecular collisions with the walls. Increasing the temperature of a gas held at constant volume increases the pressure, because the molecules move faster, so they collide with the walls more frequently and with greater force.
Work is done when a gas is compressed or expanded, and doing work on a gas increases its internal energy and can raise its temperature. This is why a bicycle pump becomes warm when used — work is being done on the air inside it.
Revision notes
Pressure and the walls
Gas molecules colliding with the container walls exert a net outward pressure.
The force acts at right angles to the wall surface. Pressure is force per unit area, so it depends on both the total force and the area over which it acts.
Temperature and pressure at constant volume
Increasing the temperature increases the average kinetic energy of the molecules, so they move faster.
They therefore collide with the walls more frequently, and each collision exerts a greater force. Both effects increase the pressure.
Doing work on a gas
Work is done when a gas is compressed or expanded.
Doing work on a gas transfers energy to it, increasing its internal energy and raising its temperature. A bicycle pump becomes warm for this reason, not simply through friction.
Key points
- Gas pressure acts at right angles to the walls.
- Pressure arises from molecular collisions.
- Higher temperature at constant volume raises pressure.
- Faster molecules collide more frequently.
- Each collision exerts a greater force.
- Doing work on a gas increases its internal energy.
Worked examples
Example 1
Explain why the pressure of a gas increases when its temperature is raised at constant volume. [3 marks]
Model answer
The average kinetic energy of the molecules increases, so they move faster (1 mark). They collide with the walls of the container more frequently (1 mark), and each collision exerts a greater force, so the total pressure increases (1 mark).
Example 2
Explain why a bicycle pump becomes warm when used. [2 marks]
Model answer
Work is done on the gas as it is compressed (1 mark), which transfers energy to the gas, increasing its internal energy and raising its temperature (1 mark).
Example 3
State the direction in which gas pressure acts on a container wall. [1 mark]
Model answer
At right angles to the surface of the wall (1 mark).
Common mistakes
Giving only one reason for the pressure increase.
Both more frequent collisions AND greater force per collision are needed.
Saying the pump warms only because of friction.
Work done on the gas increases its internal energy, which is the main effect.
Saying pressure acts in one direction.
It acts outwards at right angles to every wall surface.
Forgetting the constant volume condition.
The relationship described applies at constant volume.
Exam tips
- Give both reasons when explaining a pressure increase.
- State the constant volume condition explicitly.
- Link work done to internal energy and temperature.
- Remember this is a Triple-only topic.
Key terms
- Pressure
- Force per unit area on the container walls.
- Constant volume
- A fixed container size, needed for this relationship.
- Work done
- Energy transferred by a force, such as in compression.
- Internal energy
- The total energy of the particles in the gas.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.