Pressure in gases (physics only) — AQA GCSE Physics
Test yourself on Pressure in gases (physics only) with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Pressure in gases (physics only) explained
When a molecule hits a wall it bounces off, and the change in its motion is perpendicular to the surface.
Read the full explanation
Averaged over the huge number of impacts, the sideways contributions cancel and what is left is a net force acting at right angles to the wall, whatever the wall's orientation. Pressure is that force divided by the area it acts on, measured in pascals, where 1 Pa is 1 N/m². Because the force is always normal to the surface, a gas pushes outwards on every part of a container equally. Raise the outside pressure above the inside pressure and the gas is compressed into a smaller volume; lower it and the gas expands, as a balloon does when it is carried up a mountain.
Your focus
- Explain why the net force a gas exerts on a container wall acts at right angles to that wall.
- Calculate the pressure produced by a stated force acting on a stated area, giving the unit.
- Describe what happens to the volume of a gas when the pressure acting on it is increased at constant temperature.
Pressure in gases (physics only) exam tips
Marking Points
- one mark for stating that the force produced by the gas acts at right angles, or normal, to the surface
- one mark for stating that this force is the result of molecules colliding with and rebounding from the wall
- one mark for stating that pressure is the force divided by the area on which it acts, in Pa or N/m²
- one mark for stating that a gas is compressed into a smaller volume when the pressure on it is increased
Examiner Tips
- 💡On a diagram, draw the force arrows perpendicular to each surface, including the sides and the top.
- 💡Quote the unit as Pa, and remember that 1 Pa is 1 N/m² if a question mixes units.
- 💡If the question says net force, make it clear the sideways effects of individual collisions cancel out.
Common Mistakes
- drawing or describing the force as acting along the wall rather than at right angles to it
- saying a gas presses only downwards because of its weight
- confusing pressure measured in pascals with force measured in newtons
- assuming only the molecules nearest the wall contribute, so a bigger container has a lower pressure automatically