AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
A gas can be compressed or expanded by pressure changes. The pressure…
Full specification wording
Pressure in gases (physics only): A gas can be compressed or expanded by pressure changes. The pressure produces a net force at right angles to the wall of the gas container (or any surface).
When a molecule hits a wall it bounces off, and the change in its motion is perpendicular to the surface.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When a molecule hits a wall it bounces off, and the change in its motion is perpendicular to the surface. 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.
What earns the marks
- Stating that the force produced by the gas acts at right angles, or normal, to the surface.
- Stating that this force is the result of molecules colliding with and rebounding from the wall.
- Stating that pressure is the force divided by the area on which it acts, in Pa or N/m².
- Stating that a gas is compressed into a smaller volume when the pressure on it is increased.
Common mistakes and how to put them right
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.
Exam 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.
Try the lesson
The start of the Pressure in gases (physics only): A gas can be compressed or expanded by pressure changes. The pressure produces a net force at right angles to the wall of the gas container (or any surface). lesson, made from checked teaching notes for this specification point.
Have a go first
In which direction does the pressure of a gas produce a net force on the walls of its container?
Listen
When a molecule hits a wall it bounces off, and the change in its motion is perpendicular to the surface. 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. A strong answer earns marks for these points. stating that the force produced by the gas acts at right angles, or normal, to the surface. stating that this force is the result of molecules colliding with and rebounding from the wall. stating that pressure is the force divided by the area on which it acts, in Pa or N/m². stating that a gas is compressed into a smaller volume when the pressure on it is increased. Exam tip: On a diagram, draw the force arrows perpendicular to each surface, including the sides and the top.
Key words
Say what each word means before you turn the card.
force at right angles
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pascal
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compression and expansion
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force per unit area
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See every definition
- force at right angles
- The net force a gas produces acts at right angles to the wall of its container or any surface, whatever the wall's orientation.
- pascal
- The unit in which pressure is measured, where 1 Pa is 1 N/m².
- compression and expansion
- A gas is compressed into a smaller volume when the pressure on it is increased, and expands when the pressure is lowered.
- force per unit area
- Pressure is the force divided by the area it acts on.
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