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Edexcel GCSE Physics · 1PH0
Edexcel 1PH0 · 14.12 Check the specification (PDF) (opens in a new tab)
A gas can be modelled as a very large number of tiny particles with spaces between them. These particles are in constant random motion. They travel through the container in many different directions and repeatedly collide with one another and with its walls.
The particles are far too small to follow individually. Instead, the particle model connects their combined microscopic behaviour to a measurable macroscopic effect: gas pressure.
Consider one particle approaching a container wall. When it collides, its motion changes as it rebounds. The wall must exert a force on the particle to produce this change; the particle exerts a force on the wall in return.
A single collision produces only a tiny, brief force. However, an enormous number of particles strike each part of the wall. Their repeated impacts combine to produce a continuous force on the wall. Because particles move randomly in every direction, they collide with every inner surface—not just the bottom of the container.
Randomly moving gas particles collide with every inner surface. Their many impacts exert forces on the walls, producing pressure.
Pressure describes how concentrated a force is over a surface. For a chosen area of wall,
where is pressure, is the force acting normally—at right angles—to that surface, and is its area. Pressure is measured in pascals (Pa), with .
For a gas, the force in this relationship is the combined force exerted by particle collisions on that area of wall. Gas pressure is therefore not a substance, a single force or simply the weight of the gas. It is the force per unit area caused by countless particle impacts.
The complete explanation links two scales:
This same idea will later allow changes in pressure to be explained by changes in the motion of the particles or in how frequently they collide with the walls.
Pressure is force normal to a surface per unit area. Its unit is the pascal: .
Essential chain: particle motion → wall collisions → force on the wall → pressure.
Build explanations as a causal chain: random particle motion → collisions with the container wall → forces on the wall → force per unit area, which is pressure.
Do not say that gas pressure is caused mainly by the gas's weight or by particles simply being present. Refer to collisions with the container walls.
If a question asks about one face of a container, use the force on that face and the area of that face—not the total area of the container.
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Last updated
Pressure
The force acting normally on each unit area of a surface.
Gas particle
The tiny atoms or molecules represented in the particle model of a gas.
Collision
A brief interaction in which a moving particle strikes another object and changes its motion.
Random motion
Motion with continually changing, unpredictable directions because particles undergo collisions.
Put your knowledge into practice — try past paper questions for Physics
Pressure
The force acting normally on each unit area of a surface.
Gas particle
The tiny atoms or molecules represented in the particle model of a gas.
Collision
A brief interaction in which a moving particle strikes another object and changes its motion.
Random motion
Motion with continually changing, unpredictable directions because particles undergo collisions.