Temperature & Pressure (CIE IGCSE Physics)

• Molecules in a gas are in constant random motion at high speeds
• Random motion means that the molecules are travelling in no specific path and undergo sudden changes in their motion if they collide:
  • With the walls of its container
  • With other molecules
• Pressure in a gas is caused by the collisions with the surface (walls) of the container

Random motion of gas molecules in a container

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Gas molecules hit the sides of the container and exert a force, which creates pressure.

• A feature of gases is that they fill their container
• The pressure is defined as the force per unit area

• As the gas particles move about randomly they collide with the walls of their containers
• These collisions produce a net force at right angles to the wall of the gas container (or any surface)
• Therefore, a gas at high pressure has more frequent collisions with the container walls and a greater force
  • Hence the higher the pressure, the higher the force exerted per unit area

Gas molecules bouncing off the walls of a container

• It is possible to experience this force by closing the mouth and forcing air into the cheeks
• The strain on the cheeks is due to the force of the gas particles pushing at right angles to the cheeks