Particle Model of Gases (CIE IGCSE Physics)

• In the particle model, 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 of particles with the walls of the container
• When the particles travel faster (e.g. at a higher temperature), they collide with walls more frequently
  • This means the gas exerts a greater pressure

Random motion of gas molecules in a container. Collisions with container walls creates pressure in the particle model

Extended tier only

• Gases fill their container
• The pressure is defined as the force per unit area

• Where:
  • = pressure in pascals Pa
  •  = force in newtons N
  • = area in metres-squared m²
• This equation and the particle model can be used to explain how particles exert pressure
• As the gas particles move about randomly they collide with the walls of their containers
• These collisions produce force at right angles to the wall of the gas container (or any surface)
  • Pressure is force per unit area, so the force of these collisions exerts a pressure
• When the particles move faster, they have more frequent collisions with the container walls and these exert a greater force
  
  • If the force exerted per unit area is greater and more frequent, the pressure exerted is also greater

Gas molecules bouncing off the walls of a container exert a force perpendicular to the surface