• 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

   

• The random motion of tiny particles in a fluid is known as?Brownian motion

Random motion of gas molecules in a container

• Brownian motion provides?evidence?that air is made of small particles
• This is because when larger particles, such as smoke particles or pollen, are observed floating in the air:
  • The larger particles move with?random motion
  • This is a result of the larger particles colliding with?smaller?particles?that are invisible to the naked eye

   

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

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