Edexcel IGCSE Physics: Double Science

Revision Notes

5.2.1 Kinetic Theory

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Kinetic Theory of Gases

  • 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 particles, IGCSE & GCSE Chemistry revision notes

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

Pressure equation, downloadable AS & A Level Physics revision notes

  • 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

molecules-&-force, IGCSE & GCSE Physics revision notes

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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