CIE A Level Physics (9702) 2019-2021

Revision Notes

16.1.4 Kinetic Theory of Gases

Assumptions of the Kinetic Theory of Gases

  • Gases consist of atoms or molecules randomly moving around at high speeds
  • The kinetic theory of gases models the thermodynamic behaviour of gases by linking the microscopic properties of particles (mass and speed) to macroscopic properties of particles (pressure and volume)


  • The theory is based on a set of the following assumptions:
    • Molecules of gas behave as identical, hard, perfectly elastic spheres
    • The volume of the molecules is negligible compared to the volume of the container
    • The time of a collision is negligible compared to the time between collisions
    • There are no forces of attraction or repulsion between the molecules
    • The molecules are in continuous random motion
  • The number of molecules of gas in a container is very large, therefore the average behaviour (eg. speed) is usually considered

Exam Tip

Make sure to memorise all the assumptions for your exams, as it is a common exam question to be asked to recall them.

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