CIE A Level Physics (9702) 2019-2021

Revision Notes

16.1.3 Brownian Motion

Brownian Motion

  • Brownian motion of particles is the phenomenon when:

Small particles (such as pollen or smoke particles) suspended in a liquid or gas are observed to move around in a random, erratic fashion

  • Brownian motion:
    • Can be observed under a microscope
    • Provides evidence for the existence of molecules in a gas or liquids


  • The observable particles in Brownian motion are significantly bigger than the molecules that cause the motion
  • The collisions cause larger particles to change their speed and directions randomly
    • This effect provides important evidence concerning the behaviour of molecules in a gas, especially the concept of pressure


  • The small molecules are able to affect the larger particles in this way because:
    • They are travelling at a speed much higher than the larger particles
    • They have a lot of momentum, which they transfer to the larger particles when they collide

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