AQA A Level Physics

Revision Notes

6.6.4 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, IGCSE & GCSE Physics revision notes

Brownian motion is the erratic motion of small particles when observed through a microscope

  • 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

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top