Brownian Motion (AQA A Level Physics): Revision Note

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 atoms in a gas or liquid

• The particles are said to be in random motion, which means that they have:

  • a range of speeds

  • no preferred direction of movement

• The observable particles in Brownian motion are significantly bigger than the molecules that cause the motion

  • In most cases, these are observed as smoke particles in air

  • The air particles cause the observable motion of the smoke particles

  • This means that the air particles are small and light, and the smoke particles are large and heavy

• The collisions cause larger particles to change their speed and direction randomly

  • This effect provides important evidence concerning the behaviour of molecules and atoms in a gas, especially the concept of pressure

• The atoms 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