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 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 atoms in a gas or liquids

• The particles are said to be in random motion, this 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 were observed as smoke particles in air

  • The air particles cause the observable motion of the smoke particles that we see

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

• The collisions cause larger particles to change their speed and directions 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