Brownian Motion (OCR 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 constant, random motion

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

• Brownian motion:

  • Can be observed in a smoke cell under a microscope

  • Provides evidence for the existence of molecules in a gas or liquids

• 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 practically 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 in a gas, especially the concept of pressure

• The smaller 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