OCR A Level Physics

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5.1.4 Brownian Motion

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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, IGCSE & GCSE Physics revision notes

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

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