Lesson 3, Topic 3
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Brownian motion

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What is Brownian motion?

  • Brownian motion is the random movement of small visible particles in a suspension

  • Caused by the unequal random bombardment of molecules of liquid or gas on the visible particles


  • The zigzag movement of pollen grains in still water

  • Random movement of dust particles in still air

Brownian motion as evidence for the particle theory

  • Brownian motion is evidence for particles in matter. It is the particles in the liquid moving and colliding with the pollen grain¬† that makes them move.¬†

  • Smoke particles move in the same way because they are being hit by particles in air