Kinetic Theory of Gases Model
Properties of Matter

Modelling for Brownian motion

Practical Activity for 14-16 PRACTICAL PHYISCS


Apparatus and Materials

  • Metal tray lined with cork mat
  • Marbles, coloured, 20 - 24, about 1 cm diameter
  • Marbles, large, several, about 2 cm diameter
  • Three-dimensional kinetic model kit
  • Electric motor, fractional horsepower

  • Power supply, low-voltage, variable
  • Retort stand, boss, and clamp
  • Polystyrene, expanded, piece of

Health & Safety and Technical Notes

See apparatus entry for:

Electric motor, fractional horsepower

Beware of marbles on the floor.

Read our standard health & safety guidance

If a different vibrator is used with the original tube and rubber-sheet base from the 3-dimensional kinetic model kit, the base can soon be damaged. However, the life of the base can be prolonged considerably by sticking a small disc of aluminium foil on the rubber.

Alternative methods Adapt this experiment by adding a small piece of expanded polystyrene and following its path:

Kinetic theory model for a gas

Closely observe a suspension containing mm scale polystyrene spheres, projected onto a screen using a visualiser with a data projector.


  1. Add to the tray of marbles one or two larger marbles used in this experiment, and compare their motion with that of the smaller ones:

    Kinetic theory: two-dimensional model

  2. Alternatively, add a small piece of polystyrene to the 3D model, as used in this experiment:

    Kinetic theory model for a gas

Teaching Notes

  • The random path of a larger marble buffeted by the smaller ones suggests what you might see if you looked at, say, bits of ash in the air.
  • Similarly, the movement of the small piece of expanded polystyrene in the tube used for the experiment...

    Kinetic theory model for a gas

    ...shows a similar effect. However, here buoyancy and gravity play more noticeable parts than they would for ash in air. Nonetheless, taking the opportunity to try different sized scraps of expanded polystyrene, the students can predict the ways that smaller and larger fragments of ash will be seen to move in air, and even to realise why Brownian motion is only observed with microscopically small particles.

This experiment was safety-tested in December 2004

  • This video shows how Brownian motion can be observed in a suspension containing micrometre diameter polystyrene spheres. Using a microscope and video camera, students can observe the motion of the polystyrene spheres. The video also shows how Brownian motion can be simulated using a vibrating loudspeaker, table tennis balls and a small balloon.

  • This video shows footage of the movement of particles by Brownian motion and can be used in the classroom with your students:

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