1. Brownian motion


Brownian Motion: Evidence for a theory about the nature of gases and liquids

We're constantly surround by air molecules which are bumping into us, moving in random directions. In a liquid, the molecules or atoms are moving around each other, again, randomly and in a solid they're held in position and can only vibrate.

So how do we know that these gas or liquid particles are moving randomly? How do we know that they're particles at all?

Well, one experiment which adds evidence to support this 'kinetic' theory is called 'Brownian Motion'. To set up this experiment, we need:

  • a glass 'cell'
  • smoke from a glowing splint
  • a microscope with a light to illuminate the smoke particles

The smoke is trapped in the glass cell and the microscope is used to observe the motion of the smoke particles.

The tiny smoke particles are observed to move around in a jerky fashion - called a 'random walk'. The random movement of these smoke particles is evidence that, in this case, air molecules are randomly bombarding the smoke particles.

Sometimes more air molecules collide with the left side of a smoke particle than on the right. This means that the smoke particle gets an overall push (a resultant force) to the right, and so moves to the right.

The next moment, there may be more collisions of air molecules on another side of the smoke particle and this would make the smoke particle change direction.

This random bombardment causes the smoke particle's 'random walk', which is evidence of the kinetic theory of matter - that gases and liquids are made up of tiny invisible particles that move randomly and collide with themselves and other objects.

You can also do this experiment with a liquid by sprinkling pollen grains onto water and then viewing the 'random walk' of the pollen grains with a microscope. 'Brownian Motion' is named after the scientist Robert Brown who carried out the experiment in this way.

GCSE Physics Keywords: Gas, Liquid, Particles, Motion, Collisions, Random, Brownian motion

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