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Thermal Physics

The Kinetic Theory of Matter

The zeroth law of thermodynamics gives us a macroscopic (large scale) definition of the concept of the temperature of a body. The kinetic theory of matter can be used to give a microscopic definition of temperature.

The basic ideas of the kinetic theory are

  i) all matter consists of particles (atoms or molecules) in motion
  ii) as the temperature increases, the average speed of the movement increases.

According to this theory, the temperature of a body is a measure of the average kinetic energy of it particles.

This theory can also be used to explain the differences between the different states (or phases) of matter.

Solids

The particles vibrate around fixed positions.
They are close together and so attract each other strongly. This is why solids maintain their shape.
Liquids

The particles are still relatively close together but now have enough energy to "change places" with each other. This is why liquids can flow.
Gases

Average distance between particles typically 10 times greater than in solids and liquids. The particles now move freely at random, occupying all the space available to them.

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© David Hoult 2008