Thermal equilibrium

If there is a difference in temperature between two objects, energy is transferred from the hotter object to the cooler one. This will continue until both objects are at the same temperature. When they are at the same temperature, we say that they are in thermal equilibrium, and there is no overall transfer of energy any more between the two objects.

Energy can be transferred from a hot object to a cooler one by:

  • conduction (if they are touching each other)
  • convection
  • radiation

Conduction

When a substance is heated, its particles gain internal energy and move more vigorously. The particles bump into nearby particles and make them vibrate more. This passes internal energy through the substance by conduction, from the hot end to the cold end.

Heat from a bunsen burner is applied to a metal rod

A Bunsen burner flame heats the metal rod

This is how the handle of a metal spoon soon gets hot when the spoon is put into a hot drink.

  • A substance that transfers energy easily from the hot part to the cold part is called a conductor. Metals are good conductors.
  • A substance that does not transfer energy easily from the hot part to the cold part is called an insulator. Air and plastics are insulators.

Convection

The particles in liquids and gases can move from place to place. Convection happens when particles with a lot of thermal energy in a liquid or gas move, and take the place of particles with less thermal energy. Thermal energy is transferred from hot places to cold places by convection.

As the hot air above a radiator rises it pushes cooler air away from it. The cooler air eventually circulates back round to the radiator where it gets heated and the cycle continues.As the hot air above a radiator rises it pushes cooler air away from it. The cooler air eventually circulates back round to the radiator where it gets heated and the cycle continues.

Radiation

All objects transfer energy to their surroundings by infrared radiation. The hotter an object is, the more infrared radiation it gives off.

No particles are involved in radiation, unlike conduction. This means that energy transfer by radiation can work when objects are not touching, even in space:

  • radiation is why we are warmed by the Sun, even though it is millions of kilometres away in space
  • infrared cameras give images even in the dark, because they are detecting infrared light, not visible light
Thermogram of a pan on a stove showing the hottest parts in white, yellow or red, and the coldest parts in purple and black.
In this thermogram of a pan on a stove, the hottest parts are coloured white, yellow or red; the coldest parts are coloured purple or black

The differences between conduction, convection and radiation