# 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 , 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

## 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 , from the hot end to the cold end.

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 . Metals are good conductors.
• A substance that does not transfer energy easily from the hot part to the cold part is called an . 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.