Heat (Thermal) Energy and Heat Transfer

Convection

Convection is the movement of heat in liquids and gases. The particles in liquids and gases are not tightly packed together and so free to move around. When the particles in a liquid or gas with a lot of heat energy move to take the place of the particles with less heat energy convection is said to take place and the heat energy is transferred from the hot areas to the cold areas.

When liquids and gases are heated their atoms/molecules gain kinetic energy and move faster. As a result the particles move further apart and so take up more volume.  As more volume is taken up the density (mass per unit volume) decreases i.e. it becomes lighter in weight. Thus the area of hotter less dense liquid or gas will rise into the area of colder denser liquid or gas. The denser colder liquid or gas will then sink into the warm areas till it is warm enough to rise and so convection currents are set up and heat is transferred.

The animation below shows heat transfer by convection in a kettle.