Heat (Thermal) Energy and Heat Transfer
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.
The water molecules at the bottom of the kettle gain heat energy from the flame and vibrate faster and move further apart. Their density decreases and the hotter particle rise to the top of the kettle. The colder less dense molecules move into replace the hotter particles. This continues until all the water is the same temperature.