Convection is defined as the 'movement of molecules within fluids'. This means that it does not occur within solids but only within liquids and gases. This is because convection is mainly the bulk movement and it carries the thermal energy with them. However, solids transfer heat through vibrations and no bulk movement of the atoms/molecules occur.

Convection works differently for both liquids and gases.

Convection In Liquids
for example, when a pot of water is put to boil, this is how convection occurs. When the water at the bottom of the flask is heated, it expands. This expanded water is less dense than the water that has not been heated. As a result, it rises. The unheated or cooler body of water sinks, as a result of being denser. The movement of water as a result of the density changes is called the convection current.

Convection in Gases

if air is heated, it also gets denser than the unheated air. As a result, it rises. The cooler air sinks and replaces the warmer, less dense air. It replaces the less dense air. This is the convection current occuring in gases.

Application of convection.

Electric kettles are prime examples of convection. They have something called an electric coil. These coils are always placed at the bottom of the kettle and renders itself helpful by transferring the heat to the water via convection.

This is how it works: power is switched on, and the electrical energy heats up the heating coil and the water is heated up. The less dense water rises and the cooler water sinks.