Convection

Convection is a mode of heat transfer by actual motion of matter. It is, therefore possible only in fluids. Convection can be natural or forced. In natural convection, gravity plays an important part. When a fluid is heated from below, the hot part expands and, therefore, becomes less dense. Because of buoyancy, it rises and the upper colder part replaces it. This again gets heated, rises up and is replaced by the colder part of the fluid. The process goes on. This mode of heat transfer is evidently different from conduction where energy transfer takes place locally due to intermolecular collisions. Convection involves bulk transport of different parts of the fluid. In a liquid, you can actually see it, for example, by colouring the liquid with potassium permaganate crystals at the bottom of the container.

In forced convection, material is forced to move by a mump or by some other physical means. Heat transfer in a liquid by brisk stirring or in air by a blower or a fan is example of forced convection