There are three ways in which heat energy may get transferred from one place to another. These are conduction, convection and radiation.
Heat is transmitted through the solids by the process of conduction only. When one end of the solid is heated, the atoms or molecules of the solid at the hotter end becomes more strongly agitated and start vibrating with greater amplitude. The disturbance is transferred to the neighboring molecules.
(i) The houses of Eskimos are made up of double walled blocks of ice. Air enclosed in between the double walls prevents transmission of heat from the house to the coldest surroundings.
(ii) Birds often swell their feathers in winter to enclose air between their body and the feathers. Air prevents the loss of heat from the body of the bird to the cold surroundings.
(iii) Ice is packed in gunny bags or sawdust because, air trapped in the saw dust prevents the transfer of heat from the surroundings to the ice. Hence ice does not melt.
It is a phenomenon of transfer of heat in a fluid with the actual movement of the particles of the fluid. When a fluid is heated, the hot part expands and becomes less dense. It rises and upper colder part replaces it. This again gets heated, rises up replaced by the colder part of the fluid. This process goes on. This mode of heat transfer is different from conduction where energy transfer takes place without the actual movement of the molecules.
Application: It plays an important role in ventilation and in heating and cooling system of the houses.
It is the phenomenon of transfer of heat without any material medium. Such a process of heat transfer in which no material medium takes part is known as radiation.