Heat is a form of energy. It makes a substance hotter. We cannot see heat. We can only feel it by the effect of hotness it produces. We can define heat as energy in transit.
In solids, heat passes from one point to another through conduction. In Liquids and gases, heat transfer takes place by convection. Heat transfer takes place by the process of radiation when there are no particles of any kind which can move and transfer heat. So, in an empty space or vacuum heat is transferred by radiation. We shall now study heat transfer by conduction, convection, and radiation in detail.
If we heat one end of a metal bar by keeping it over a gas burner, we find that its other end also gets hot after some time. So heat is transferred from hot end of the bar to its cold end. In this case of heat transfer, there is no movement of molecules (or particles) of the material of metal bar as shown below in the figure.
“Conduction is the transfer of heat from the hotter part of the material to its colder part without the actual movement of the particles. What I want to say here is that in the process of conduction there is no net movement of the particles of the body. Heat passes through solids by conduction only.
Thermal conduction plays an important role in our daily life. Some of the practical applications of thermal conduction include:-
(a) Cooking utensils are provided with wooden handles.
(b) Eskimos make double walled houses of the blocks of ice.
Learn more about heat conduction and thermal conductivity.
Convection involves the bodily movement of the heated molecules. Convection is the process in which heat is transmitted from one place to other by the actual movement of heated particles. It is prominent in the case of liquids and gases. Land and sea breezes are formed due to convection. It also plays an important part in ventilation, electric lamps, and heating of buildings by hot water circulation. The figure below shows the transfer of heat by convection when water in a cooking pot is heated.From this figure, you can clearly see convection current. So, when water is heated in a cooking pot then its heated molecules become lighter and moves upwards. As heated molecules of water move upwards, the denser and colder molecules of water begin to move downwards. So warm water rises and colder molecules of water descend. This sets up convection current as shown in the above figure.
Radiation is the process in which heat is transmitted one place to other directly without intervening medium. We get radiation of sun without an intervening medium. Heat radiations can pass through a vacuum. Their properties are similar to light radiations. Heat radiations also form a part of electromagnetic spectrum. Transfer of heat by radiation does not need any material medium. Energy from the sun comes to us by radiation.