We have defined heat as energy that flows. As far as two objects or systems are in thermal contact heat will transfer from the hotter to a cooler region. Heat energy can be transferred from one place or body to another in three different ways. These are: conduction, convection, and radiation.

Heat transfer: Conduction

If you hold one end of a copper rod and place the other end in a flame (Figure 3), the end you are holding gets hotter and hotter, even though it is not in direct contact with the flame. When we heat one end of an object (say a metal bar) the molecules there vibrate faster and faster. As a result they collide with their slower moving neighbors. During the collision they transfer some of their kinetic energy to molecules having less kinetic energy. These in turn transfer some of their energy by collision to their neighboring molecules. In this way energy is transferred from a high temperature region to a lower temperature region. A process of heat transfer in which energy is passed Heat transfer by conduction from molecule to molecule through a material by successive molecular collision is called conduction.

Although heat is transmitted through a material medium, there is no transfer of the particles of the medium in the process of conduction.

Heat transfer: Convection

Suppose you want to boil a tea using a tea kettle, and place the kettle on a store (Figure 5). When the water is heated it expands and hence its density decreases. This causes the hot water to rise up, because it is surrounded by more dense cold water. At the same time the colder water moves down toward the source and takes the place of the displaced hot water. In this way the whole water in the kettle will get heat from the source. This means of heat transfer is called convection.

Heat transfer: Radiation

Conduction and convection require the presence of matter as a medium to carry the heat from the hotter to the colder region. But a third type of heat transfer occurs without any medium at all. This form of energy transfer through vacuum by use of electromagnetic waves is referred to as radiation. This is how energy reaches us from the sun. It is proved experimentally that all bodies at temperature above absolute zero are continuously radiating energy. When the wave strikes an object, it gets hotter.