Transference of Heat

There are three distinct ways in which thermal energy is transferred from one place to another. They are called conduction, convection and radiation.

Conduction is the only means of heat transfer in solids. If a tea spoon is placed in a cup of hot tea, the other end of the spoon soon becomes hot as heat is conducted along the spoon. Similarly saucepan placed on fire soon becomes hot all over even though heat is supplied only at the bottom: heat is conducted along the material of the saucepan to all parts of its body.

Conduction can also take place in fluids, but the chief means of heat transfer in liquids and gases is by convection. In conduction, we may say that heat is flowing, trough the heated body as one part of the body in at a higher temperature than another.

There is no movement of the heated part to another part: rather heat energy is simply passed on from the hotter to the cooler parts. On the other hand, in convection, heat energy is carried from one place to the other by the movement of the heated substance. The a vessel containing a liquid is heated at the bottom, a current of hot liquid moves upwards while a current of cold liquid moves downwards to take its place. Convection currents are soon set up with the liquid and the heat spreads to all parts of the liquid. The same process takes place when a gas is heated.

For both conduction and convection, a material medium is needed within which to transfer of heat takes place. Radiation is the mode by which heat reaches us from the sun after having travelled across about 150 million of kilometers of empty space. If you stand in frond of a brightly glowing fire or near a burning bush, you feel the warmth coming from the fire.

You must also have noticed that as soon as you stand away from the direct rise of the sun on a bright day you do not feel the burning heat. If clouds pass across the sky to obscure the overhead sun temporarily instant relief from the sun's heat is also felt. Radiant heat travels in much the same way as light and they are in fact different forms of the same phenomenon. When the readings of the thermometers becomes steady after steam has been passed trough for some time, it will be found that there is a gradual fall of temperature as we move further from the heated end.