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Transfer of Heat

If one end of a long metal rod is heated, the other end also becomes hot after some time. If a liquid in a vessel is heated by placing a burner below the vessel, we find that the top surface of the liquid also becomes warm after some time. We receive the heat from the sun directly. These three examples suggest three different methods by which heat can be transferred from one place to another. The process of heat transfer in the first example is called conduction. In the second example heat is transferred by convection and in the third by radiation. We will study these processes in more detail.


When two bodies or two different parts of the same body have different temperatures and are brought into thermal contact, they exchange heat energy and tend to equalize the temperature.

The mechanism of heat transfer can be understood in terms of the thermal motion of the atoms and molecules in them. We know that temperature is a manifestation of the thermal motion of molecules and is related to the average kinetic energy per molecule. Suppose two adjacent parts A and B of a body are at different temperatures TA and TB with TA > TB. This means that the average energy of a molecule in part A is greater than that in part B. When the more energetic molecules in part A collide with the less energetic molecules in part B, they transfer some heat energy to the latter. The parts A and B exchange energy by collision between atoms or molecules.

The net effect of a large number of such collisions is to equalize their energies on an average. Energy is thus transferred from A to B via collisions till TA becomes equal to TB. The converse of this is also true. If two parts of a body are to be kept at different temperatures, a steady flow of heat energy must be maintained. Or, if two parts of a body are at different temperatures, there is a steady flow of heat energy from one part to the other. This process of transfer of heat from one place to the other via collisions is called conduction.


Fluids (liquids and gases) are heated mainly by a process called convection in which buoyancy and gravity play an important role. When a fluid is heated from below, the hot portion at the bottom expands. Since the mass of a given portion remains unchanged, its density decreases. According to Archimedes' principle this lighter portion will rise upwards. The denser fluid from above will take its place by moving downwards. Thus a convection current is set up in the fluid. This movement of the liquid can be seen by colouring it, say, with potassium permanganate crystals at the bottom of the fluid.

The example of natural convection is the steady surface wind on the earth blowing in from north-east towards the equator, the so called trade wind. A reasonable explanation is as follows: the equatorial and polar regions of the earth receive unequal solar heat. Air at the earth's surface near the equator is hot while the air in the upper atmosphere of the poles is cool. In the absence of any other factor, a convection current would be set up, with the air at the equatorial surface rising and moving out towards the poles, descending and streaming in towards the equator. The rotation of the earth, however, modifies this convection current. Because of this, air close to the equator has an eastward speed of 1600 km/h, while it is zero close to the poles. As a result, the air descends not at the poles but at 30° N (North) latitude and returns to the equator. This is called trade wind.


All bodies emit heat radiation from their surfaces by virtue of their temperature. This radiation is called radiant energy. The heat that we receive from the sun is transferred to us by a process which, unlike conduction or convection, does not require the help of a medium in the intervening space. It is well-known that most of the space between the sun and the earth is a vacuum and that there is almost no trace of matter in that space and yet we receive heat from the sun.

The heat travels in straight lines as does light. This is evident from the fact that the shape of a tree coincides with its shadow. Because heat and light travel with the same speed, they are both simultaneously cut off in an eclipse of the sun. Since light is propagated by waves of some kind, it is reasonable to expect that heat is also propagated by similar waves. Thus, heat is propagated by wave motion and we say it is radiated. We shall use the term radiant energy or thermal radiation to denote heat radiated by an object.

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