Transmission of heat

Modes of transmission

There are three distinct methods by which heat can be transferred or propagated: (1) conduction, (2) convection and (3) radiation.

Conduction is the mode of transmission of heat in which heat energy travels from one particle to another in the direction of decreasing temperature without any bodily movement of the material particles from their normal positions. Maxwell defined it as “the flow of heat through an unequally heated body from places of higher to places of lower temperature.” The power of transmitting heat in this way is possessed by all substances, solids, liquids and gases, to a varying degree.

Convection

Convection is the mode of heat transference in which the material particles conveying the heat are carried from one place to another place until the whole mass of the substance becomes uniformly heated. Maxwell defined it as “the motion of the hot body itself carrying its heat with it.” The power of transmitting heat in this manner by the actual motion of the heated particles is possessed by liquids and gases.

Thus the transference of heat either by conduction or convection requires essentially a material medium (intervening medium).

But there is yet another mode of transference of heat in which no material medium is needed. We get heat from the sun on earth, although there is no continuous material medium between the sun and the earth. This mode of transmission of heat is called radiation. Radiation is that mode of transmission of heat in which heat energy travels from the source of heat to its recipient without any material medium taking part in it. Maxwell defined it as follows: “In radiation, the hotter body loses heat and the colder body receives heat by means of a process occurring in some intervening medium which does not it self thereby become hot.” The processes of conduction and convection are necessarily slow processes due to the action of the intervening medium, while radiation travels with the enormous velocity of light, that is, with 3x10¹º cm./sec.

The present chapter deals with conduction and convection and radiation is discusses in the last chapter.

Conduction

According to the kinetic theory, the ultimate particles or molecules of a body are in a state of incessant motion to and fro. When a body is heated, the molecules there vibrate more vigorously and this increased agitation (i.e. the increased heat energy) is passed on by collision from particle to particle. Consider the mechanism of conduction of heat to the other end of a metal bar heated at one end. Here heat is first communicated to particles of the bar in contact with source of heat. These particles, as a result, vibrate more vigorously about their respective mean positions of rest and transfer the energy to adjacent particles by collision; and these in their turn to the next layer of particles and so on. The energy of vibration so transferred from layer to layer is nothing but the heat transferred by conduction. Some substances conduct heat better than others. There is an approximate rule that good electrical conductors are also good thermal conductors and so metals are generally good conductors, while substances like glass, mica, ebonite, felt, etc. are all bad conductors. Air and other gases are bad conductors of heat. The good thermal conductivity of metals has been partly ascribed to the presence of free electrons in them. The free electrons behave as mobile molecules of a perfect gas and as such help in the rapid transference of heat from one part of the metal to the other.

Convection

Convection molecule

If a hot piece of metal is suspended in air, the air immediately around it is heated by conduction and radiation. This causes an expansion and a decrease in density so that air rises and cold air flows in from below and from the sides to take its place. This continuous until the metal is cooled to room temperature. Such air movement is termed natural convection. If a streaming is maintained by some external agency, such as a fan or a pump, the process is called forced convection.

As detailed study of the phenomenon of convection is beyond the scope of this book, we shall illustrate here only some of its practical applications in science and technology and some natural phenomena where convection plays an important role.

Application in science:

Newton’s law of cooling

When a liquid is cooled by a drought of air, the rate of loss of heat is proportional to the difference between the temperature the liquid and that of the surroundings.

Natural phenomena:

Winds

Winds are caused by convection currents set up in the atmosphere as a result of unequal heating of the earth by the sun.

Land and sea breezes

Convection currents account for land and sea breezes.

Sea breeze

Land having a lower a specific heat and a greater absorbing power than water becomes more heated than the sea by the sun’s heat during the day time. Consequently in the evening the air above the land becomes more heated and rises up and cooler air from the sea blows towards the land to take its place by convection, causing what is known sea breeze.

Land breeze

Since good absorbers are good radiators, during the night the land loses more heat than the sea. On account of its lower specific heat, the temperature of the land will become lower than that of the sea in the early hours of the morning. As a result, the air above the land becomes cooler than that above the sea. So colder air from over the land blows towards the sea by convection causing what is called the land breeze.

Trade winds

The earth in the tropics becomes more heated and this heat is communicated to the air which is consequently heated. This heated air being lighter rises up and to replace this colder air from the north and the south flows towards the equator, but owing to the rotation of the earth from west to east, the wind gets a resulting velocity in the north-eastern direction in the northern hemisphere and south-eastern direction in the southern hemisphere. The first is known as the north-east trade wind and the other as the south-east trade wind.