Transmission of Heat
There are three ways of heat transmission:
(i) conduction,
(ii) convection, and
(iii) radiation.
● Conduction :- If we hold one end of an iron rod in a flame, the other end soon becomes too hot to be held in hand. Heat enters one end of the rod and is transmitted along its whole length. This process of heat transmission is called conduction and takes place mainly in solids. The actual mechanisms of heat transfer differ in metals (iron, silver, etc.) and non-metals (such as wood). Among solids there are good as well as bad conductors of heat. Substances such as wood, cotton, wool and glass are bad conductors (good insulators) of heat. Liquids and gases, in general are bad conductors. Air is a very bad conductor of heat. The good insulating properties of wool, cotton, etc. are mainly due to the air spaces they contain. Woollen clothes do not allow the heat of our body to escape and we feel warm. Sawdust is a bad conductor of heat. Therefore, ice slabs are covered with sawdust to minimise melting.
In airconditioned rooms, double windows, consisting of two panes of glass with a thin layer of air in between, serve as a better insulators of heat than windows with single, thick panes.
On a cold night two thin blankets are preferred to a single thick blanket because the layer of air between the two blankets serves as a better insulator. Ovens, geysers, etc. have double walls with glass wool or straw in between to reduce the loss of beat by conduction. Refrigerators and ice-boxes have similar double walls to minimise heat gain by conduction
When we touch an iron hammier lying in the sun, it appears much hotter than its wooden handle because iron is a good conductor of heat and conducts heat rapidly to the hand. Wood being a poor conductor of heat, conducts heat from the touched surface only.
In winter, a stone floor feels cold to the bare feet, but a carpet on the same floor feels warm even though both are at the same temperature. Stone, being a good conductor. conveys heat, away from the feet rapidly. Carpet is a poor conductor and conveys little heat. Consequently, the feet feel cold on the stone but not on the carpet.
A refrigerator has to be switched off for defrosting whenever a thick layer of ice deposits on the outside and inside of the freezer. Ice, being a poor conductor, affects the cooling action of the freezer. Thus defrosting helps in the efficient functioning of a refrigerator.
During severe winter, Eskimos live in snow huts called igloos. Snow, being a poor conductor shields them from cold. It prevents the heat they generate from escaping and keeps them warm.
● Convection :- In liquids and gases beat is transmitted by convection. In this process heat is carried from one place to another by the actual movement of liquids and gases. If we heat a liquid in a vessel from below, the liquid at the bottom gets heated and expands. The hot liquid rises due to its lower density and its place is taken by cold liquid from above. "Convection currents are thus set up and the whole liquid gets heated to a uniform temperature. Convection currents are set up in gases and air in a similar way.
Heating elements in geysers and water heaters are fitted near the bottom so that water can be heated by convection currents. Heating elements in electric ovens are fitted near the bottom to heat the entire enclosed air by convection. An element near the top of an oven would heat only the top layers of air, leaving the cool layers below almost unaffected.
The cooling unit (freezer) in a refrigerator is fitted near the top to cool the whole of the interior. The air near the top cools and descends due to increased density. Its place near the top is taken by warm air and in this way convection currents are set up, which cool the entire interior
Convection currents in the atmosphere result in winds (see section on geography). Sea and land breezes can be explained on the basis of convection. During daytime the seashore (land) warms up much faster than sea water. Air over the shore rises and cooler air from water takes its place resulting in a sea breeze.
At night land cools faster than water, resulting in a land breeze.
● Radiation :- Both conduction and convection require a material medium for conveying heat from one part to another. Radiation, on the other hand, does not require any medium. The earth receives radiant energy from the sun in the form of electromagnetic waves which can pass through vacuum.
All bodies are continuously emitting and absorbing radiant energy. If a body emits more energy than it absorbs its temperature falls. On the other hand, if a body absorbs more energy than it emits, its temperature rises. A body at a constant temperature, then, absorbs as much energy as it emits
The rate at which a body emits or absorbs radiant energy depends on its temperature and the nature and area of its surface. A rough surface is a better absorber than a smooth one, because microscopically a rough surface has more surface area.
Good absorbers are good emitters and poor absorbers are poor emitters.
If we pour hot coffee simultaneously in two metal cups of the same size and shape, but with one having a rough black surface and the other a bright polished surface, the coffee will cool faster in the black cup because the rough black surface is a better radiator. Thus coffee or ten remains hot in a shining bright cup for longer. When iced water is poured in these empty cups, the water in the black cup will warm up faster since black is also a better absorber of radiant energy.
The base of an electric iron is highly polished so that it does not lose heat by radiation. Houses which are white washed or painted in tight colours keep cooler in summer. because light surfaces do not absorb much solar radiation. Newton's Law of Cooling This law states that the rate ut which a hot body loses heat is directly proportional to the difference between its temperature and the surrounding temperature. For example, hot water takes much less time in cooling from 90°C to 80°C than in cooling from 40°C to 30°C
If hot water and fresh tap-water are kept in a refrigerator. the rate of cooling of hot water will be faster than the tap water.
Suppose, a person is served hot coffee with separate cream (at room temperature), but he wants to drink it after a while. It is then advisable to add cream right in the beginning rather than at the time of taking the coffee because this way, the coffee will remain hotter.
Cooling at Night The earth and other objects on it receive solar radiation during the day and become warm. At night they start emitting radiant energy and become cool. Objects such as stones, metals, etc. which are good conductors of heat, keep receiving heat from the earth by conduction and maintain their temperature. However, bad conductors like grass and wood do not receive the earth's heat by conduction and get colder than the air, resulting in the formation of frost on them.
Cloudy nights are warmer than clear nights because clouds reflect the radiations emitted by the earth at night and keep it warm. Clouds act like a blanket.
◆ Greenhouse Effect :- A greenhouse acts like a radiation trap. In a greenhouse, heat radiation from the sun passes through the glass and keeps the plants and the air inside warm. The glass prevents warm air from escaping. Moreover, radiation emitted by objects in the greenhouse cannot escape through glass.
A car parked in the sun with its windows closed gets terribly warm due to the greenhouse effect.
◆ Solar Cooker :- A simple solar cooker is a box made of insulating material like wood, card-board etc. The box has. a glass cover to retain heat inside by the greenhouse effect. The inside of the box is painted dull black to increase heat absorption. The cooking vessel is kept inside the box which is then kept in the sun. Generally, this type of cooker is used only for warming food but can sometimes be used for cooking rice, pulses, etc.
◆ Thermos Flask :- A thermos flask is double walled with a vacuum between the walls. The two inner glass surfaces facing each other are silvered. It has a plastic or cork stopper. In a thermos flask heat transfer by conduction is almost nil through the vacuum. The stopper, being a poor conductor, conducts very little heat. The vacuum also prevents heat loss by convection. Silvered surfaces of the walls prevent heat loss by radiation. Thus in a thermos flask, the transmission of heat by conduction, convection, and radiation is minimised and, therefore, its content remains at nearly the same temperature for a long time.
