The global greenhouse effect The earth’s atmosphere behaves like a greenhouse. The sun’s radiation easily passes through it and is absorbed by the earth’s surfaces. As it warms up, the earth re-radiates energy of longer wavelengths. This becomes absorbed by water vapour, carbon .dioxide and other gases in the atmosphere. The warm atmosphere, in turn, radiates back to the […]
The greenhouse Anyone who walks into a greenhouse, even on a day when the sunlight is rather dull, realizes how efficiently it acts as a radiation trap. Very hot bodies such as the sun emit most of their radiation in the form of visible light and short wavelength infrared rays which easily pass through glass without being absorbed. (The […]
The vacuum flask The vacuum flask is also commonly known as a Thermos flask, which is the trade name used by a large manufacturing firm. Originally, it was devised by Sir James Dewar for the purpose of storing liquefied gases. Liquid oxygen, for example, boils at the very low temperature of – 183 °C (90 K), so […]
Practical applications of radiation The investigations on radiation and absorption described above have a umber of useful applications. Buildings which are whitewashed or painted in light olours keep cooler in summer, since the light surfaces reflect ‘radiation from the sun. Many factory roofs are now aluminium-painted. The bright surface reduces the heat lost in winter, and keeps the interior […]
Absorption of radiation by a surface As stated earlier in this chapter, radiation falling on a surface is partly absorbed and partly reflected. The absorbing powers of a dull black and a polished surface may be compared by using two sheets of tinplate, one polished and the other painted dull black. On the reverse side of each […]
To compare the radiation from different surfaces The rate at which a body radiates energy depends on its temperature and the nature and area of its surface. It is found that, for a given temperature, a body radiates most energy when its surface is dull black and least when its surface is highly polished. A comparison of the […]
The detection of radiation. The thermopile Radiant energy from a hot body may be detected by converting it into electric energy. A simple experiment serves to show how this is done. A copper and an iron wire are twisted together to form a junction, while the free ends of the wires are connected to the terminals of a […]
RADIATION Both conduction and convection are ways of conveying heat from one place to another which require the presence of a material substance, either solid, liquid, or gas. There is a third process of heat transmission which does not require a material medium. This is called radiation, and is the means by which energy travels from the sun across […]
land and sea breezes At places on the coast in summer-time it is noticeable that a breeze generally blows from the sea during the day, while at night the direction of the wind is reversed. These breezes are local convection currents. During the day the land is heated by the sun to a higher temperature than the […]
The domestic hot water supply system The domestic hot water supply system consists of a boiler, a hot water storage tank and a cold supply tank interconnected by pipes arranged as shown in Fig. 17.13. When the system is working a convection current of hot water from the boiler rises up the flow pipe A while cold […]