Heating laws

Heating laws

Energy flux

Energy flux is the energy flowing per second per unit area normal to any surface. Its units are watt/m2.

Perfectly Black Body

Perfectly black body absorbs the entire radiation incident upon it. Thus absorptive power of a perfectly black body is unity (i.e. 100%). When such a body is heated to high temperature, it would emit radiations of all wavelengths. The nature of radiation emitted by a perfectly black body would depend on its temperature only and not on mass, size, density or nature of the body. For an ideal black body, reflectance and transmittance must be 0. Nobody in actual practice can be perfectly black. The nearest examples of ideal black bodies are lamp black (96%) and platinum black (98%). They absorb visible and near infra red radiations, but cannot absorb far infra red radiation.

Kirchhoff’s law

According to this law, at a given temperature and far a given wavelength, the ratio of spectral emissive power to spectral absorptive power for all bodies is constant, which is equal to spectral emissive power of perfectly black body at the same temperature and far the same wavelength. The law implies that at a particular temperature, a body can absorb only those wavelengths, which it is capable of emitting.

Wien’s law

According to this law the wavelength corresponding to which energy emitted/sec/area by a perfectly black body is maximum, is inversely proportional to the absolute temperature of the black body.

Stefan’s law

According to this law the total energy emitted emitted/sec/area by perfectly black body corresponding to all wavelengths is directly proportional to fourth power of the absolute temperature of the body.

Stefan Boltzmann law

According to this law the net amount of radiation emitted per second per unit area of a perfectly black body at temperature is equal to difference in the amounts of radiation emitted/sec/area by the body and by the black body enclosure at T0.

Newton’s law of cooling-

According to this law when difference in temps of a liquid and its surroundings is small (~ 30C), then the rate of loss of heat of the liquid id directly proportional to difference in temperature of the liquid and the surroundings.