Laws Related to Heat and Temperature

All bodies emit thermal radiations. Prevost’s theory of heat exchange says that substances at all finite temperatures emit radiations, the amount of which increases with the increase in temperature. It is not even affected by the presence of surrounding bodies. The temperatures of a body could either fall or rise on account of the exchange of radiant energy with the surrounding bodies.
When two bodies exchange equal amount of heat, a state of thermal equilibrium is attained. Prevost’s theory defines thermal equilibrium between two bodies as a dynamic equilibrium as the bodies constantly exchange heat radiations.
Every body emits heat radiation due to its temperature. Kirchhoff’s law of heat radiation states that the ratio of emissive power of a body to its absorptive power at any temperature, for a particular wavelength, is always constant and is equal to the emissive power of a perfect black body. Total amount of heat leaving the body is given by the sum of heat reflected and heat emitted. In equilibrium state, heat leaving the body must be equal to the heat incident on the body.
The Stefan–Boltzmann law also known as Stefan's law describes the power radiated from a black body in terms of its temperature. Stefan–Boltzmann law states that the thermal energy radiated by a black body radiator per second per unit area is proportional to the fourth power of the absolute temperature.
Emissivity of a body is the ratio of the radiant emittance of the body to that of a perfect blackbody when both are at the same temperature.
Knowing the solar constant, Stefan’s Law is applied in determining the surface temperature of the sun. Solar constant is the amount of radiant energy received per second per unit area by a perfect black body kept on earth in a direction perpendicular to the direction of rays.
Newton’s law of cooling states that the rate of loss of heat of a body varies directly as the temperature difference between the body and the surroundings. Newton’s law of cooling is valid only when there is small temperature difference between the body in consideration and the surroundings.

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