[Half-Inch Stud]

bobzdar, moor-daddy, I stand by my definition of heat radiation as light.
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Radiation: The rate at which an object radiates energy has been found to be proportional to the fourth power of its absolute temperature T, and the surface of the object.

DQ/Dt = esAT4
where s is a universal constant called the Stefan-Boltzmann constant which has a value of 6.67x10-8 Wm-2K-4; and e is the emissivity, a characteristic of the material, it takes values between 0 and 1.

Any object not only emits energy by radiation, but it also absorbs energy radiated by other bodies. The net radiant heat flow from the object at T1 to an environment at T2 is then given by the equation

DQ/Dt = esA(T14-T24)

Prevost law of exchange states, that in a state of thermal equilibrium the amount of energy radiated per second from an object, is equal to the energy absorbed by it, in the form of radiation from surrounding objects.

Thus a good emitter is a good absorber.

An ideal absorber, called a black body, absorbs all the energy incident on it. Its emissivity is equal to unity, e = 1. The human body is a very good approximation to a black body, e = 0.95.

Radiative heat transfer can take place through a vacuum, unlike conduction and convection processes. It is found experimentally that the wavelength corresponding to maximum energy radiation decreases with temperature.

This relation is known as Wien displacement law

l at max energy = B/T

where proportionality constant B is equal to 2.9x10-3Km. Earth’s surface (~300K) emits mainly infrared radiation.
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