Physics > Heat Transfer > 3.0 Radiation
Heat Transfer
1.0 Conduction
2.0 Convection
3.0 Radiation
4.0 Kirchhoff's Law
5.0 Wein's displacement law
6.0 Stefan-Boltzman law
7.0 Newton's laws of cooling
3.1 Basic Definitions
Black body
A perfect absorber as well as a perfect radiator of heat energy is known as black body. The radiation emitted by the black body is known as black body radiation.
From the black body, we can know that good absorbers are also good emitters.
There is no absolute black body.
Generally, lamp black is considered close to the ideal black body.
Emissive power ($E$ )
Emissive power is defined as radiant energy emitted per second per unit area of the surface.
It is denoted by $E$.
For a black body, $$E = \sigma {T^4}$$ Its units are $$W/{m^2}\,or\,J/s - {m^2}$$
Absorptive power ( $a$ )
Absorptive power is defined as the ratio of the ratio of the radiant energy absorbed in a given time to the total radient energy incident on it in the same interval of time. $$a = \frac{energy\ absorbed}{energy\ incident}$$ A perfectly black body absorbs all the radiation incident on it, the absorptive power of a perfectly black body is one. $${a_{black\,body}} = 1$$
