Emissive Power : E
The rate of radiation per unit surface area of an object is called its emissive
power.
Emissivity or Coefficient of emission : e
The ratio of emissive power of any object (E) to the corresponding emissive power of a
perfectly black body (E_{b}) at same temperature is called emissivity.
Stefan's law of heat radiation
The emissive power of a perfectly black body is proportional to the fourth power of its
absolute temperature.
i.e. E = s
T^{4}
where s = Stefan's constant = 5.67
´ 10^{8} watt / m^{2}
´ K^{4}
For any other object E = e s T^{4}
Newton's Law of cooling
The rate of cooling of an object is directly proportional to the excess temperature of the object
over that of its surrounding objects; provided that the excess temperature is sufficiently small.
K > q & constant for the object under a given temperature of the surroundings
q : temperature of the object is ^{0}C
q _{0} : temperature of the surrounding objects in
^{0}C
(The surrounding object is assumed to be perfectly black bodies)
If T << 2T_{0} where T and
T_{0} are temperatures of the object and the surrounding objects in
^{0}K respectively, then from Stefan's law we can show that
**********
