Question

Define emissive power and coefficient emission of a body.

Answer 1

Emissive power is the radiant energy emitted by the Body. Body should be anything either it is black or gray, coefficient of emission is the fractional part of power of a perfectly black Body at the same temperature.

Complete step by step answer:
Emissive power (E) of a material at a certain temperature is defined as the rate of radiation emitted through a unit area of the material.
or Emissive power is the power of a body at a given temperature emitted as radiant thermal energy from a unit surface area of a hot body per unit time. It is also known as spectral emissive power. Basically, it is the total amount of thermal energy emitted per unit area per unit time.
Let Q is the amount – of thermal energy emitted.
‘A' is the surface area of the body
‘t’ is the time for which body emits energy
Thus,
$\text{Emissive power}=\dfrac{Q}{At}$
SI unit of emissive power is watt per square meter$\left( W/{{M}^{2}} \right)$
Coefficient of emission of a body can be defined as the emissive power of a body to emissive power of perfect black body.
Hence, $\text{Emissivity(e)=}\dfrac{\text{rate}\,\text{of}\,\text{radiation}\,\text{emitted}\,\text{through}\,\text{a}\,\text{unit}\,\text{area}\,\text{of}\,\text{material}}{\text{rate}\,\text{of}\,\text{radiation}\,\text{emitted}\,\text{through}\,\text{a}\,\text{unit}\,\text{area}\,\text{of}\,\text{blackbody}}$
The object and the blackbody should be exposed in the same condition.

Note:
Emissive power a Body is the thermal Energy emitted per unit area per unit time and emissive coefficient is the ratio of emissive power of anybody to emissive power of a black body.