Question

Define emissive power and emissivity.

Answer 1

The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation and it may include both visible radiation (light) and infraredradiation, which is not visible to human eyes. The thermal radiation from very hot objects (see photograph) is easily visible to the eye. Quantitatively, emissivity is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature as given by the Stefan–Boltzmann law. The ratio varies from 0 to 1. The surface of a perfect black body (with an emissivity of 1) emits thermal radiation at the rate of approximately 448 watts per square metre at room temperature (25 °C, 298.15 K); all real objects have emissivities less than 1.0, and emit radiation at correspondingly lower rates.[1]

Answer 2

Answer:

(1) Emissive power or radiant power of a body (symn-

bol, R): The emissive power or radiant power of a

body at a given temperature is defined as the

quantity of radiant energy emitted by the body per

unit time per unit surlace area of the body at that

temperature.

(2) Coefficient of emission (or emissivity) of a body

(symbol, e): The coefficient of emission (or

emissivity) of a body is defined as the ratio of the

emissive power of the body (R) to the emissive

power of a perfect blackbody (R,) at the same

temperature as that of the body.