Learn About Heat It's Thermodynamics! Part 3 | Definitions, Formulas,Units

To view  Learn About Heat It's Thermodynamics! Part  2 Please click the Link below

Learn About Heat It's Thermodynamics! Part 2

Emissive Power And Absorptive Power

Emissive Power: The Power of a body at a given temperature is defined as the quantity of radient energy emitted by the body per unit time per unit surface area of the body at that temperature

If Q = amount of radiant energy emitted 

    A = surface area of body

    t  =  time for which body radiates energy

Formula

Emissive Power of body at given temperature

                E = Q

                       At

S.I. unit of emissions power is J/m^2s or  W/m^2

Dimensions : [M^1 L^0 T^-3]

Emissive Power of a body depends on:

1. Temperature of the body
2. Nature of body
3. Surface area of body
4. Nature of the surrounding

Note:- Emissive Power of a perfectly black body is always greater than any other body at same temperature. Emissive Power of some surfaces are Lampblack 98%, Aluminum Paint 33%, Platinum 11%, Copper 5%, Silver 3%.

Coefficient of Emission (emissivity)

Coefficient of Emission of a body is the ratio of emissive power of the body at the given temperature to the emissive of a perfectly black body at same temperature

Formula

Coefficient of Emission e = E

                                                Eb

Where, E= Emissive power of ordinary body at given temperature.

             Eb = Emissive power of perfectly black body at same given temperature.

For perfectly black body, e = 1

For perfect reflectors, e = 0

For ordinary bodies, e < 1

Note :- Good absorbers are Good emitter of heat.

Absorptive Power

Absorptive Power of a body at a given temperature is defined as the amount of radiant energy absorbed per unit area per unit time by a suface at that temperature. A body which absorbs all radiation of all wavelengths would be called 'Perfectly Black Body'

Kirchhoff's Law of Radiation 

It states that 'the coefficient of absorption of a body is equal to its coefficient of emission at any given temperature.'

                                                                   a = e

But coefficient of emission e  = E

                                                      Eb

                         a = E    or  E = Eb

                               Eb        a 

Hence Kirchhoff's law can be stated as " At any given temperature, the ratio of the emissive power (E) to the coefficient of absorption (a) is constant for all bodies and the this constant is equal to emissive power (Eb) of a perfectly black body at the same temperature."

Stefan's Law of Radiation

It states that the amount of radiant energy emitted per unit time per unit surface area of perfectly black body is directly proportional to the fourth power of its temperature.

 Formula

                                          Q ∝  T^4  Or  Eb ∝ T^4

Let,   Q  =  Amount of radiant energy emitted by perfectly black body.

         A  = Area of the perfectly black body.