Stefan's Law of Radiation:
The total radient energy emitted E per unit time by a black body of surface A is propertional to the fourth power of its absolute temperature.
E  T4
or E =  AT4 = Stefan's const
For abody which is not black body
E =  AT4 = emmisitivity of te Black Body.
Note: Emmisitivity and absorptive power have the same value.
Net loss of thermal Energy[secondary information]:
If abody of surface area A is kept at absolute temp T in asurrounding of temperature T0(T0 > T) Then the energy emitted by the body per unit time.
E = AT4
And the energy absorbed per unit time by the body
E0 = AT04
Net loss of thermal energy per unit time.
 E = E - E0 = A[T4 - T04]
Newton's Law of Cooling:
For a small temperature difference between a body and its surroundings, the rate of cooling of the body is dirctly propertional to the temperature difference. If abody of temperature T and surface area A is kept in a surrounding temperature T0(T0 < T). Then net loss of thermal energy per unit time.
 = A(T0 - T00)
If the temperature difference is small
T = T0 + T
=> A[(T0 + T)4] = A[T04 (1 +  )4 - T04]
=> AT04[1 + 4 + higher powers of  - 1]
= 4AT03T
Now, Rate of loss of heat at temperature T
mc = - 4AT03[T - T0]
= - K[T - T0]
K = 
(T - T0)
WIENS LAW:
At a const temperature T, when wavelength  is increased , the energy emitted E, first increases, reaches a maximumand then decreases i.e at a particulartemperatuure the spectral radiancy  is a maximum at a particular wavelength m.
As the temperature increases , the maximum radiancy of energy occurs at shorter wavelength i.e
mt = b
b is a constant.
This is called wiens Displacement Law.
Dumb Question
:
Q: At a particular wavelength the bodies A, B, C, have their emmisive Power in Ratio 1:2:3. what is the ratio of the absorptive power ?
A: 1:2:3 became
= constant
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