Physics-Heat Transfer, Radiation , Laws of Radiation, Advantages, disadvantages.

1. HEAT TRANSFER
LAWS OF RADIATION

2. What is Radiation?
• Radiation is the transfer of energy by rapid oscillations of
electromagnetic fields
• The most important general characteristic is its wavelength ( λ )
• Radiation travels through space at the speed of light
3 × 108
𝑚𝑠−1
or 670,616,630 𝑚𝑝ℎ

3. Laws of Radiation
• Kirchhoff’s Law
• Wien’s Law
• Stefan-Boltzmann Law
• Planck’s Law

4. Kirchhoff’s Law
A black body, by definition, absorbs radiation at all wavelengths
completely. Real objects are never entirely “black” – the cannot absorb all
wavelengths completely, but show a wavelength-dependent absorptivity
ε(λ) (which is < 1).
According to Kirchhoff’s Law the emission of a body, Eλ (in
thermodynamic equilibrium) is : E λ (λ ,T) = ε(λ) B λ ( λ,T)
For a given wavelength and temperature, the ratio of the Emission and
the absorptivity equals the black body emission.

5. Wien’s Law
According to Wien’s displacement law the dominant wavelength at which
a blackbody emits electromagnetic radiation is inversely proportional to
the temperature of the object.
λm ∝
1
𝑇
λmT = constant
This shows that with increase of temperature of the blackbody the maximum
intensity of radiation shifts towards shorter wavelength.

6. According to Wien’s distribution law E λ dλ =
𝐴
λ
5 𝑒−𝐵 λT dλ

7. Stefan-Boltzmann Law
The Stefan-Boltzmann law states that a blackbody radiates
electromagnetic waves with a total energy flux ( E ) directly proportional
to the fourth power of the temperature T of the object:
E = T4
i.e. ,
P = AeT4

8. Plank’s Law
According to Planck’s Law the energy emitted by a black body
equals:
E λ dλ =
2𝜋ℎ𝑐
λ
5
1
exp
ℎ𝑐
λ𝑘 𝐵
𝑇
−1
dλ
c = Speed of light (in vacuum) = 299,792,458 m𝑠−1
h = Planck constant = 6.626 × 10−34 Js
kB = Boltzmann constant = 1.380 × 10−23 𝐽𝐾−1

9. Advantages of Wien’s and Stefan-Boltzmann Law
• The intensities of radiation emitted at various wavelengths by a blackbody
at a given temperature are shown by a blackbody curve.

10. Advantages of Wien’s and Stefan-Boltzmann Law
• A blackbody is a hypothetical object that is a perfect absorber of
electromagnetic radiation at all wavelengths
• Stars closely approximate the behavior of blackbodies, as do other hot,
dense objects