introduction of laws of radiation

1. Prepared by:
Chauhan vishal (12me243)
Kapadiya nirav (12me242)
Laws of radiation

2. Heat can be transferred in three
basic mode
Conduction Convection Radiation
All the modes of heat transfer required the existence of a
temperature difference .

3. - Conduction refers to the transfer of heat
from one end to another end of the same
substance or between the two substance
having physical contact with them via
molecular vibration
Conduction

4. -Convection refers to the mode of heat
transfer in which the heat is transfer by the
moving fluid particles.
Convection
Force
convection
Natural
convection

5. - Radiation is the transfer of heat through the
space or matter by means other than
conduction and convection .
- Radiation is refers to the transfer of heat from
the body or from the fluid for which no
medium is required.
Radiation

6. Heat transfer takes place by electromagnetic
waves or by energy pockets (called photons)
no medium is required for heat transfer.
Vacuum is the best medium for the heat
transfer by radiation.

7. Governing
Laws
For
Radiant
Heat
Transfer
Kirchhoff's law
Stefan Boltzmann's law
weins displacement law
Planck's law

8. Example of heat transfer by radiation
-Heat libration during nuclear explosions
- Space application.
- Heat leakage from furnaces.
- Solar energy incident upon the earth.
-Combustion chambers

9. In short………..
All the substance with body temperature
above the absolute zero Kelvin level
continuously emits energy in the form of
radiation.

10. First we see some properties
The rate of heat transfer by radiation from
the given body depends upon
1 .temp of the body
2 .nature of surface of body
3 .frequency or wavelength of
radiation
Surface emission properties-

11. Total emissive power(E)
At a given temp the total amount of heat emitted by a
surface in all the direction over entire wavelength per
unit area per unit time is called emissive power.

12. emissivity(€)-
It is define as a ability of the surface
to radiant heat.
OR
It is the ratio of total emissive power
of any body to the emissive power of
black body of equal temp.
€=E/Eb

13. The emissivity may vary with variation
in temp and wavelength.
The value of emissivity for a diff
substance ranging 0 to 1.
For a white ,emissivity =0.
For a black ,emissivity =1.

14. Monochromatic emissive power (E)
The monochromatic emissive power define as
the amount of radiant energy emitted by surface
at a given temp per unit area per unit time and
per unit wavelength.
Radiosity: -it is define as a total energy leaving
from the surface from all the direction per unit
area per unit time, expressed in W/m^2.

15. Incident
radiation(G) Reflected radiation(Gr)
Material
surface Absorbed
radiation(Ga)
Transmitted
radiation(Gt)
Absorptivity , reflectivity & transmissivity

16. When incident radiation(G) on a material
surface ,part of that reflect back(Gr),some
part is transmitted(Gt) and remaining
amount of energy absorbed(Ga) by the
material depending upon the characteristics
of the body
By conversion of
energy principal
G=Gr+Ga+Gt
G/G=1=Gr/G+Ga/G+Gt
/G
1=ƍ+α+Ṭ

17. α=Ga/G=fraction of incident radiation
absorbed or absorbtivity.
Ṭ=Gt/G=fraction of incident radiation
transmitted or transmissivity
ƍ=Gr/G=fraction of incident of radiation
reflected or reflectivity

18. Stefan-Boltzmann´s law:
The emissive power of black body is proportional
to forth power of its absolute temperature.
Eb = σ⋅T^4
where Eb = total power radiated per unit area from
a blackbody (W/m2)
σ = 5.669 ⋅ 10-8 W/(m2⋅K4). (Stefan-Boltzmann
constant)
T = absolute temperature (K)

19. black body is an
object that absorbs
all the radiant
energy reaching its
surface from all
direction with all
the wavelength.
It is a perfect
absorbing body .
For black body:-
α=1,ƍ=0,Ṭ=0
Black body
if all the
incident
radiation falling
on the body are
reflected it is
called the white
body.
For a white
body:-
α=0,ƍ=1,Ṭ=0
White body
when all
irradiation is
transmitted
threw the
body it is
called…..
α=0,ƍ=0,Ṭ=1
Transparent body
The concept of different bodies

20. PLANK’S LAW
Plank’s law describes the electromagnetic radiation
emitted by a blackbody in thermal equilibrium at a
definite temperature.
The law is named after the max plank.
Plank radiation is the greatest amount of radiation that
any body at thermal equilibrium can emit from it’s
surface whatever its chemical composition or surface
structure

21. 1
12
)( /5
2

 kThc
e
hc
I 



where h=6.62x10^-34 Js is the Planck’s constant.
=Wavelength
c=velocity of light in vacuum=3x10^8 m/sec
K= Boltzmann constant=1.3805x10^-23 j/k
T=Absolute temperature
The intensity (I) of electromagnetic radiation at a given
wavelength () is a complicated function of the
wavelength and the temperature (T).

23. For the given value of  ,the (E)b increases with
increases in absolute temp
The peak (E)b shifts towards a smaller wavelength
at a higher temp. it means at higher temp the
energy is emitted in form of smaller wavelength for
the maximum value of monochromatic emissive
power for the T<=800k.the energy emitted in the
infrared region and thus the radiation is not visible
to eye.
The area under the curve gives the rate of radiant
energy emitted within the wavelength interval d.