To see Learn About Heat It's Thermodynamics! Part 1 please click the link below
Absorption, Reflection, and Transmission Of Heat Radiation
When radiant energy is incident on the surface of any body, part of this energy is absorbed, part is reflected and another part is transmitted by the body. This ability is measured by the coefficient of absorption (Qa), reflection (Qr), transmission (Qt).
Formula
Qa + Qr + Qt = Q
Where Q is total radiant energy incident upon the surface of body.
Coefficient of adsorption (a)
The coefficient of absorption of a body is defined as the ratio of quantity of radiant energy absorbed by the body in the given time to the quantity of radiant energy incident on the body in the same time.
Coefficient of adsorption, a = Qa
Q
Black surfaces are good absorbers of radiant heat.
For perfectly black body a = 1 and for ordinary bodies a < 1
Coefficient of reflection (r)
The coefficient of reflection of a body is defined as the ratio of quantity of radiant energy reflected by the body in the given time to the quantity of radiant energy incident on the body in the same time.
Coefficient of reflection, r = Qr
Q
Polished surfaces are good reflectors of radiant heat.
Coefficient of transmission (t)
The coefficient of transmission of a body is defined as the ratio of quantity of radiant energy transmitted by the body in the given time to the quantity of radiant energy incident on the body in the same time.
Coefficient of transmission, t = Qt
Q
Note: Coefficient of absorption, reflection, transmission has no Unit and Dimension
Law of conservation of heat energy
Total amount of radiant energy is equal to sum of energy absorbed, reflected, and transmitted from the body.
Q = Qa + Qr + Qt
Dividing both sides by Q, we get
Q = Qa + Qr + Qt
Q Q Q Q
∴ 1 = a + r + t
You can also remember this as "art"
From this relation, it is clear that good absorbers are poor reflecters and transmitters of radiant energy.
Athermanous substances
The substance which do not transmit any incident heat radiation are called athermanous substances.
Examples - Wood, Iron, Copper, Lampblack. Water vapour are athermanous substances
For Athermanous substances a + r = 1 & t = 0
Diathermanous substances
The substances which are transparent or pass through to heat radiations are called diathermanous substances. They are neither good absorber nor good reflector
Examples - Glass, Quartz, Sodium chloride, Hydrogen, Oxygen, Dry air, rock salts.
Perfectly Black Body
Lampblack
It is a body which absorbs all radiant energy incident on it. Qa = 1, Qr + Qt = 0. Hot black body emits more energy per unit area than cold black body. It practically does not exist in nature. Lampblack absorbs 98% of radiant heat.
Ferry's Black Body
Scientist Ferry a body which can be considered to be perfectly black body
1) It consists of a double-walled hollow metal sphere having small aperture from which the radiant heat will come in.
2) Inner surface is coated with lampblack, outer surface is nickel polished and has a conical projection diametrically opposite to aperture.
3) The space between two walls is evacuated to avoid loss of heat.
4) The radiant heat inside sphere suffers multiple internal reflection after a large number of reflection almost all the radiant heat is absorbed. Thus it acts as perfectly black body.
Spectrum of black body radiation in terms of wavelength
When temperature of black body is increased,
1) Energy distribution curve continues to be non-uniform.
2)Peak of Eλ Vs λ curve shifts towards the left means as temperature increases value of λmax decreases
3) At higher temperature total energy per sec unit area corresponding to all wavelength increases
To see Learn About Heat It's Thermodynamics! Part 3 please click the link below
Learn About Heat It's Thermodynamics! Part 3
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