“Kirchhoff’s Law of Radiation” MHT-CET Class 12 Physics Crash Course | Chapter Kinetic theory of Gases and Radiation
In this session G.S. Khairnar discussed the theory and MCQ’s of “Kirchhoff’s Law of Radiation”. The session will be useful for all the aspirants of MHT-CET 2022. The session will be in Marathi and English.
Kirchhoff’s Law of Radiation:-
Hello students let’s recall some important terms given below!
Emissive power (E):
The quantity of radiant energy emitted per unit time per unit area of a body at given temperature is called as emissive power of a body at that temperature.
∴ E =Q / A t
Its SI unit is: Kcal/m2S or J/m2S and
[E] = [M1L0T-3]
At given temperature the emissive power of the perfectly black body is greater than any other body.
Emissive power of a body depends on:
- Temperature of body,
- Nature of body,
- Surface area of body,
- Nature of surroundin
Coefficient of emission or emissivity (e): The ratio of emissive power of a body to the emissive power of perfectly blackbody at the same temperature.
Being the ratio of same quantities the coefficient of emission has no unit.
Absorptive power
The amount of radiant energy absorbed per unit area per unit time by a surface at that temperature.
Coefficient of absorption (a): The ratio of absorptive power of a body to the absorptive power of perfectly black body at the same temperature.
Let’s understand the concept of Kirchhoff’s law of radiation in detail….!
Consider two objects one is ordinary body and other black body of equal surface area A, are enclosed in constant temperature enclosure as shown in fig below. When heat is given to the objects, it can absorb the heat as per its absorptive power.
Let,
Q = amount of heat incident on object
a = coefficient of absorption of ordinary object
Qa = amount of heat absorbed by ordinary body
E = Emissive power of ordinary body
Eb= Emissive power of black body
Case 1) When heat is incident on ordinary body-
Ordinary body will absorb the heat according to the magnitude of its coefficient of absorption, and then if ‘Q’ is heat incident on ordinary body, heat absorbed can be given as,
Qa=a.Q ……………….(1)
According to principle of het exchange the amount of heat absorbed by ordinary body is always equal to amount of heat emitted by black body, therefore
Case 2) When heat is incident on black body-
When heat is incident on black body, it absorbs all of incident heat. So from principle of heat exchange we have, heat absorbed = heat emitted by black body
Q=Eb……………………(4)
From equation (3) and (4)
i,e, According to Kichhoff’ s law of radiation
coefficient of absorption is always equal to coefficient of emission.
Kirchhoff’s Law of Radiation:-
Let’s solve some MCQ’s…!
Q.1) Emissive power of body is ………
a) directly proportional to heat incident on the body
b) inversely proportional to surface area of black body
c) inversely proportional to time
d) all of above
Answer
d) all of above
Q.2) The coefficient of emission of body is………
a) ratio of emissive power of a body to the emissive power of perfectly blackbody at the same temperature
b) ratio of emissive power of a black body to the emissive power of ordinary body at the same temperature
c) ratio of emissive power of a body to the emissive power of perfectly blackbody at the different temperature
d) ratio of emissive power of a black body to the emissive power of ordinary body at the different temperature
Answer
a) ratio of emissive power of a body to the emissive power of perfectly blackbody at the same temperature
Q.3) Heat emitted at the rate of 2400 watt by the cube of length 2 cm. Emissive power of the material of cube is,
Q.4) The ratio of emissive power of bodies of on which heat incident in ratio of 3:5 on the surface areas in ratio 2:3for same time is………
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