Tag: kirchoff's laws

Questions Related to kirchoff's laws

Statement-1:Good reflectors are poor emitters of thermal radiation.
Statement-2:The ratio of the emissive power (e) and absorptive power(a) is constant for all substances at any given temperture and for radiation of the same wavelength

kirchoff's laws black body radiation heat transfer thermal properties physics

  1. Statement-1 is true, Statement-2 is true and Statement-2 is the correct explanation for Statement- 1

  2. Statement-1 is true, Statement-2 is true but Statement-2 is not the correct explanation for Staement-1

  3. Statement-1 is true, Statement-2 is false.

  4. Statement-1 is false, Statement-2 is true.

Show answer
Correct Option: A
Explanation:

From the krichoff's law we know that absorptivity is equal to emissivity,so the body which emitt more wil absorbe more nor reflect more. So good reflector are poor emittor.
Now, by taking there ratio it validate krichoff's law because at any fixed or constant temperature both are equal.

In a dark room with ambient temperature $T _o$, a black body is kept at a temperature $T$. Keeping the temperature of the black body constant (at $T$), sunrays are allowed to fall on the black body through a hole in the roof of the dark room. Assuming that there is no change in the ambient temperature of the room, which of the following statement(s) is/are correct?

kirchoff's laws black body radiation heat transfer thermal properties physics

  1. The quantity of radiation absorbed by the black body in unit time will increase

  2. Since emissivity = absorptivity, hence the quantity of radiation emitted by black body in unit time will

    increase

  3. Black body radiates more energy in unit time in the visible spectrum.

  4. The reflected energy in unit time by the black body remains same.

Show answer
Correct Option: A,B,C,D
Explanation:

Since the radiation is continuously falling on the black body, the quantity radiation absorbed per second will increase.

The reason given in question is self explanatory.
With an increase in temperature, the entire Plank's curve shifts upwards and hence radiation in any spectrum will increase.
Reflected energy per unit time will be zero since black body has 0 reflectivity and hence it will remain constant.