1
$\begingroup$

My text book has a question which says that the emissive power of a black body isn't one but the answer states that the absorptive power is 1, considering that $$e=a \tag{Kirchoff's law}$$

and a black body is defined as an object which has $$e=1$$ Then why isn't $a=1$

  1. Choose correct options

(a) Good absorbers of a particular wavelength are good emitters of same wavelength.This statement was given by Kirchoff.

(b) At low temperature of a body the rate of cooling is directly proportional to temperature of the body.This statement was given by Newton.

(c) Emissive power of a perfectly black body is 1

(d) Absorptive power of a perfectly black body is 1

The answer is given as (a,d)


Waves and Thermodyanamics by DC Pandey 15th edition

$\endgroup$
2
  • $\begingroup$ Can you quote the textbook question for context and accuracy? $\endgroup$
    – J. Murray
    Nov 11, 2018 at 15:18
  • $\begingroup$ @J.Murray Done.. $\endgroup$
    – hjhjhj
    Nov 11, 2018 at 15:29

2 Answers 2

1
$\begingroup$

For blackbody $a=e=1. a=1$ implies that the blackbody absorbs all radiation falling on it. For anything that is not a blackbody, $0≤e=a<1$

$\endgroup$
1
$\begingroup$

The emissivity ($e$) of a perfectly black body is 1. Emissivity is the ratio of energy emitted by a body to the energy emitted by a black body. The emissive power is the energy emitted per unit area per unit time. The emissive power of a black body is not 1, and it varies with temperature.

It is true that emissivity = absorptive power = 1 for a black body, i.e., $e=a=1$

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.