Re: Directional emissivity?

Thanks for the excellent reply. I have one question. One atmospheric
physics textbook that I have (which, unfortunately, I don't have with
me at this moment) explicitly states that Kirchoff's law is also valid
for passive materials that _aren't_ in radiative equilibrium (where
emitted irradiance does not equal absorbed irradiance), contrary to
what I think you implied in your post. Or does "thermal equilibrium"
in this context mean something different from "radiative equilibrium"?
More specifically, the book uses the example of an infinite blackbody
plane in equilibrium with an infinite graybody plane, but the authors
go on to say that equilibrium is just a convenient assumption for the
purpose of illustrating Kirchoff's law, and that it's not a necessary
condition for the law to be valid.

Later tonight I can get an exact quote from the textbook since I may
have bungled it here.

.

Relevant Pages

  • Re: Directional emissivity?
    ... physics textbook that I have explicitly states that Kirchoff's law is also valid for passive materials that _aren't_ in radiative equilibrium, contrary to what I think you implied in your post. ... More specifically, the book uses the example of an infinite blackbody plane in equilibrium with an infinite graybody plane, but the authors go on to say that equilibrium is just a convenient assumption for the purpose of illustrating Kirchoff's law, and that it's not a necessary condition for the law to be valid. ...
    (sci.physics)
  • Re: Optimised antenna
    ... shows that a radiator can be any size,shape or elevation as long as it ... This is because the result of additions to Gauss's static law results ... while attaining equilibrium. ... result of that weak force by placing a radiator tipped from right ...
    (rec.radio.amateur.antenna)
  • Re: Lizard engines and rat engines
    ... >> law of thermodynamics. ... >> equilibrium - especially if they are supposed to apply to ALL ... for predicting the behavior of non-equilibrium systems. ... Jaynes proposed that Gibbs approach could ...
    (sci.bio.evolution)
  • Re: Directional emissivity?
    ... In qualitative terms, Kirchoff's law ... in thermal and radiative equilibrium. ... frequency of individual absorption and emission events. ... only one particular intensity of monochromatic irradiance. ...
    (sci.physics)
  • Re: 2nd law of thermodynamics in question
    ... I already told you I do not adhere to such a ridiculous law. ... he zeroth law of thermodynamics begins with a simple definition of thermodynamic equilibrium. ... During the process of reaching thermal equilibrium, heat, which is a form of energy, is transferred between the objects. ...
    (sci.physics)