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Quoting from my text book - "a blackbody is a perfect radiator of radiant energy. It is in thermal equilibrium with it's surroundings and radiated as much energy per unit area as it absorbs from it's surroundings at any given time"

Is a blackbody always in thermal equilibrium with it's surrounding temperature? What is the meaning of "radiant energy"? Why should a blackbody be in thermal equilibrium with it's surroundings?

Nisha Prakash
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  • You have like three questions in one post. The norm here is that one question per post. – tryst with freedom Oct 07 '20 at 06:23
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    Does the textbook you quote indeed have an apostrophe in the possessive pronoun "its"? Then throw it away. – Peter - Reinstate Monica Oct 07 '20 at 14:43
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    What strikes me as odd is that all bodies (not only the "black" ones) move towards thermal equilibrium (it just may be somewhere else, I think, and/or they arrive there later). At the same time nothing ever is exactly at the equilibrium, given that the universe is not static -- it's just that everything moves toward equlilibria, not only in thermodynamics. – Peter - Reinstate Monica Oct 07 '20 at 14:46

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Is a blackbody always in thermal equilibrium with it's surrounding temperature?

Any body would be in thermal equilibrium with surroundings only if their temperatures are same. If the black body was at different temperature, then it would radiate/absorb heat to gain the same temperature and this will take some time.

What is the meaning of "radiant energy"?

Every body radiates some energy via Electromagnetic Radiations and this is called here as "radiant energy".

Why should a blackbody be in thermal equilibrium with it's surroundings?

Forget about black body and take any body. This body will have a natural tendency to try and maintain a thermal equilibrium with its neighbours. If it were not so, that is, the temperature of the body was different from its surroundings, then in a net exchange with its surroundings, the body will radiate/absorb heat energy from its surrounding to get same temperature . This fact is both logical and experimentally verified.

More experimentally, when the body is at different temperature,it radiates more/less heat than it gains from the surroundings and hence its temperature changes. This net exchange is given by Stefan Boltzmann Law.

$$\Delta E_{body} = eA\sigma (T^4_{body}- T^4_{surroundings})$$

Tony Stark
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  • "Any body would be in thermal equilibrium with surroundings only if their temperatures are same." Isn't that "not even wrong"? (The "surroundings" of a planet is the vacuum of space and as such doesn't have any temperature!) And it is typically not the case that the bodies visible from another body (which for astronomical objects are typically not part of its surroundings) have the same temperature, even if this body is in thermal equilibrium. It's just that they are small and far away. – Peter - Reinstate Monica Oct 07 '20 at 14:42
  • Well I am answering for someone who was new to this stuff. Your argument is correct. But I (and probably the OP) had the picture in mind that the body was in a room on Earth. Are you suggesting to explicitly mention that? – Tony Stark Oct 07 '20 at 14:46
  • You can discuss radiant energy best with bodies in a vacuum. No pun intended. – Peter - Reinstate Monica Oct 07 '20 at 14:49
  • Please give me some suggestions about how I should reflect that in my answer, if you have any. – Tony Stark Oct 07 '20 at 14:51
  • Simply forget about "equal temperature", it is a very special case (for example, you couldn't see anything in such an environs -- all photons would have the same frequency and density from all directions). What is important is that (any!) body moves towards a state where the amount of radiative energy it receives equals the one it radiates away (in a vacuum). From where it comes and where it goes is irrelevant. For example cold rocks in space (no radioactivity) at some point reach such an equilibrium but of course the visible stars are much hotter (but far away). – Peter - Reinstate Monica Oct 07 '20 at 14:56
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    True they move to such a state but what reason's motivate them to such a state if not temperature? It'd be great if you could see this related question which I had posted

    here @Peter-ReinstateMonica

     – tryst with freedom Oct 07 '20 at 18:01