# What does it mean when we say "a blackbody is in thermal equilibrium with it's surroundings"?

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?

• You have like three questions in one post. The norm here is that one question per post. Oct 7, 2020 at 6:23
• Does the textbook you quote indeed have an apostrophe in the possessive pronoun "its"? Then throw it away. Oct 7, 2020 at 14:43
• 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. Oct 7, 2020 at 14:46

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"?

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.

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

• "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. Oct 7, 2020 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? Oct 7, 2020 at 14:46
• You can discuss radiant energy best with bodies in a vacuum. No pun intended. Oct 7, 2020 at 14:49
• Please give me some suggestions about how I should reflect that in my answer, if you have any. Oct 7, 2020 at 14:51
• 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 Oct 7, 2020 at 18:01