Does a Black Body Absorb or Release Energy?

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Discussion Overview

The discussion revolves around the behavior of black bodies in terms of energy absorption and emission, particularly focusing on whether they release energy as heat or only absorb energy. The scope includes theoretical concepts related to thermal radiation and practical applications in radiation heat transfer.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that a black body emits as much radiation as it absorbs, suggesting a steady state condition.
  • Others argue that while a black body absorbs all incident electromagnetic radiation, it specifically releases energy as electromagnetic radiation rather than heat.
  • A participant clarifies that energy flows from a hotter black body to a colder one until they reach the same temperature, indicating that steady state is only achieved at equilibrium.
  • It is noted that the spectrum of radiation emitted by a black body depends solely on its temperature, regardless of the material composition.
  • One participant raises a question about the use of the term "heat," indicating that it can be ambiguous and is better defined in the context of energy transfer between bodies at different temperatures.
  • Another participant mentions the complexities introduced by view factors when applying the Stefan-Boltzmann law in practical scenarios.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of energy absorption and emission by black bodies. There is no consensus on whether the term "heat" is appropriately applied in this context, and the discussion remains unresolved regarding the nuances of these concepts.

Contextual Notes

Limitations include potential confusion surrounding the definitions of heat and energy, as well as the assumptions made about steady state conditions and the specific contexts in which black bodies are discussed.

gfd43tg
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Hello

I am wondering, does a black body release energy as heat, or does it only absorb all energy that basically comes into contact with it?

Thanks
 
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Maylis said:
Hello

I am wondering, does a black body release energy as heat, ...
No, it releases energy as electromagnetic radiation.
... or does it only absorb all energy that basically comes into contact with it?

Thanks
Not really. It absorbs all electromagnetic radiation that is incident upon it.
 
UltrafastPED said:
A black body emits as much radiation as it absorbs; it is a steady state condition.
No, that's not correct. Maybe in some sense it is correct, but that will only be confusing. If you put two black bodies in the universe, energy will flow from the hotter one to the colder one and you only reach steady state after they reach the same temperature.

A black body emits a particular spectrum of electromagnetic radiation which only depends on the temperature of the black body. In other words, any two black bodies of the same temperature, no matter what they are made of, have exactly the same surface brightness and look the same through a spectrometer.

http://en.wikipedia.org/wiki/Black-body_radiation

It's a useful concept because most sufficiently hot and absorbent objects with enough internal degrees of freedom (like stars) have spectra which are close to the black body spectrum.

Maylis, are you possibly confusing a black body with a black hole? A black hole is a black body, but they are very different concepts.

A black body releases energy through all available avenues, but usually when people are talking about a black body, they only care about the electromagnetic emission.
 
No, I'm learning about black bodies in the context of radiation heat transfer, so I want to calculate the heat energy released or absorbed by a black body
 
Ok, "heat" is a tricky term, and there's nothing wrong with how you used it in the previous post, but in your original post, when you use heat as a noun without much context, it is very unclear what you mean. Heat doesn't describe a particular type of energy, but rather categorizes energy by how it is behaving statistically. Heat is energy that travels from hot things to colder things.

A black body will absorb any light hitting it, and it will emit a blackbody spectrum.
 
Maylis said:
No, I'm learning about black bodies in the context of radiation heat transfer, so I want to calculate the heat energy released or absorbed by a black body

Have you derived, or rather seen the derivation of, the Stefan-Boltzmann law yet?
 
I'm familiar with it, yes. Complexities are added when one must consider view factors and such.
 

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