# Black body

1. Apr 30, 2014

### Maylis

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

2. Apr 30, 2014

3. May 1, 2014

### Redbelly98

Staff Emeritus
No, it releases energy as electromagnetic radiation.
Not really. It absorbs all electromagnetic radiation that is incident upon it.

4. May 1, 2014

### Khashishi

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.

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.

5. May 1, 2014

### Maylis

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

6. May 2, 2014

### Khashishi

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.

7. May 2, 2014

### WannabeNewton

Have you derived, or rather seen the derivation of, the Stefan-Boltzmann law yet?

8. May 2, 2014

### Maylis

I'm familiar with it, yes. Complexities are added when one must consider view factors and such.