Radiation heat of a gas

fluidistic · May 11, 2009

Can gases lower their temperature by heat radiation?
For example in a common room (at around 30°C), I know that air transfers heat by convection, but does it also transfers heat by radiation? (Like a solid body)
The Sun does, but its surface is a plasma I think.

Bob S · May 11, 2009

The heat (thermal) radiation goes as the fourth power of absolute temperature, so it is very small at room temperature.

fluidistic · May 11, 2009

Bob S said: ↑

The heat (thermal) radiation goes as the fourth power of absolute temperature, so it is very small at room temperature.

Ah ok.
But does fluids radiate? For instance in wikipedia they say

Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature.

which seems to exclude the case of any fluid radiation. As if only solid bodies could radiate.
I do not really care that air at room temperature radiates very few or a lot, but I do care if it does radiate or not. I'm just curious if it's a property of only solids and plasma, solids-plasma-gases and liquids.

Wikipedia's reference : http://en.wikipedia.org/wiki/Thermal_radiation#Properties

Count Iblis · May 11, 2009

It can be related to how well the gas absorbs thermal radiation. Consider a gas in a box in thermal equilibrium with a photon gas. The gas has a certain optical depth, so a photon has a cerain probablity of being absorbed by the gas per unit time and per unit volume. This then means that the gas must emit the same amount of radiation pr unit time and volume as it is in equilibrium. But since what the gas emits does not depend on whether or not the photon gas is actually present, this is a general result.

In case of a solid, it will absorb or reflect all the radiation incident on its boundary and then the above argument (so-called detailed balance argument) yields the familar Stefan Boltzmann law.

russ_watters · May 11, 2009

fluidistic said: ↑

But does fluids radiate? For instance in wikipedia they say which seems to exclude the case of any fluid radiation. As if only solid bodies could radiate.
I do not really care that air at room temperature radiates very few or a lot, but I do care if it does radiate or not. I'm just curious if it's a property of only solids and plasma, solids-plasma-gases and liquids.

That's just sloppy wording or a simplification by wiki, as obviously, fluids can have surfaces. And a well-defined surface isn't really even necessary....

The earth's atmosphere, for example, radiates energy into space: http://earthobservatory.nasa.gov/Features/EnergyBalance/page6.php

fluidistic · May 11, 2009

Thank you very much to all of you.

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Radiation heat of a gas

fluidistic

Bob S

fluidistic

Count Iblis

russ_watters

Staff: Mentor

fluidistic

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