Heat Transfer: Conduction vs. Convection vs. Radiation

[jobyts]

As I remember learning in school, heat cannot flow from cold to hot. But isn't that true only for conduction and convection? Radiation heat is purely Infrared waves, and there should not be any issue with that. Please correct if I am wrong.

 

[Shooting Star]

Every object radiates EM energy. When it's infrared radiation, we call it radiant heat.If two bodies are at different temperatures, and both are emitting mainly infrared which is incident on the other, then in general the colder body will receive more radiation than it emits, and thus heat up, even though it's also emitting heat rays which are being absorbed by the hotter body. (Shapes have also to be taken into account.)

But if their temperatures are widely different, then which one heats up may depend on other factors, because the main radiation may not be in the infrared band.

 

[cesiumfrog]

Cue the perpetuum mobile using Faraday isolator...

 

[MrXow]

Heat can flow from cold to hot,
http://en.wikipedia.org/wiki/Maxwell's_demon
there is no law that says it cannot.

 

[clustro]

Heat an most definitely flow from cold to hot. That's what a refrigerator does after all -_-

The thing is, it will not flow in such a manner spontaneously.
Heat cannot flow from cold to hot without work input.

 

[MrXow]

It has never been observed not to flow from hot to cold but there is no law that says it cannot

 

[FredGarvin]

It has never been observed not to flow from hot to cold but there is no law that says it cannot

You are correct. However, the odds of that happening are very very very small.

 

[MrXow]

like 10^-54

 

[Shooting Star]

The OP is asking about radiant heat. Read his query.

 

[russ_watters]

The OP is asking about radiant heat. Read his query.

Yes, so let's answer it:

Radiation between a cold and a hot object involves both objects radiating photons towards each other, but since the hot object is hotter, it radiates more (via the Stefan-Boltzmann law) and the net energy transfer is from the hot object to the cold one. How much is determined by plugging both temperatures into the equation.

 

[stewartcs]

Heat can flow from cold to hot,
http://en.wikipedia.org/wiki/Maxwell's_demon
there is no law that says it cannot.

Is there a law that says it can?

 

[Shooting Star]

Yes, so let's answer it:

Radiation between a cold and a hot object involves both objects radiating photons towards each other, but since the hot object is hotter, it radiates more (via the Stefan-Boltzmann law) and the net energy transfer is from the hot object to the cold one. How much is determined by plugging both temperatures into the equation.

I have already given an answer in my first reply.

What if the colder body transparent to infra-red, which the hotter body mostly emits?