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Heat flow

  1. Oct 24, 2007 #1
    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.
  2. jcsd
  3. Oct 24, 2007 #2

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    Every object radiates EM energy. When it's infra-red radiation, we call it radiant heat.If two bodies are at different temperatures, and both are emitting mainly infra-red which is incident on the other, then in general the colder body will recieve 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 infra-red band.
  4. Oct 24, 2007 #3
    Cue the perpetuum mobile using Faraday isolator...
  5. Oct 24, 2007 #4
  6. Oct 24, 2007 #5
    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.
  7. Oct 24, 2007 #6
    It has never been observed not to flow from hot to cold but there is no law that says it cannot
  8. Oct 24, 2007 #7


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    You are correct. However, the odds of that happening are very very very small.
  9. Oct 24, 2007 #8
    like 10^-54
  10. Oct 24, 2007 #9

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    The OP is asking about radiant heat. Read his query.
  11. Oct 24, 2007 #10


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    Yes, so lets 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-Boltzman 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.
  12. Oct 24, 2007 #11


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  13. Oct 24, 2007 #12

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    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?
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