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Fundamental question about thermal radiation

  1. Jul 21, 2011 #1

    phinds

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    I've always thought that the following was true but posts in another thread lead me to believe I might have a fundamental misunderstanding. The other thread was a different topic and I didn't want to hijack it, so here's my question.

    What I THOUGHT was true but am not now sure about: an object is the only thing inside a room. It's magically off the floor so no conduction and I've sucked all the air out of the room, so no convection. The body is at 300 degrees Kelvin. All the surfaces of the room are at 350 degrees kelvin. My belief has been that the body radiates and the walls radiate and because the walls are hotter, they radiate MORE, so the net result is that the body absorbs some head from the walls and it gets hotter and the walls get cooler. By exactly what amounts is not relevant.

    What I gathered from the other thread is that the only radiation is the NET radiation. That is, in this situation, the body does not radiate at all. The walls radiate until both the walls and the object are the same temperature.

    I'd appreciate it if someone who knows what they're talking about, as I apparently don't, could clarify this for me.

    Thanks
     
  2. jcsd
  3. Jul 21, 2011 #2

    Drakkith

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    That is incorrect. Both the walls and the object will radiate at any time they are above 0k. The walls radiate more energy, so eventually both will reach equilibrium.
     
  4. Jul 21, 2011 #3

    phinds

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    Well, hm ... that's exactly what I THOUGHT was the case. What lead me to think I might have it wrong was a thread on Hawking radiation out of a black hole where there were seemingly authoritative statements and (references supporting material) saying that Hawking radiation had never been observed because black holes are colder than the CMB so even if that's the only thing radiating into a black hole, it STILL couldn't have Hawking radiation because of the temperature difference.

    I guess I must have misunderstood the other thread.

    Thanks
     
  5. Jul 21, 2011 #4

    Drakkith

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    I think they mean that the radiation is drowned out from the amount of background radiation. But I'm not sure.
     
  6. Jul 21, 2011 #5

    phinds

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    I thought so too but posted a specific question that addressed that and it lead to this thread.

    I'll have to go back and review that other thread. Still haven't done that.
     
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