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Heat radiated

  1. Mar 26, 2016 #1
    1. The problem statement, all variables and given/known data
    A spherical body of area A and emissivity 0.6 is kept inside a perfectly black body. Total heat radiated by the body at temperature T is
    (A) 0.4σAT4
    (B) 0.8σAT4
    (C) 0.6σAT4
    (D) 1.0σAT4
    2. Relevant equations
    Stefan-Boltzmann's Law: P = AεσT4

    3. The attempt at a solution
    If it wasn't inside the black body, the answer would be (C). But it is inside one and I don't know what to do about it. Wouldn't the black body just absorb all the power radiated? We also don't know what temperature the black body is. And the question asks for total heat radiated, not power which is confusing because the answers will have dimensions of power, unless they've just omitted saying total heat per second. Honestly, I've always been a bit confused with emissivity and absorptivity. Please help.
  2. jcsd
  3. Mar 26, 2016 #2


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    I'm as puzzled by the question as you are. Yes, clearly they mean rate of heat emission. I do not interpret total heat as net heat, so do not see how the black body affects it. Maybe that is the point of the question, that it is not affected. If it means the net heat then indeed you would need to know the temperature of the black body.
  4. Mar 27, 2016 #3
    The answer is D, if it helps. Otherwise, it seems like a pretty weird question, so I guess I'll skip it.
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