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Homework Help: Radiation problem

  1. Feb 15, 2008 #1
    1. The problem statement, all variables and given/known data

    Black-body radiation, at temperature Ti fills a volume V. The system expands adiabatically
    and reversibly to a volume 8V. The final temperature T_f = xT_i, where the factor x is equal to
    (a) 0.5 (b) 2.8 (c) 0.25 (d) 1

    2. Relevant equations
    3. The attempt at a solution

    Correct answer should be (d)1 because, photons do not feel attraction or repulsion potential to each other.So, increasing volume adiabatically should not reduce or increase its temperature.

    Please check if I am correct.
  2. jcsd
  3. Feb 15, 2008 #2


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    This is incorrect. The Stefan-Boltzmann Law tells us that the energy and entropy of blackbody radiation in a box are [itex]U=bVT^4[/itex] and [itex]S=\frac{4}{3}bVT^3[/itex], respectively, where [itex]b[/itex] is a constant (reference: Callen's Thermodynamics). I'll leave the rest to you.
    Last edited: Feb 15, 2008
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