Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    Mapes

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook