Entropy Change in Blackbody Radiation System in Thermal Equilibrium

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An evacuated container with volume V and at a temperature T contains black body radiation with an energy density equal to 4T4/c


S(T,V)=16VT3/3c

The container is placed in thermal contact with a heat bath at temperature Tr. If the heat capacity of the cavity material itself is negligible, show that the overall change in entropy of the universe after the system and heat bath have reached thermal equilibrium is

Stot=(4VTr3/3c)(1-t3(4-3t))

t= T/Tr

i am so stuck. please help.i have tried expanding it out to see whether it looks like anything familiar, but no

 

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r u from ucl??