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Homework Help: Heat transfer and entropy

  1. Apr 14, 2010 #1
    1. The problem statement, all variables and given/known data
    Consider two identical Einstein solids. One solid has been placed in boiling water and the other in ice water. Then the two solids are placed in good contact with each other within an insulated box. Using the concept of entropy, prove that heat will spontaneously transfer from the hotter block to the colder block. Explanations that use concepts other than entropy will receive zero credit. Make sure the logic of your explanation is clear. Equations (and perhaps graphs) should make up the bulk of your answer. Use only a minimum number of words.

    2. Relevant equations
    deltaS(total)=deltaS(A)+deltaS(B), where A is the hot solid and B is the cold solid
    deltaS=deltaQ/T, where Q is energy due to heat transfer, and T is temperature

    3. The attempt at a solution
    deltaS(total)=deltaS(A)+deltaS(B)=deltaQ(A)/T(A) + deltaQ(B)/T(B)
    T(A) much greater than T(B)
    -so deltaS(total) will be negative, which implies that the heat should transfer from the higher entropy object to the lower entropy object
    -this is what I have so far, but I turned it in and got zero credit... any suggestions? I'm wondering if I need to use the second law of thermodynamics somewhere...

  2. jcsd
  3. Apr 14, 2010 #2
    How does the total change in entropy imply what is happening with the individual bodies, let alone how is it negative? And I do suggest the second law of thermodynamics, especially since you broke it.

    Also, you might not want to use delta, since the temperature isn't constant through out the whole process. So it is best to use differentials, such as dS and dQ.
  4. Apr 15, 2010 #3
    Awesome, thanks!

    Yeah, I realized yesterday that the deltaS(total) couldn't be negative, since entropy can't be destroyed, so that was a slip on my part. Thanks for pointing it out!
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