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Homework Help: Cherry Pie Entropy

  1. Dec 5, 2013 #1
    1. The problem statement, all variables and given/known data

    When 1.70 ✕ 105 J of heat enters a cherry pie initially at 20.0°C, its entropy increases by 470 J/K. What is its final temperature?

    2. Relevant equations

    3. The attempt at a solution
    I have no clue
  2. jcsd
  3. Dec 5, 2013 #2
    Sorry, but you have to try something. Do you know the equation for how the heat that enters is related to the temperature rise? Do you know the equation for how the entropy change is related to the initial and final temperatures. Both these equations involve the heat capacity.
  4. Dec 5, 2013 #3
    Generally, S = Q/T. Kind of simplistic, but that is probably what you're going to use to find the net heat transfer into the cherry pie. Then, you'll need to know the heat capacity of the cherry pie to determine the change in temperature. Does the problem statement give the temperature of the surroundings? If so, I'd use that as the temp at which the heat transfer occurs. And if this is a college thermo class, dS = Cavg*ln(t2/t1)
  5. Dec 5, 2013 #4
    Something like the last equation is the one to use, but I'm sure they want you to assume that the heat capacity is constant. You don't need to know the surroundings temperature, so don't waste your time looking for that.

    So again, in terms of the mass of the pie, the heat capacity, and the initial and final temperatures, what is the change in entropy? You don't need to know the mass or the heat capacity, because they are going to cancel out. And again, in terms of the mass of the pie, the heat capacity, and the initial and final temperatures, what is the amount of heat that enters? Can you combine these two equations?

  6. Dec 5, 2013 #5
    You know, I somehow completely missed the fact that Q was given in the problem statement. I thought the poster was only given T1 and dS. If Q is known, it should be easy enough to substitute something like Q=C(T2-T1) into the first equation, and then C will cancel.
  7. Dec 5, 2013 #6
    According to PF rules, we really shouldn't be doing the problem for suchjoe. We should just be giving him little hints so that he can analyze it for himself. I feel like I've already revealed too much about how to do it.
  8. Dec 5, 2013 #7
    Sorry. I'm new here.
  9. Dec 5, 2013 #8
    No problem. I went through the same thing when I was just starting. Anyway, welcome to Physics Forums!!!

  10. Dec 5, 2013 #9
    thanks guys got the answer
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