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Homework Help: Find net entropy change

  1. Oct 27, 2015 #1
    1. The problem statement, all variables and given/known data
    A heat engine operates between a high-temperature reservoir at 634 K and a low-temperature reservoir at 320 K. In one cycle, the engine absorbs 6540 J of heat from the high-temperature reservoir and does 2200 J of work.

    2. Relevant equations
    What is the magnitude of the net change in entropy as a result of this cycle?

    3. The attempt at a solution
    Find Qc=Qh-W=4340J
    Sc=Qc/Tc= 4340J/320K=13.56J/K
    Sh=Qh/Th= 6540J/634K= 10.32 J/K
    Net S= Sc-Sh= 3.2 J/K
    Please help. I feel like i am doing this problem correctly but I seem to be missing something. Thank you!
  2. jcsd
  3. Oct 27, 2015 #2
    Since the engine is operating in a cycle, what is the change in entropy of the working fluid?
    What is the change in entropy of the hot reservoir? What is the change in entropy of the cold reservoir? What is the total change in entropy of the working fluid and the two reservoirs?

  4. Oct 27, 2015 #3
    So the hot reservoir has a net (-) charge because it's energy is absorbed. The net change in the cold is (+) because work is being put into the system and needs to be exhausted..? I am not exactly sure of how a "cycle" would influence the entropy unless it is referring to a reversible engine.
  5. Oct 27, 2015 #4
    In any cycle, the change in any state function (like entropy), for a system experiencing the cycle, is equal to zero. The hot reservoir loses heat to the engine and the cold reservoir receives heat from the system. So, the change in entropy of the hot reservoir is negative and the change in entropy of the cold reservoir is positive.

  6. Oct 27, 2015 #5
    The correct answer is not zero :/
  7. Oct 27, 2015 #6
    I didn't say it was. I said that the entropy change of the working fluid is zero. The total entropy change, including the reservoirs, is not zero. Your original answer was correct, although you obtained this answer without understanding what is happening fundamentally. I was trying to explain the correct way to analyze the problem.

    Last edited: Oct 27, 2015
  8. Oct 27, 2015 #7
    So yeah I understand the Sh is negative that is why i subtracted it from the positive Sc. Is my equation incorrect?
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