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Entropy change homework

  1. Apr 23, 2010 #1
    Now then, a past paper question worth 3 marks.

    What is the entropy change of a working substance taken around 1) a completely reversible closed cycle and 2) a closed cycle containing irreversible steps? Explain your reasoning.

    Now this question has me a tad confused. I think that for part 1 the entropy change is zero. My reasoning was that if you take the substance along some reversible path, say from 1 to 2, giving an entropy change of ds(1 to 2) then to go back along would give an entropy change of -ds(1 to 2). If that make sense.

    For part 2, i think ds could be > 0 or < 0. My reasoning is that the entropy change for the 'universe' must be > 0 but that the change in entropy for the working substance could be positive or negative, as long as the entropy change of the system + surroundings is > 0 .
    Hope thats clear.
    Any ideas?
    Thanks
     
  2. jcsd
  3. Apr 23, 2010 #2

    Mapes

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    Think "state variable."
     
  4. Apr 24, 2010 #3
    I know its a state variable, hence my reasoning for part 1).
    Im not sure what your hinting at to be honest. Part 2) has me confused, mainly due to the fact its talking about the working subtance itself, and not a 'system', and how to reason for part 2).
     
  5. Apr 24, 2010 #4

    Mapes

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    So does the working substance return to its original state in part 2?
     
  6. Apr 24, 2010 #5
    Iv made this question sound more complicated than it really is, havent I?!
    Since its a closed cycle i presume there is no net change in the working substance after the cycle, and therefore the entropy change in the substance is zero?
     
  7. Apr 24, 2010 #6

    Mapes

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    Sounds good to me. And as you said, the total entropy increases due to irreversibility. You've got it!
     
  8. Apr 24, 2010 #7
    Thanks muchly mapes, i was getting the old knickers in a twist for no reason there.
     
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