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Homework Help: Why can't this process run forever?

  1. Feb 2, 2014 #1
    A cyclic cycle occurring in a system immersed in surrounding, all inside an isolated box. Each cycle, net heat flow into the system from isolated surroundings in the box and the box has an inlet which just enough heat flows to balance that consumed by the cycle. why can't this process run forever?


    I know that Q=-W in this case. But I don't know how to explain.
    Last edited: Feb 2, 2014
  2. jcsd
  3. Feb 2, 2014 #2

    Simon Bridge

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    The situation is incompletely described:

    ... is there any other kind?
    What sort of cycle? What causes the cycle?

    ... "in surrounding" what?

    The box is all by itself? Or do you mean an isolating box? i.e. a box with adiabatic walls?

    Heat can flow from outside the box - so not adiabatic walls - so the box is not isolating??
    How can heat energy get "consumed" by the cycle? (i.e. what does that mean?)

    The place to start with this is to be more careful about describing the system under study.
    Be pedantic about it.
    Last edited: Feb 2, 2014
  4. Feb 2, 2014 #3
    I'm just as confused as you are. That's how the question was given to me by my teacher...
  5. Feb 2, 2014 #4
    The question just says "consider a cyclic change occurring in a system that is immersed in surroundings, all inside an isolated box.
  6. Feb 2, 2014 #5

    Simon Bridge

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    I think you'd better post the question verbatim - i.e. copy it out word for word.
    Mind you - you won't always be given everything you need to know as part of the problem statement: some of it will be in the course notes and/or delivered in class.

    But if it is really that vaguely worded, I don't think we can help you much - have you tried discussing it with your classmates? If everyone has the same problem then you can confront the teacher as a group.

    Anyway there are still some things you can figure out, like:
    What do you know about cyclic systems? (review course notes: have you covered heat engines?)
    Consider how entropy factors into this?
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