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Thermodynamic cycle and entropy

  1. Mar 30, 2010 #1
    Alright, I'm confused about why thermodynamic cycles can never have 100% efficiency. There reversible processes right? And a reversible process has zero change in entropy, so then why can't they achieve 100% efficiency. I thought entropy was the measure of energy that cannot be used. How would you measure efficiency if the system increases in entropy?
  2. jcsd
  3. Mar 30, 2010 #2


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    Homework Helper

    In a thermodynamic cycle, not all of the energy inputted is converted into work. Some heat is always rejected. Not all processes are 100% reversible.
  4. Mar 30, 2010 #3


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    Staff: Mentor

    .....so why no try buiding a cycle with all reversible processes, so no heat is rejected? You'll find you can't. Real processes that can be combined into real thermodynamic cycles will always result in a cycle with some entropy.

    Try drawing a cycle with no heat rejection on a t-s diagram and see if you can find real processes (devices) that do what the t-s diagram suggests.
    Last edited: Mar 30, 2010
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