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Homework Help: Mono-thermal source

  1. Feb 14, 2006 #1
    Applying the 1st and 2nd law of thermodynamics to a cycle, show that it is impossible to build a heat engine with only one heat resevoir (T ext.)

    First law : dU=dQ - dW

    2nd law : It is impossible for any system to undergo a process in which it absorbs heat from a reservoir at a single temperature and converts the heat completly to mechanical work, with the system ending in the same state in which it began.

    Where do I begin and is T ext.?
  2. jcsd
  3. Feb 14, 2006 #2

    Andrew Mason

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    First Law: Consider the change in internal energy if the heat is flowing from one reservoir to another at the same temperature. What does that say about the relationship between dQ and dW?

    Second Law: All the heat cannot be converted to work. So what does that tell you about the value of dQ (and dW)?

  4. Feb 14, 2006 #3
    Okay so dU must be 0 and if there were true then by first law

    W = -Q but if temp is constant than

    Q = mCdelta T => Q =0

    So W = 0

    So the efficiency is 0

    Is that why?
  5. Feb 14, 2006 #4

    Andrew Mason

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    There is no work done. So it is not an engine. (The efficiency is undefined).

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