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Homework Help: Find Qh of a heat engine

  1. Mar 18, 2013 #1
    1. Thot reservoir= 600K, Tcold reservoir= 300K, Output= 0.5 J of work for every J of thermal energy extracted from the hot reservoir. What is Qh?

    2. efficiency= W/Qh; Carnot efficiency= 1- (Tcold/Thot);

    3. I have found the Carnot efficiency to be 0.50. I don't know where to go after this.
  2. jcsd
  3. Mar 18, 2013 #2

    rude man

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    1. what is the 1st law?
    2. what is the 2nd law? (the assumption seems to be that you're running a Carnot engine.)
    3. what is the definition of efficiency? oops, you have that equation already I see ...

    3 equations, 3 unknowns.
  4. Mar 18, 2013 #3
    First Law: dU = Q-W; dU=0.0J because it returns to initial state (cyclic engine)

    I am NOT running a Carnot engine, it's the only thing that I could solve for.
    Second Law: thermal energy will flow spontaneously from a hot object to cold object but cannot spontaneously flow from cold to hot object.

    More on efficiency:
    Qhot= W + Qcold
    Efficiency= 1-(Qcold/Qhot)
  5. Mar 18, 2013 #4


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    So what is the efficiency?
  6. Mar 18, 2013 #5

    rude man

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    You're right - you don't need to make the Carnot assumption. Answer Dr Claude's question!
    Combine your two equations above and you have the answer!
    Last edited: Mar 18, 2013
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