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Heat, Force?

  1. Mar 17, 2009 #1
    1. The problem statement, all variables and given/known data
    The Internal energy of fuel in a combustion chamber increases by 75J due to 130J of heating. How much work can be done by the piston? How far does the piston move while pushing with 5000N of force?

    2. Relevant equations

    3. The attempt at a solution
    Since they gave us the info to find efficiency i divided 75J by 130J and got 57.7% eff. I plugged that into (e= w/QH) and got work = 75.01J and i plugged it into w=F *(delta x) and got .015m. I'm not sure if this is right, but the units seem to be correct.
  2. jcsd
  3. Mar 18, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Not quite.

    As you said, efficiency is
    e = w/QH

    However, you have used ΔU, not w, to calculate e=57.7%. So that's incorrect.

    What is another equation that relates U, W, and Q? It should be in your text book, early in the section or chapter on thermodynamics.
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