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Homework Help: Carnot heat problem

  1. Feb 4, 2010 #1
    1. The problem statement, all variables and given/known data
    Problem: A heat engine is connected to a 300watt motor. If we power the motor for 4hrs and heat engine is 40% efficient,how much heat is extracted from heat source in?

    2. Relevant equations
    Given: Q= heat QC=waste heat QH=W+QC and Power=E/t or Work/t

    3. The attempt at a solution This is a diagram equation of heat.
    QH-----l----->QC
    l
    l
    V
    300w

    300W(3.412btu/1W)= 1023.6btu (4hr) = 4094.4 btuh
    ----- 4094.4btu(40%)=1637.8btuh
     
  2. jcsd
  3. Feb 5, 2010 #2

    ehild

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    Homework Helper

    The engine uses more heat energy than the work it yields: the work is 40 % of the heat: W=0.4*Q.

    Do you really use btu units for heat instead of joule?

    ehild
     
  4. Feb 5, 2010 #3
    So, are you saying that time is not used here (4hrs)? its just the starting heat loss.

    yes btu, this is more about residential usage in US.
     
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