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Homework Help: What is the engine's thermal efficiency?

  1. May 13, 2012 #1
    1. The problem statement, all variables and given/known data

    A heat engine using 110g of helium as the working substance follows the cycle shown in the figure.

    http://session.masteringphysics.com/problemAsset/1074114/5/19.P59.jpg

    What is the engine's thermal efficiency?

    2. Relevant equations

    n=W/Q_h

    3. The attempt at a solution

    Alright so up to this point I have figured out p1,p2,p3, T1,T2,T3, V1,V2, and V3.

    I tried doing W = area of triangle = .5(5-1)atm*(5.0*10^-3-1.0*10^-3)m^3 and got .008J. I assumed Q_h was 2200K because that is what I got for the temperature between T2 and T3, and that is where the heat should be going in. So I divided and 36.36% but that is wrong. Any help here? Thanks!
     
  2. jcsd
  3. May 13, 2012 #2
    Bump any help?
     
  4. May 13, 2012 #3
    Work is not a triangle in this case. You'll have to integrate dW = P*dV, where P = nRT/V.

    Also, why is Q_h a temperature of 2200 K and not a heat transfering into the engine cycle?

    Heat units = Joules

    Temperature units = Kelvin
     
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