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Homework Help: More PV Graph Questions

  1. Mar 9, 2010 #1
    1. The problem statement, all variables and given/known data
    A diatomic ideal gas originally at pressure Po and volume Vo is heated at
    constant volume until its pressure increases by a factor of 3. Next it is
    isothermally expanded until its pressure returns to Po. Finally it is
    isobarically compressed to its initial volume.

    a) Show the process on a PV diagram.

    b) Find Q, W, and delta E for each of the 3 legs [All entries should be in terms of the “givens”—
    Po and Vo.]


    2. Relevant equations



    3. The attempt at a solution

    Part A) Note the very crude attachment.

    Part B) I think i have this generaly correct, but im having trouble using the given terms

    Leg 1 (AB)

    Q = E = nC_v(delta T), W = 0

    Leg 2 (BC)

    Q = W, E = 0

    Leg 3 (CA)

    Q = E + W, W = P(delta V)
     

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  3. Mar 9, 2010 #2

    ideasrule

    User Avatar
    Homework Helper

    That's right. Just express n, Cv, and delta-T in terms of gamma, P0, and V0. Remember that PV=nRT.

    Do you know how to find the work done by an isothermal process?

    Yeah. Now express delta-V in terms of V0.
     
  4. Mar 9, 2010 #3
    "That's right. Just express n, Cv, and delta-T in terms of gamma, P0, and V0. Remember that PV=nRT."

    but PV isnt constant, how can is use the pv equation
     
  5. Mar 9, 2010 #4

    ideasrule

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    Homework Helper

    PV isn't constant, but P1V1-P0V0 = nR(T1-T0).
     
  6. Mar 9, 2010 #5
    ok so i just solve that equatio for n and delta-T and plug those in....

    n = (PV - initial PV)/R(detaT)..... delta T = (PV-initial PV)/nR...

    C_v is constand volume so is that just C_v = V_o
     
  7. Mar 9, 2010 #6
    leg 2....


    Q = W = nRT(ln V/V_o), E = 0

    leg 3.....

    Q = E + W, W = P(delta V) = P(v-v_o), can E be expressed in P & V
     
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