Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Work by an ideal gas in a thermodynamic cycle

  1. Feb 12, 2008 #1
    1. The problem statement, all variables and given/known data

    Two moles of an ideal gas are carried around the thermodynamic cycle shown in Fig. 18-29. The cycle consists of (1) an isothermal expansion A to B at a temperature of 700 K, with the pressure at A given by pA = 9 atm; (2) an isobaric compression B to C at PC = 4 atm; and (3) an isochoric pressure increase C A. What work is done by the gas per cycle? (See attached picture)

    2. Relevant equations

    pv=nRT

    W=pdv

    P=nRT/V


    3. The attempt at a solution

    W=nRT*ln(v2/v1)

    For Pa:
    9 atm= [2 mol*(.08205784 L*atm/k*mol)*700K]/V2

    For Pb:

    4 atm= [2 mol*(.08205784 L*atm/k*mol)*700K]/V1

    V2/V1= 4/9

    W=nRT(ln(4/9)) = 93.16 J (for A to B)

    Here is where I got stuck. I can't seem to find the work for B to C without having variables in the answer. Once I can find the work from B to C I can add the work quantities to find the total (and ignore the value for C to B because the total work with no volume change is zero).

    Any help in finding work from B to C would be appreciated.

    Thank you
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Feb 12, 2008 #2

    George Jones

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What is the pressure at C? What is the volume at C?
     
  4. Jan 28, 2011 #3
    You can use PV=nRT to find the volume at point A and C
    Then, pluge into the formula W=nRT*ln(V2/V1)
    Final subtract it with the area below that triangla W - 3*101325*(V2-V1)
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook