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Homework Help: PV Diagram calculations

  1. Jan 19, 2010 #1
    1. The problem statement, all variables and given/known data

    1 litre of a monatomic ideal gas starts from an initial state (i) at T = 300K and P = 10^5 N/m^2. It (the gas) is changed to a final state (f), such as that its pressure decreases 20% and its volume double.

    a) Draw a PV Diagram mark its initial state and final state upon it

    b) Calculate the temperature of its final state

    c) Calculate the change in internal energy caused by transition from the initial state to the final state.

    d) Calculate the work done during the transition.

    2. Relevant equations

    Ideal Gas Laws
    N = n*Na
    (3/2kT) * number of molecules

    3. The attempt at a solution

    a) Find initial volume by using V = 1/P so 10^-5 m^3 then to find the final volume find double this so 1/50000 m^3 and find the final pressure by finding 4/5 of 10^5 which is 80000. Then go ahead plotting these point on the graph I have drawn with relevant scales.

    b) Find the number of molecules in the gas with N = n*Na (6.02*10^-23) so after using n = PV/kT (10^5 * 10^5 / 1.38*10^-23 * 300 = 2.5*10^20) to get n. This gives 0.015 (2.5*10^20 * 6.02*10^-23)

    Then I did Tf = (PfVf)/kN ((80000*1/50000)/1.38*10^-23 * 0.015) to get 7.73*10^25K (I'm sure this is wrong)

    c) Use (3/2kT) * number of molecules but I don't know whether to use the initial or final temperature.

    d) Use W = integrate sign v2 v1 PdV (do you use the initial or final pressure?)

    Help would be appreciated very much.
    Last edited: Jan 19, 2010
  2. jcsd
  3. Jan 19, 2010 #2
    The problem says the volume is initially 1L, which is .001 m^3
    n = PV / RT (R is the gas constant)

    It asks for the change so you're looking for U_final - U_initial
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