1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Energy and Entropy from P-V Diagram

  1. Mar 1, 2012 #1
    1. The problem statement, all variables and given/known data
    An ideal monatomic gas is taken through the above cycle where p1= 2 X 105 Pa and V1 = 5 cm3. During the cycle both the pressure and volume will change such that p2 = 2 p1 and V2 = 4 V1.

    a) How much work is done by the gas in going from state a to state c along path abc?
    Wabc = 3 J
    OK

    b) What is the change in internal energy in going from b to c?
    Ubc= J

    c) What is the change in internal energy during the complete cycle?
    UTOT = 0 J
    OK

    d) What is the change in entropy per mole of gas in going from b to c?
    ΔSbc = J/K

    e) What is the change in entropy per mole during the complete cycle?
    ΔSTOT= 0 J/K
    OK

    I am having problems with Parts B and D.

    2. Relevant equations
    PV=NRT
    Q = nCvdt


    3. The attempt at a solution

    I only have P and V. I cannot use the Ideal Gas Law to calculate n or T because I have two unknowns. I cannot use my Heat Equations because I am still missing n and T. I'm just stuck. If I can calculate B, I think I can get D...I just don't see any way to calculate the Heat in part B. HELP!

    Thanks.
     

    Attached Files:

  2. jcsd
  3. Mar 1, 2012 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Use: ΔU = nCvΔT = n(3R/2)ΔT

    Since PV = nRT, you can express ΔT in terms of ΔP and V (V is constant).

    Use: dS = dQ/T. What is the heat flow in terms of dT and Cv? Integrate that to get ΔS.

    AM
     
  4. Mar 1, 2012 #3
    I can't believe I didn't see that. Once I read "express ΔT in terms of ΔP and V" it all fell together. It took me less than 2 minutes to solve the problems. Thanks! :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Energy and Entropy from P-V Diagram
  1. P-V diagram question (Replies: 2)

  2. P-V diagram (Replies: 2)

  3. P-v diagram (Replies: 1)

Loading...