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!

Work, reversible expansion

  1. Dec 6, 2011 #1
    1. The problem statement, all variables and given/known data

    Okay i feel like i'm going crazy. Here is a sample problem my professor gave me. But I cannot figure out for my life how he got his answer and it hsould be very simple.


    A 10.0 mol sample of a perfect gas having Cv = 2.0R undergoes a reversible expanion form 5.00 L to 20.0 L at 5.00 atm of pressure. Calculate q, w, delta U, and delta H for this process..

    Solving for work, he has...


    w = -PdV = -5(20-5) = -22.5 L atm = -22.8 J

    HOW ON EARTH does he get those numbers?







    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Dec 6, 2011 #2
    dV is the change in volume, which is (20.0-5.00 L)=15.0 L
    -P is the given pressure x(-1), -P = -5.00 atm
    w = -PdV= -5.00*15.0 L atm = -75.0 L atm
    Since L atm is a unit of work, it can be converted to J:
    1 L atm = 101.325 J, so -75.0(101.325) J = -7.60x10^3 J = w

    I'm not sure what else your professor did to get to his answer, but this is what I got based on what was given.
     
  4. Dec 6, 2011 #3
    dV is the change in volume, which is (20.0-5.00 L)=15.0 L
    -P is the given pressure x(-1), -P = -5.00 atm
    w = -PdV= -5.00*15.0 L atm = -75.0 L atm
    Since L atm is a unit of work, it can be converted to J:
    1 L atm = 101.325 J, so -75.0(101.325) J = -7.60x10^3 J = w

    I'm not sure what else your professor did to get to his answer, but this is what I got based on what was given.
     
  5. Dec 6, 2011 #4
    thank you! I just needed confirmation that I wasn't missing something. That was exactly what I thought the answer should be
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook