1. Not finding help here? Sign up for a free 30min 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 done on compressed gas

  1. Oct 2, 2012 #1
    1. The problem statement, all variables and given/known data

    Pt. A) A gas is compressed at a constant pressure of 0.632 atm from 7.11 L to 5 L. In the process,
    440 J of energy leaves the gas by heat.
    What is the work done on the gas?
    Answer in units of J

    Pt. B)

    What is the change in its internal energy?
    Answer in units of J

    2. Relevant equations

    PV = nRT
    U = Q + W
    1 atm = 101325 pa
    1L = .001m^3

    3. The attempt at a solution
    A)

    .632atm = 64037.4 pa
    7.22L = .00711 m^3
    5L = .005 m^3

    W = P(Vf -Vi)
    =
    64037.4(.005-.00711)
    =
    -135.189

    U = Q+W

    U = -135.189 -440
    =
    -575.1189

    OR (done other ways)
    : 304.881
    : -304.881
    : 575.118914

    All of these were wrong. Please help?

    B) ...
     
    Last edited: Oct 2, 2012
  2. jcsd
  3. Oct 2, 2012 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Is this the work done ON the gas or the work done BY the gas? What does the question ask for?
    What is W in U = Q + W? (ie is it work done ON or BY the gas?).

    AM
     
  4. Oct 2, 2012 #3
    It asks for the work done ON the gas. The reasons its W = P(Vf-Vi) and not W = -P(Vf-Vi) is because its work ON gas not BY the gas. In U = Q + W, W is work on gas presumably, though I'm not 100% sure.
     
  5. Oct 3, 2012 #4

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    This is important. Positive work is done BY the gas when it expands. The work done BY the gas is PΔV where ΔV>0. Positive work is done ON the gas when it is compressed. So the work done ON the gas is -PΔV.

    So, in the expression U = Q+W, the W is work done ___ the gas.(hint: if W is positive is there an expansion or compression?).

    AM
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Work done on compressed gas
  1. Work done by a gas: (Replies: 5)

  2. Work done by a gas (Replies: 6)

Loading...