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!

Change in Internal Energy

  1. Aug 31, 2013 #1
    Hello, this is my first post, so I apologize in advance if it's not quite up to par.

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

    A gas is compressed at a constant pressure of 0.639 atm from 5.41 L to 2.3 L. In the process, 470 J of energy leaves the gas by heat.
    a) What is the work done on the gas (J)?
    b) What is the change in its internal energy(J)?

    2. Relevant equations

    a) ##W=-PΔV##
    b) ##ΔU=Q+W##

    3. The attempt at a solution

    a)
    ##P = 0.639atm {\frac{1Pa}{9.872*10^-6atm}} = 64728.52512Pa##
    ##ΔV = V_f - V_i = 2.3L - 5.41L = -3.11L {\frac{1*10^{-3}m^3}{1L}} = -0.00311m^3##
    ##W = -P ΔV = -6.4728.52512Pa * -0.00311m^3 = 201.3057131J##

    I submitted my answer to part a and it was correct. Part b is what I need help with. I believe that I am doing it correctly, but my answer is returned as incorrect. My incorrect work and answer appears below.

    b)
    ##ΔU = Q + W = -407J + 201.3057131J = -205.6942869J##

    Any insight into why my work/answer for part b is incorrect would be greatly appreciated!
     
  2. jcsd
  3. Aug 31, 2013 #2
    If your answer to part a is correct, then I can't see what's wrong with your answer to part b (see EDIT)

    Is this a multi choice question that is automatically assessed?

    EDIT: Just noticed that in the question statement you say that 470J of energy leaves, but in the calculation for part b you use the figure 407J
     
    Last edited: Aug 31, 2013
  4. Aug 31, 2013 #3
    Unfortunately, this isn't a multiple choice question. I have to submit my answers electronically and a computer grades them. I have confirmation that part a is correct and that my answer for b is incorrect.
     
  5. Aug 31, 2013 #4
    Ok. Can you confirm which value of heat leaving the gas is correct: 407 J or 470 J
     
  6. Aug 31, 2013 #5
    It is 470, thus this would be my error in part b. Thank you very much for helping me find my silly mistake. The correct answer would be -268.6942869J.
     
  7. Aug 31, 2013 #6
    You're welcome. It happens to everyone now and then!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Change in Internal Energy
Loading...