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Homework Help: 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

    ##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.

    ##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!
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