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Homework Help: Finding the change of internal energy

  1. Aug 4, 2005 #1
    In moving out of a dormitory at the end of the semester, a student does 2.69 x 104 J of work. In the process, his internal energy decreases by 5.52 x 104 J. Determine each of the following quantities (including the algebraic sign): (a)W, (b) delta U, and (c)Q.

    I get the following using formula: delta U = Q - W

    (a) 2.69E4 J

    (b) -5.52E4 J

    (c) -28300 J

    I know that part a and c are right, but part b isn't for some reason I do not understand. I think it should be right, but it marks it wrong. It's probably the easiest problem I've ever encountered, but for some reason I'm having trouble getting it. Does anyone know what the answer should be?
    Last edited: Aug 4, 2005
  2. jcsd
  3. Aug 5, 2005 #2
    perhaps the change in potential energy is his initial potential energy minus the work he does. If I have 5 units of energy in me, and I mow the lawn using two, I have 3 units of energy left for the day....make sense?
  4. Aug 6, 2005 #3

    Doc Al

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    I don't understand how b could be wrong either; after all, dU is given in the problem statement. It looks correct to me. (And if b is wrong, how could c be right, since c depends on a and b.)
  5. Aug 6, 2005 #4


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    If a and c are right and the formula is right then b is right.

    If b is not right then either a is wrong, and/or c is wrong, and/or the formula is wrong.
  6. Aug 6, 2005 #5
    for some reason, my other friends use the same formula and get the answers that are the same as mine, and they get marked right, but i get marked wrong. :(
  7. Aug 7, 2005 #6


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    I would go talk to the professor who taught the class and gave the exam. Ask a friend of yours to be with you and take both sets of answers when you go to see the prof.

    What is Q, by the way?
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