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Homework Help: Baffling Gas Energy Question

  1. Apr 9, 2006 #1
    If the change in internal energy of a gas during a process is 1500 J and the heat transferred to the gas during the process -400 J, what is the work done by the gas?

    I immediately though the answer to this question was 1100J, but it's not, I am completely confused by this. How can it not be 1100?
  2. jcsd
  3. Apr 9, 2006 #2


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    Q(in) + W(in) = DeltaE

    Q(in) is NEGATIVE , but DeltaE is POSITIVE.

    so, what's W(in)?

    Now notice that they're asking for W(out) ...
  4. Apr 9, 2006 #3
    Q(in) + W(in) = DeltaE
    -400 + W(in) = 1500
    W(in) = 1900

    So now how do I find W(out)? I figured that [W(in) + W(out)]/2 = change in internal energy which is how I came up with my original answer of 1100J, but that's incorrect.
    Last edited: Apr 9, 2006
  5. Apr 9, 2006 #4


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    HINT: W(out) is the opposite to W(in)
  6. Apr 9, 2006 #5


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    Work transfers Energy OUT of one object and INto another ...
    I was refering to Work that transfers Energy Into the gas as W(in).
    Energy is conserved, so that W(in) would be W(out) for some other object.

    Textbooks are notoriously flaky about which W they mean.
    I envision all Energy transfers going into my sample, which raises its T.
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