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Homework Help: Isothermal Expansion Question

  1. May 10, 2009 #1
    1. The problem statement, all variables and given/known data
    During an isothermal expansion, a confined ideal gas does -150J agaisnt its surroundings. Which of the following describes the heat transfer during this process?

    A 150J of Heat was added to the gas
    B 150J of Heat was removed from the gas
    C 300J of Heat was added to the gas
    D 300J of Heat was removed from the gas
    E No heat

    2. Relevant equations
    U = Q + W
    In isothermal expansion, Q = -W

    3. The attempt at a solution
    For this multiple choice problem, I thought the answer would be B. This is becaise the gas does -150J of work agaisnt its surroundings. This means that the surrounding did 150J of work into the gas. Thus, I would expect that if we Followed Q = -W, that 150J of heat was removed from the gas. However, the answer is (A).
  2. jcsd
  3. May 10, 2009 #2
    Q must be positive. Because the gas does work against it's surroundings, the gas loses energy. So W is negative.

    Q = -(-150) = 150

    If you still cant see it
    so W = -Q
    -150 J = -Q
    So Q = +150J.

    Giving the answer A to be correct.

    Hope this helped?
  4. May 10, 2009 #3


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    Staff Emeritus
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    jetpackman, welcome to Physics Forums.

    Something isn't right here. If the gas is expanding, it is doing a positive amount of work on the surroundings. Are you sure the problem statement says the gas is expanding?

    At any rate, Q = -W as you said, since ΔU=0 for an ideal gas & isothermal process.
  5. May 11, 2009 #4

    Andrew Mason

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    Science Advisor
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    The convention is to write the first law as [itex]\Delta Q = \Delta U + W[/itex] where Q is the heat flow into the gas and W is the work done by the gas. U is the internal energy of the gas. In this case, there is no change in U, and there is an expansion of the gas so the gas does positive work on its surroundings. This means that the negative sign for work done by the gas on the surroundings is an error. The magnitude of the work is 150J.

    [tex]\Delta Q = W = 150 J[/tex]

    So the answer is A. But it was not your fault. Bad question.

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