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Homework Help: Isothermal Expansion: Work and Heat

  1. Apr 21, 2009 #1
    Which of the following statements are true?

    1. Heat is converted completely into work during isothermal expansion.
    2. Isothermal expansion is reversible under ideal conditions.
    3. During the process of isothermal expansion, the gas does more work than during the isobaric expansion (at constant pressure) between the same initial and final volumes.

    Carefully consider all options and choose the best one.

    A only
    B only
    C only
    A and B only
    A and C only
    B and C only
    A and B and C

    I can't seem to figure this one out. I think the answer is A only because that definitely works. I'm very unsure about the other two statements though. I believe B is incorrect because only adiabatic processes are reversible. And I believe C can be correct or incorrect based on the situation so I pick A.

    Thanks for the help guys!
  2. jcsd
  3. Apr 21, 2009 #2

    Andrew Mason

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    For A, apply the first law. What is the change in internal energy in any isothermal change? What does that tell you about the relationship between work and heat flow (W and Q)?

    For B, consider the classic four parts of the Carnot engine cycle: is isothermal expansion part of that cycle?

    For C, draw a PV diagram for isothermal expansion and for isobaric expansion. Which graph as the most area under it? What does that area represent?

  4. Apr 21, 2009 #3
    so I'm guessing that A works because the law applies W=Q.

    B actually works because in the carnot engine cycle, isothermal expansion can be reversed.

    C, the area represents work. I would say that more work is done on the isobaric.

    So A and B only is the correct answer. Am I right?
    Last edited: Apr 21, 2009
  5. Apr 21, 2009 #4
    i've noticed no one has answered yet. I kind of need this by tonight. thanks!
  6. Apr 23, 2009 #5

    Andrew Mason

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    You are. But you don't seem very confident. Is the isobaric path above the isothermal? If so, it has more area and represents more work.

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