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Thread: Thermodynamics: heat transfer?

  1. #1 Thermodynamics: heat transfer? 
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    Hi,
    Can you help me figuring this exercise on thermodynamics?

    "A cylinder with a movable piston contains a certain quantity of helium. With a very slow transformation - represented in a V-p graph as a straight line - it goes from state to state . It is then subjected to a costant-volume transformation followed by a costant-pressure one, returning to the initial state".

    I had a graph but couldn't post it. Anyway, from there it was pretty simple to find the work done (area of the triangle of the three states):
    .
    It was a counterclock-wise transformation, hence the negative work.

    It then asks for , which I calculated from the ideal gas law. I didn't know the amount of substance, but I took that from state A.
    then .

    Finally, it wants the heat transfer with the outside during .
    At first, I wanted to use , but the specific heat gave me some doubts: which one should I use when the transformation is not particular (I'm referring to the fact that there are specifc specific heats when the transformation is with costant pressure/costant volume)?
    Then I thought I could use . I could calculate the difference in internal energy:
    .

    Since it is work done on the gas, it is more internal energy, not less: . Also, we are only talking about the work done from A to B, which includes the area underlying the triangle: . But I can't figure out the sign of the Q: the work causes a compression, but I don't know how much. I mean: it could be so much that the temperature went much higher than 375K, arriving then at that level giving off heat; but it could also be that the work brought the temperature under 375K and that it arrived at that level absorbing heat.
    And anyway, whatever sign I use the answer , the book says) isn't right. And I'm stuck here.

    Where is my mistake?
    Thanks in advance!
    (If something isn't clear, please tell me)


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