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A small, easy algebraic question about the First Law of Thermo for SISO

  1. Mar 28, 2012 #1

    o_O. I know this is a really stupid question, but how the heck did the Q - W get to the other side of the equation? Shouldn't it be W - Q? I'm going to feel really stupid when I hear the answer, I'm sure, and I'm already done with the class so it's ultimately irrelevant, but I'd love to know.
  2. jcsd
  3. Mar 29, 2012 #2


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    Gold Member

    Since you don't have an expert answer yet, I will try for an easy one. I am sure the experts will shoot me down if my thinking is wrong...

    Q(dot) - Ws(dot) is still on the left in the final equation.

    Substitute each term for a letter. Let us use u, v and w. Put a pair of brackets round Q(dot) - Ws(dot) and that is u.

    Then you start with u + v - w = 0

    Adding w gives u + v = w

    Subtracting v gives u = w - v

    u still equals Q(dot) - Ws(dot)

    (The term on the right hand side has been simplified from w - v for your final equation).
    Last edited: Mar 29, 2012
  4. Mar 29, 2012 #3
    Dude, that makes total sense. It simultaneously explains where the "change in" signs on enthalpy and kinetic and potential came from. Thanks a ton.
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