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Simple fluid

  1. Mar 12, 2008 #1
    1. The problem statement, all variables and given/known data
    http://ocw.mit.edu/NR/rdonlyres/Phy...49-44D6-96D2-75602F179856/0/notes_dfnitns.pdf
    There it says tht the number of independent variables of a simple fluid is 2 (see the "Complete Specification" section). But the ideal gas law is PV=NkT which has 4 variables, so why are there not 3 independent variables for an ideal gas which is an example of a simple fluid?


    2. Relevant equations



    3. The attempt at a solution
     
    Last edited: Mar 12, 2008
  2. jcsd
  3. Mar 12, 2008 #2

    Mapes

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    There is another equation of state for ideal gases, [itex]U=Nc_vT[/itex]. The constraint from this additional equation reduces the number of independent variables to two.
     
  4. Mar 12, 2008 #3
    That equation will only work if V is constant.
     
  5. Mar 12, 2008 #4

    Mapes

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    No. Every student of thermo (including me) claims that at first, but no. It applies to all ideal gases under all processes.

    EDIT: To give a little more explanation, [itex]c_v[/itex] is a constant. [itex]U=Nc_vT[/itex] is an equation of state that uses that constant; it's not a process subject to constraints. We could just as well write [itex]U=N(c_p-R)T[/itex].
     
    Last edited: Mar 12, 2008
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