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Simple fluid

  • Thread starter ehrenfest
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  • #1
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Homework Statement


http://ocw.mit.edu/NR/rdonlyres/Physics/8-044Spring-2004/7A20B047-A049-44D6-96D2-75602F179856/0/notes_dfnitns.pdf [Broken]
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?


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The Attempt at a Solution

 
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Answers and Replies

  • #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.
 
  • #3
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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.
That equation will only work if V is constant.
 
  • #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].
 
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