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Thermodynamics, specific heats

  1. Mar 20, 2009 #1
    1. The problem statement, all variables and given/known data
    A piston-cylinder device contains 1.2kg Nitrogen gas at 120 kPa and 300K. The gas is now compresses slowly in a polytropic process during wich PV^1.3 = constant.
    The process ends when the volume is reduced by one-half.
    Determine the entropy change of Nitrogen during this process.

    My question here is about what specific heat constant to use.


    2. Relevant equations
    I thought i had to use Cv when the volume is constant.
    And Cp when the pressure was constant.


    3. The attempt at a solution
    As I can see, in this process. Neither the pressure or the volume is constant?
    But in the solution provided, tey are using the Cv .
    Why is that? The volume did get reduced by one-half:confused:

    Im new to Thermodynamics, and not very good at it..
    But this is confusing me. Can someone explain this to me? How to know when to use what constant :)
     
  2. jcsd
  3. Mar 21, 2009 #2

    Mapes

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    Science Advisor
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    Gold Member

    Could the process have been divided into constant-volume and constant-pressure parts to simplify? This is a common strategy. But it's hard to say without seeing the solution.

    Another possibility is that the constant-volume specific heat is being used in the equation for internal energy for an ideal gas,

    [tex]E=mc_VT+E_0[/tex]

    which is always true (and can be used to find the change in internal energy of an ideal gas for any process if one knows the change in temperature).
     
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