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Homework Help: 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


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    Science Advisor
    Homework Helper
    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,


    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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