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First law of thermodynamics applied to Compression and Expansion

  1. Sep 20, 2012 #1
    How does the first law of thermodynamics apply to the Compression And expansion of a gas at constant temperature ?

    If the gas is at a constant temperature and is being compressed, then
    ΔQ = 0
    ΔU = +
    W(on the system) = +


    Are those(^^) correct ?

    The equation being :
    ΔU = ΔQ + W(on the system)
     
    Last edited: Sep 21, 2012
  2. jcsd
  3. Sep 20, 2012 #2
    Compression or expansion at constant temperature means heat is extracted or added.
     
  4. Sep 20, 2012 #3
    At constant temperature ΔQ is not zero. ΔU is zero, for ideal gas.
     
  5. Sep 21, 2012 #4
    would u explain "why" ?
     
  6. Sep 21, 2012 #5
    Originally this was an experimental fact about gasses. Nowadays there is maths to back it up, and we know that it only applies to ideal or perfect gasses.




    http://demonstrations.wolfram.com/JouleExperimentOnFreeExpansion/

    If you can cope with the maths google has lots on the Joule experiment and the Joule-Thompson experiment.

    If not you can take it as a definition of a perfect gas.

    The internal energy of a perfect gas depends only on its temperature.
     
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