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2nd Law of Thermo: Specific entropy

  1. Feb 6, 2012 #1
    1. The problem statement, all variables and given/known data

    A parcel of dry air is lifted isothermally such that its pressure decreases from 1000 hPa to 850 hPa. Calculate the change in specific entropy of the parcel.

    2. Relevant equations

    ΔS=n*R*ln(p1/p2)

    3. The attempt at a solution

    Using R=8.314 J/mol K
    p1=1000hPa
    p2=850hPa

    And assuming n=1

    ΔS=1.35J/K

    ---

    The problem is that's just the change in entropy. I need the change in specific entropy, which units are in J/g*K.

    The problem gives no information on the amount of mass within the parcel. Even if I had the mass, I'm not sure how I would find the specific entropy with it. What am I missing?

    edit: Nevermind. R is specific for dry air which is R=287 J/kg*K
     
    Last edited: Feb 6, 2012
  2. jcsd
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