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Homework Help: Entropy/ 2nd law of thermodynamics

  1. Apr 29, 2007 #1
    1. The problem statement, all variables and given/known data
    A mass of water at T1 is isobarically and adiabatically mixed with an equal mass of water at T2. Show that the entropy change in the universe is 2*m*Cp*ln[(T1 + T2)/(2*SQRT(T1*T2))]

    2. Relevant equations
    I'm assuming
    dS = dq/T
    q = m Cp dT

    3. The attempt at a solution
    At the very begining I was just using q = cpdT and getting total entropy = to m Cp ln(T2/T1) but that might be because that takes the process as reversible and this is an irreversible function

    Next, I tried to find the equilibrium temperature by setting q1(using T1 and Tf) and q2 (using T2 and Tf) = 0 but I dont believe that is true and I couldnt figure out a way to get the equation given in the question.

    It seems like it would be simple but I must be missing something.

    Please help
    Last edited: Apr 29, 2007
  2. jcsd
  3. Apr 29, 2007 #2
    Is this answer as simple as finding the equilibrium temperature then using that to find the entropy change in both masses and subtracting those?? If thats the case, then how does one figure out the equilibrium temp because I tried that to no avail.
  4. Apr 29, 2007 #3

    Andrew Mason

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    Science Advisor
    Homework Helper

    It is not a simple question.

    I think you are expected to take into account the vapour pressure of the water and assume that it is saturated at all times.

  5. Apr 29, 2007 #4
    I thought I might have to do that but it is isobarically mixed so I do not know how to deal with that
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