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Thermodynamics - obtaining quenching entropy

  1. Oct 26, 2008 #1
    I'm having a problem determining the total entropy change in a metal quenching problem. Here is the information I am given (subscript 'm' is for metal, 'w' is water):

    mm = 20 kg
    mw = 1000 kg
    T1m = 800 C
    T1w = 30 C
    Cpm = .4 kJ/kg K
    Cpw = 4.18 kJ/kg K

    With this information, I found T2 to be 304.6 K.

    How can I obtain the entropy change with this information? I have no pressure values, and all my entropy equations involve pressure, or specific volume which depends on pressure. Thanks for any help.
     
  2. jcsd
  3. Oct 26, 2008 #2

    Mapes

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    Do you know of an equation that connects dS to Cp that you might be able to integrate?
     
  4. Oct 26, 2008 #3
    I did find a relationship, but it is returning negative values for me, so something is amiss...

    s2 - s1 = Cp ln(T2/T1)

    I'm trying to find the total entropy change. Would that mean that I sum the entropy change in the quenched metal and the water?
     
  5. Oct 26, 2008 #4

    stewartcs

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    Yes.

    CS
     
  6. Oct 26, 2008 #5

    Mapes

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    Yes. It's not unusual for entropy to decrease; it happens whenever a hot object cools down.
     
  7. Oct 26, 2008 #6

    stewartcs

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    You'll need to consider the mass of each as well in your equation.

    CS
     
  8. Oct 26, 2008 #7

    stewartcs

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    Caveat:

    But the total change in the universe will always increase.

    CS
     
  9. Oct 26, 2008 #8
    Wow, thanks for all the responses everyone :). OK so here is what I came up with, and it seems to be giving me the right answer, but just for future reference:

    s2-s1 = mmCpm ln(T2/Tm1) + mwCpw ln(T2/Tw1)
     
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