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Thermodynamics brain teaser

  1. Aug 4, 2005 #1
    perhaps I'll get more replies here

    "explain how liquid water can go from 25C/1 atm to 30C/1 atm in a process for which q<0"

    I was thinking perhaps that since , [tex]q=mc_p \Delta T [/tex] how can q be negative when the temperature change is positive? , maybe it has something to do with the specific heat, however the only other factor which effects cp is temperature right? Well since the tempreature change is already given, I'm not quite sure how one can say that it is due to cp, me thinks it shouldn't be. unless the average change in cp is negative for an increase in temperature.

    I guess it has to do with internal energy, but how is it that q is not always dependent on temperature then when we are given the q equation? How can we explain the situation in terms of the heat equation (above).
  2. jcsd
  3. Aug 5, 2005 #2
    I have no idea. I thought specific heat was a constant??

    Is this a legit question?
  4. Aug 5, 2005 #3


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    Just because heat flux is zero or negative, doesn't mean work input is zero or negative. If you put work into a fluid, you raise the temperature.
  5. Aug 5, 2005 #4


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    I was reading the question and I couldn't see Q_Goest's response, but I was thinking the same thing. Energy equation Qin - Wout = U2-U1
  6. Aug 5, 2005 #5
    ahh that sounds plausible, thanks for the suggestions so far, any other suggestions?
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