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B Temperature increase with stirring?

  1. Apr 4, 2016 #1
    Let's say that I have a spoon and a cup of coofee, I stir it with the spoon does its temperature decrease or increase ? (There is friction force so a bit of energy should turn into heat but it's strange).
     
  2. jcsd
  3. Apr 4, 2016 #2

    berkeman

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    Staff: Mentor

    What do you think happens? There is a classic experiment where you put energy into the liquid by stirring and measure the starting and finishing temperatures. Have you seen info about that experiment?
     
  4. Apr 4, 2016 #3

    jfizzix

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    As the macroscopic fluid flow of the stirred coffee diffuses because of internal viscosity and so forth, the energy has to go somewhere, and it does. The large scale motion of clumps of fluid diffuses into small scale molecular motion, which we usually associate with thermal energy.

    Assuming all other things constant, the temperature would increase, but by a really tiny amount. It would not be worth trying to heat up coffee by mechanical motion.

    2 kilojoules of energy is about what you'd need to heat a glass of water by just 1 degree Celsius.
    2 kilojoules of energy is also the amount of kinetic energy in a 4 kilogram bowling ball rolling at highway speeds (100kph)

    The amount of kinetic energy in a swirling pot of coffee is a lot less than that of a bowling ball
     
  5. Apr 5, 2016 #4
    I know Joule's experiment but you need to consider that the spoon has a lower temperature than coofee
     
  6. Apr 5, 2016 #5
    In the real world, I would think you would have to stir really fast and furious to keep your coffee warm, and even then it would not help much. Afterall, the coffee is loosing heat through its container, and evaporation, and stirring would help that along. I don't think realistically you could keep up.
     
  7. Apr 5, 2016 #6
    Quantum mechanics is the best way to heat up your coffee. Subject the water molecules to microwave radiation, the increase in rotational energy will warm your coffee just fine. Much better than Joule heating, and no spoon to cool things down, just the cup pulling heat out of the system. (Of course I am being tongue in cheek here.)
     
  8. Apr 6, 2016 #7

    A.T.

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    The answer also will depend on the initial temperatures of the cup and the surrounding air.
     
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