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A simple thermodynamic question

  1. Jul 25, 2010 #1
    1. The problem statement, all variables and given/known data
    When a sealed Thermos bottle of hot coffee is shaken, what changes, if any, takes place in the temperature of the coffee and its internal energy? Justify your answer.

    2. Relevant equations
    Internal energy = random kinetic energy + random potential energy

    3. The attempt at a solution
    The answer given says that because work is done on the system there is an increase in internal energy. But isn't the work done used to increase the mechanical energy of the system, not the internal energy. There should be no increase in the temperature, shouldn't it?
  2. jcsd
  3. Jul 25, 2010 #2
    Logically I would think that by shaking the bottle the particles kinetic energy increases (they move faster due to the shaking) thus increasing temperature?
  4. Jul 25, 2010 #3
    but if i remember correctly the internal energy of a system excludes the energies due to the state of the body as a whole such as the (translational) kinetic energy due to the motion of the body and the (gravitational) potential energy due to the height of the body. Internal energy includes only 'random' energy, doesn't it?
    Last edited: Jul 25, 2010
  5. Jul 25, 2010 #4


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    After the thermos and the coffee inside have stopped moving, the mechanical energy is the same as what it was originally. Friction has converted the additional mechanical energy into thermal energy (internal energy).
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