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A question about heat transfer

  1. Apr 20, 2010 #1
    Hi. If there are two objects A and B, the temperature of A is TA, the temperature of B is TB, and my questions are:

    1). If TA = TB, their internal energy (which is the kinetic energy plus some other forms of energy) might not be the same. In this case, A and B are in thermal contact. Since TA = TB, no heat flow happens. Am I correct? The internal energies of A and B remain the same although the internal energy of A may be larger than the internal energy of B.

    2). If TA > TB, heat flow is from A to B. Here, I know that the internal energy of B will increase (TB might not be increased though in the case of phase transition). However, how about the change of the internal energy of A? Is that all the heat flow from A is from the kinetic energy of A's molecules? or some of the heat flow come from the kinetic energy of A and some of it come from the other forms of energy of A such as the potential energy?

  2. jcsd
  3. Apr 20, 2010 #2
    1) Is completely correct.

    2) Has merits and is nearly there but internal energy can also appear as strain energy as well as kinetic energy of the molecules. So if we take two strips of differing metals and fix them rigidly together and then heat one strip. We will find that the heat will be transferred from one to the other until their temperatures equalise. However the resulting compound strip will have bent due to the differing coefficients of expansion of the metals. The resulting bent strip will have a higher stain energy than the unbent strip.
    This strain energy can only have come from the supplied heat.

    Note for the above strip

    [tex]Q = \Delta U + W[/tex]

    W = zero since no work is done on or by the strip on the external environment.
  4. Apr 21, 2010 #3
    Thank you for your reply. I still have a question for you second point. If you heat strip A, A absorbs heat and some of the energy turn into the kinetic energy of A and some of it turn into the strain energy of A. Since strip A and strip B are in temperature difference now, heat flow from A to B is started. It is clear that the heat flow B absorbed turns into both the kinetic and strain energy of B since B also bent, but how about the energy lost of A? Is that all the energy A transferred to B originated from the kinetic energy of A? If this is not true, what is the microscopic mechanism of transfer the non kinetic energy as heat to other objects?
  5. Apr 21, 2010 #4
    Elementary thermodynamics does not tell us about the intermediate states only the end result. Intermediate states not normally definable and the subject of modern research.
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