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Understanding entropy

  1. Nov 3, 2013 #1
    I posted this question a couple days back, but it got removed because it looked like a homework question (which, I suppose, is flattering, since I came up with it on the way home from work, and I'm not even a student, let alone a teacher)...so i'm going to try to rephrase it -- but because this is the most concise formulation I could think of, here's the original:

    I'm not even sure there's enough information there to get the entropy; because, if I understand correctly, the entropy is a measure of the number of "microstates" (in this case, the coordinates/momenta of each ball) that give rise to a given "macrostate"...and I'm not sure what a macrostate would be here: I can't think of any "macro" variables analogous to heat, etc.
    The closest I can think of would be that you would get the same "macrostate" by swapping any of the identical balls, or rotating the bucket...

    I picked the state in (1) because it seemed like it would be the state with the highest entropy, because intuitively, if you dropped three cueballs in a bucket and rattled it around, eventually the balls would settle down next to each other on the side...but I'm trying to figure out away to express this quantitatively.

    So, any guidance, corrections, thoughts, musings, etc would be appreciated.

  2. jcsd
  3. Nov 4, 2013 #2


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    S = k log(W), where W is the number of microstates accessible to the system.

    For each microstate available the total energy of the system is unchanged. Different microstates are rearrangements of the energy of the system. See:
  4. Nov 4, 2013 #3


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    In the OP, there are two macrostates. In each, all the microstates have the same energy, so you can compare the entropies of the macrostates.
    jjustinn, I think you need to specify more details. The way you describe (1), the three balls only just fit in the bottom of the bucket. That being so, the momenta of the same three balls moving in the bottom of the bucket are not independent.
  5. Nov 4, 2013 #4


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    I don't plan to solve his question - instead I provide tools for him to do his own analysis.

    Also macrostates, as I understand them, are measured by temperature, pressure, etc. When you have to count things, they are microstates.
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