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Increasing entropy vs groundstate

  1. Dec 6, 2009 #1
    Atoms and molecules like to be in their ground state, that is its most probabilistic for them to be in the ground state, but the entropy of an isolated system is always increasing. Which implies, to my understanding, all states should be equally likely. There is something i'm definitely missing. How do I reconcile these two concepts?
  2. jcsd
  3. Dec 6, 2009 #2
    Here's an interesting link on the concept of a negative entropy source (the Sun):


    I recall a similar description in the book A Brief History of Time.

    My problem with entropy theory is that, if my brain recognizes a pattern, I call it "order" and if it doesn't, I call it "chaos" or "disorder." So the line between the two is perhaps not a property of the universe, but perhaps a natural limit on the power of pattern recognition.
  4. Dec 6, 2009 #3
    Not "always increasing" just "practically never decreasing".

    Consider: there are only a few "ways" for an atom (with a particular amount of energy) to be in an excited state. This system can decay into a photon and a ground state atom, and then there will be an infinite range of "ways" that those two could be arranged. (The thermodynamic microstates correspond to all those "ways"/arrangements of the entire system, not to the atomic electron states by themselves; the latter doesn't even correspond to a closed system.)
    Last edited: Dec 6, 2009
  5. Dec 7, 2009 #4


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    When an atom drops into its ground state, it gives off a photon. The entropy of the photon makes up for the entropy loss of the atom itself, so that the total entropy always increases. Remember that once the atom gives off that photon, it's not isolated.

    (cesiumfrog was right, though, it technically is "almost never decreases" rather than "always increases." But the probability of seeing entropy decrease in a real system is astronomically low, i.e. we have every reason to expect that it's never happened.)
  6. Dec 7, 2009 #5
    Atoms and molecules only like to be in the ground state, when their are not an isolated system. You are right that if they were isolated, then all states would be equally likely. However atoms are in thermal contact with the environment. All state of the combined system enviroment+atom are indeed equally likely. One can derive that this is equivalent to saying that the free energy of the atoms is the smallest possible.
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