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Notion of Entropy

  1. Jul 13, 2013 #1
    Everybody says entropy of the universe is increasing.

    But after the big bang, matter formed, it came together, stars and galaxies were formed, on a small planet called Earth atoms/molecules came together to form unicellular organisms and then higher life. Our brains are organizing even more and more as our intelligence level is increasing.

    Isn't this order?
     
  2. jcsd
  3. Jul 13, 2013 #2
    :/
    Get yourself together man.
    Are all these irreversible processes?
     
  4. Jul 13, 2013 #3

    Nugatory

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    Staff: Mentor

    You're letting yourself be misled by the pop-sci description of entropy as "disorder".

    Entropy is a measure of the number of internal states that a system can have. Not all increases in entropy correspond to our intuitive notion of "increasing disorder".
     
  5. Jul 13, 2013 #4
    What Nugatory said. One can associate some kind of "disorder" to entropy, but this view is very weak in describing the state of the system. Entropy is characterized by the multiplicity, i.e. the number of microstates associated with a macrostate.

    As for life forming on earth, the entropy of earth does not need to increase. A simple argument is as follows:
    most of the radiation earth receives from the sun is in the green and the earth emits in the infrared. Since the earth is in steady state, energy in = energy out. Thus for every photon earth receives, it emits two photons (the energy of an infrared photon is about half of the energy of a green photon). Two photons have a higher multiplicity than a single photon. Thus the change in entropy does not need to be greater than zero.
     
  6. Jul 13, 2013 #5
    From the engineering point of view (see the book Thermodynamics; An engineering approach by Cengel and Boles) "entropy generation can be used as a quantitative measure of irreversibilities associated with a process".
    Sgen > 0 --> irreversible process
    Sgen = 0 --> reversible process
    Sgen < 0 --> impossible process
     
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