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B How to distinguish order and disorder?

  1. Jun 20, 2016 #1
    Thinking of entropy as a measure of disorder, how do we distinguish order and disorder. Entropy should increase with time resulting in more and more disorder with the passage of time. The solar system evolved with time. Now, do we consider the formation of the solar system as getting ordered or disordered?

    If I have not misconceived it, the same question should also be relevant to formation of stars, galaxies as well as to formation of life on earth (through abiogenesis), all of which take/took huge amounts of time to be formed. So are these formations a result of getting ordered or disordered?
     
  2. jcsd
  3. Jun 20, 2016 #2
    Hi mitrra:

    I am not sure I am describing this with complete technical accuracy, but I think the general idea is OK.

    If you think of the state of the solar system before there were planets, and compare that to the current system of planets and moons, the planets and moons represent an increase in order which is a decrease of entropy. However, the changes which produced planets and moons converted free energy into entropy, with a total net of entropy increase. Whenever there is free energy available in a system, some of it will become an increase in entropy, but some may be converted into order. Another example would be the effect of sunlight in increasing living stuff as order while much of it just becomes an increase in temperature as entropy.

    Hope this helps.

    Regards,
    Buzz
     
  4. Jun 20, 2016 #3

    Nugatory

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    Staff: Mentor

    More disordered. If it doesn't look that way to you, chances are that you're only thinking of the sun and the stuff orbiting it, overlooking all the energy that was present in the initial ball of gas but was radiated away over the past billions of years. That energy is now spread across an enormous volume of space in an very disorderly way, so if you look at the system as a whole instead of picking out one tiny piece of it you get a better sense of how much disorder there is.
    It's also important to understand that entropy has a proper mathematical definition, which google will find for you. This idea that "entropy is disorder" is just a convenient oversimplification for people who aren't prepared to take on the math. It's OK if you want an intuitive picture of what high-entropy system and low-entropy systems look like, but it can be very misleading if you're trying to understand how a system might evolve from one state to another.
     
    Last edited: Jun 20, 2016
  5. Jun 20, 2016 #4
    Also to be thought of is a precise statement of the second law of thermodynamics. It does not say "Entropy should increase with time." It says that in an isolated system, the entropy does not decrease with time. If the isolated system is already in equilibrium, then the entropy remains constant. If it is not in equilibrium, the entropy increases until equilibrium (maximum entropy) is reached.
     
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