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Why changes in entropy?

  1. Jun 18, 2006 #1
    For a gas I know that entropy can change due to changes in energy, number of particles and the volume of space it occupies. How else can entropy of a substance change? What about liquid or solids?
     
  2. jcsd
  3. Jun 18, 2006 #2
    Any other interaction with the gas will change its entropy as well.
    Electric fields can change it, magnetic fields, interactions with light could change it, ...
    For liquids and solids it is exactly the same, or for the blackbody radiation, or for plasmas, ...
    There maybe practical differences, of course.

    It would be interresting and useful to take the time to analyse what happens for some of these interations.

    For example, what happens when you switch on a magnetic field, how does the material respond, how and why does the entropy change.

    As another example, what happens inside a ruby laser, how does the entropy of the ruby crystal change when excited, and why, and when, ...

    Thermodynamics has connections to most if not all other branches of physics. Look at the energy-side of a branch of physics, like electromagnetism, and you have a connection to thermodynamics.
     
    Last edited: Jun 18, 2006
  4. Jun 20, 2006 #3
    If entropy is regarded as the number of microstates of a system (or logarithm - blah blah blah), then it is easy to see that any set of objects can have an entropy.
     
  5. Jun 21, 2006 #4

    DaveC426913

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    Entropy represents of the number of indistinguishable states something can be in.

    An unbound manuscript with all its pages in order has low entropy.

    Throw it on the floor and many of it pages will be mixed up. Its entropy rises.

    Do the Moonwalk on the floor for a half hour, and all its pages will be shuffled. It will be in a state where there are a huge number of states (some factorial of the number of pages) it can be in, allof which that are pretty much indistinguishable from each other. It's entropy is at maximum.

    The entropy of the universe will reach maximum when the atoms are distributed such that there is no way of distinguishing any unique state.

    At least, that's what I got from Brian Greene's 'Fabric of the Cosmos'...
     
  6. Jun 21, 2006 #5

    Tom Mattson

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    In macroscopic, classical thermodynamics we say that entropy changes are entirely due to the irreversibilities (internal or external) of a system. Any time a system undergoes an irreversible process, it suffers an increase in entropy. So which processes are irreversible? Strictly speaking, all of them. But we can conceive of reversible processes as theoretical ideals, and think about what kind of characteristics they would exhibit. Just think back to Physics I when you dealt with pendulums that swung on frictionless pivots through resistanceless air. Reversible processes are those hypothetical processes that, if they were executed in reverse, would leave no trace on their surroundings. Irreversible processes do leave a trace when they proceed in the 'backward' direction. Examples of irreversible energy transfers are work done by friction (wet or dry) and heat transfer due to a temperature gradient.
     
  7. Jun 22, 2006 #6
    I don't understand these two examples. What are the "indistinguishable states" of an unbound manuscript with it's pages in order?
     
  8. Jun 23, 2006 #7

    DaveC426913

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    There are no two indistinguishable states of an ordered manuscript. If you tried to switch two pages - it would be immediately obvious. Any change in the page order of the manuscript could be easily described. It is at zero entropy.

    Now, after doing your moonwalk on them, take two pages and swap them. Is the manuscript in a noticably different state now? Pretty hard to tell. High entropy.


    Compare it to a universe filled with galaxies and planets and humans. This is a universe with low entropy. It can be well described. Switch an atom at the core of a star with an atom in a human. The change would be immediately apparent (at least to the human :biggrin: the kinetic energy alone...)

    In a few quadrillion years, the atoms that WERE in all the humans and stars and galaxies will have all reached the same temperature and distance from each other. Switch two atoms now. Notice a difference? Not really. You could swap atoms till the cows come home and the universe would look pretty much exactly the same. This is a state of high entropy.




    It also becomes apparent why entropy is on the increase. Natural forces (such as wind, or a Moonwalker) will conspire to shuffle the pages of the manuscript, trending towards maximum entropy. The only thing that will lower the entropy within the system (the manuscript) is an external force (such as a human who night decide to re-sort the manuscript). Once the humans have dissipated because their Sun died, the manuscript will not get any more organized.

    Overall entropy always increases. You can lower the entropy locally, but only at the expense of the overall entropy of the universe.
     
    Last edited: Jun 23, 2006
  9. Jun 23, 2006 #8
    Hi Dave*,
    Thank you for clarifying that the manuscript with numbered pages has zero entropy. That makes sense.

    But I don't see how shuffling the pages increases the entropy! You see, by numbering the pages, every state of the manuscript is distinguishable from every other!

    To posess any indistinguishable states, some of the pages would need to be unnumbered, or indistinguishable from others.
     
  10. Jun 24, 2006 #9
    What I learned about entropy is the following: entropy is a measure for the disorder of a system. The higher the disorder, the greater is the entropy value of that particular system. So in that case the example with the manuscript pages becomes quite clear. A nice ordened book has a lower entropy value (=few disorder) than when every page is spread on the ground (=more disordered even if they are numbered , and a greater entropy value).

    Processes in which the entropy of the studied system rises (=greater disorder at the end) are often said to be spontaneous. So you can say for example, it doesn't take much effort to make a messy room from a tidy room (spontaneous, rise in entropy value), but it wil take much more effort to make a tidy room from a messy one (decrease in entropy, not-spontaneous).
    Apologies for the sometimes bad english.

    Jelle V
     
    Last edited: Jun 24, 2006
  11. Jun 24, 2006 #10
    It's a common misconception that entropy is disorder in the usual sense: a gas of molecules in a box cannot be said to be any more ordered than a gas of molecules in a box twice the original box's size, yet the latter will have twice the entropy of the first.

    Entropy can be defined in two ways:

    (a) The statistical mechanics way: it's proportional to the logarithm of the number of available microstates that lead to the same macrostate.

    (b) The sum of infinitesimal heat supplied to the system divided by the temperature at which it is supplied, during a reversible process which starts and ends at macrostates A and B is the change in entropy of the system as it moves from macrostates A to B (NB. this definition does not tell us what zero entropy is; for that we must rely on definition A).

    As for the first definition, microstate means the exact specification of the position and momentum of every molecule and macrostate means the specification of thermodynamic variables such as volume, pressure, temperature, etc.

    The second definition is motivated by Clausius' theorem, and I could provide more information if anyone wants.

    It's easy to see how the first definition can be confused with disorder, and often saying that a system can access more microstates while being in the same macrostate is confused to the system possessing more disorder, but that is, in many opinions, quite misleading.
     
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