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Brief explanation of what entropy is?

  1. Oct 19, 2006 #1
    Hi, could someone please give me a brief explanation of what entropy is? Due to my limited understanding what entropy actually is, most of the things I've read about it seem very vague. I've only been seeing entropy popping up in integrals and in sometimes in comments along the lines of entropy being a property of the system.

    But is that all there is to it? I mean surely there must be some kind of 'physical interpretation.' I can relate things like heat transfer and work done to other things I know about but I just can't grasp what entropy is.

    Any help would be good thanks.
  2. jcsd
  3. Oct 19, 2006 #2
    Entropy is the randomness in a system. For example in chemistry, the randomness goes like this:

    the formula is: dS = δQ / T where S is the entropy, δQ is the amount of heat absorbed in a reversible process, and T is the temperature.
  4. Oct 19, 2006 #3

    Andrew Mason

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    Entropy is not an easy concept to grasp. There are a lot of subtleties. I would suggest you read as much as you can about it, starting with the history (see http://en.wikipedia.org/wiki/Entropy" [Broken], which is pretty good).

    Absolute entropy is not really a very useful concept. So don't worry too much about trying to understand what 'entropy' is physically. Mathematically, it is Q/T. But change in entropy is useful. It tells us which direction a thermodynamic process will naturally go. It tells us how much of the heat can be used to perform useful work.

    Good luck.

    Last edited by a moderator: May 2, 2017
  5. Oct 21, 2006 #4
    Thanks for the explanations guys, much appreciated.
  6. Oct 21, 2006 #5
    The 2nd Law of Thermodynamics states that the amount of entropy always increases with time in the universe. As the universe expands, it gets more and more disordered.

    Its sort of something to do with the idea that if you leave your room unmanaged, pretty much you'll find it quite messy after a week. Similarly, a cup amde of china is in an ordered state, but when you break it entropy has increased since it is less ordered.
  7. Oct 21, 2006 #6

    Andrew Mason

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    You should be very cautious about equating entropy to disorder. You have to define disorder. For a thermodynamic system which, by definition, is constantly changing, it is not easy to see how a change in 'order' occurs from one to another. It is hard to see "order" in an inherently chaotic system. It is better, in my view, to think of entropy as a measure of how disperse the energy is. As energy becomes more dispersed, the entropy increases. As a hot small object transfers its heat to a large cooler object, the energy is less concentrated - entropy increases.

    Have a look at the paper http://www.entropysite.com/teaching_entropy.html"

    Last edited by a moderator: Apr 22, 2017
  8. Oct 21, 2006 #7


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    You appear to be ascribing order through a humanist point of view which isn't correct, remember the amount of 'energy' that's required to fix the cup in the analogy you used. This is fundamental in understanding entropy within a physical system.

    Somebodies been reading 'A Brief History' :wink:
  9. Oct 22, 2006 #8
    I know that, but entropy has been quite often defined as a 'measure of disorder', has it not, though the accuracy of this is apparently questionable. Indeed, I don't think there is any particular quantity that actually measures disorder, but surely you can admit that it comes close.o:)

    Probably, it would be better to say that entropy would show what is most probable than not.

    Well, I would have thougt a 'humanist's' POV albeit a less mathematical one than the formal matematical definition would be a nice starter rather than jumping in and trying to understand it.

    However, perhaps someone might explain to me exactly why quite a number of people (including Hawking:wink: ) attempts to describe it using disorder then if it is misleading.
  10. Oct 22, 2006 #9

    Andrew Mason

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    That was the popular way to explain entropy when he was taught thermodynamics. It was not clear then and it is still not clear.

    Notice that Hawking does not provide a definition of disorder. Indeed some of his examples are misleading. A container of air with all the air in one half is not necessarily more ordered than a container with air evenly distributed throughout the whole volume. There is chaos at the microscopic level in both. But the energy is more concentrated in the smaller volume and more dispersed in the larger volume.

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