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Question bout entropy

  1. May 10, 2003 #1
    In school we just started a unit on thermodynamics. So far entropy was only briefly talked about. On first impression it seemed to me that the idea of entropy sort of goes against the definition of energy. Energy being the ability something has to do work, and entropy being energy that isnt available to do work. Doesn't really make sence to me..can anyone explain?

    Also, if the entropy in a closed sytem is suppost to go up whenever an energy change takes place, wouldn't that go against energy can never be created or destroyed?
     
  2. jcsd
  3. May 10, 2003 #2

    mathman

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    Mix hot water and cold water. The net result is warm water (same amount of energy). Entropy has gone up - you can't unmix into hot and cold in the closed system.
     
  4. May 11, 2003 #3

    FZ+

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    The fundamentals of thermodynamics is that when we do work, we exploit an energy gradient. (usually temperature) The nature of energy is to move towards a point where all the energy is in equilibrium, or equally distributed in all places - that is said to be at maximum entropy, or disorder. Heat engines use this as power. Consider a hydroelectric plant... it uses the imbalance in gravitational potential to generate power. All of our other engines work this way. Now, at a point of maximum equilibrium, no work can be done. The energy is present, but there are no longer any relative potentials. In order to restore imbalance, work must be done INTO the system ie. raise the water up. To do this, you would have to raise entropy somewhere else, and lose additional power from equilibrium. Hence the Second Law.
     
  5. May 11, 2003 #4

    marcus

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    Can someone give an intuitive reasoning for why the entropy of a black hole is equal to the (event horizon) surface area?

    This is the 1974 Bekenstein result that kicked off all the current work in quantum gravity (hawking's BH temperature and radiation result followed close on Bekenstein.) It is a major landmark in contemp physics. There has to be some, at least
    *some*, intuitive content.

    Sometimes entropy is associated with the idea of information. Why is this?
     
  6. May 11, 2003 #5
    Entropy is simply a measure of spread of energy. Say, let a billiard ball bounce withing half of pool table (say, by having a stick dividing table on two and not letting ball into the other half), and then remove the stick and let the ball to take over entire pool - the ball will do that. We call this "entropy increase".

    Whenever you give to a physical system more states, it'll occupy them too along old states. Arithmetic of this is what we label as "law of increase of entropy" or "law of increase of disorder" or "heat goes from hot to cold", etc.

    Energy is not lost, it is just spread over those states which become available. Just becase there is no difference between "new" and "old" states.
     
  7. May 12, 2003 #6
    FZ,
    Would you mind giving an example of that?
     
  8. May 13, 2003 #7
    As an example, an accidentally dropped egg splatters in a cup. the reverse process, a splatered egg reforming into a whole egg and jumping up to an outstretched hand, will never happen on it....
    if you puncture a helium-filled balloon in a clsed room, the helium gas spreads throughout the room--but the individual helium atoms will never clump up again into the shape of the balloon.
     
  9. May 13, 2003 #8
    That part makes sence. What I don't get is in FZ's post he said work needed to be done into the system to restore imbalance or potential difference. Doing that work would raise entropy somewhere else. How does doing work in a system raise entropy in some part of it?
     
  10. May 13, 2003 #9

    marcus

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    Why should the entropy of a black hole be proportional to its surface area and not proportional, for instance, to its mass, or volume?

    There is an idea here----the log of the number of microstates all of which appear the same on a macro level----or? what has happened to the idea of entropy? It must be evolving, getting broader or deeper, for it to be possible to identify it with the area of a black hole. this is one of the most influential results in contemporary physics, it has to mean something. you cant just keep on talking about popping a balloon full of helium and ignore this other aspect of entropy.

    Anybody have a clue? Mentors?
     
  11. May 13, 2003 #10

    jb

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    let's get some of mc hawking's expertice on the subject:

     
  12. May 13, 2003 #11
    Entropy is spread of energy over all available states.

    Give a system more states - energy will spread over them too (system does not know which states are "new" states and which ones are "old" states).

    So, it is all just statistics (=arithmetic of combinations and permutations):

    http://www.physics.ubc.ca/~birger/boltzmann/node3.html
     
  13. May 13, 2003 #12

    LURCH

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    The explanations you've gotten so far are pretty good. A simpler thing would be to say that energy goes from where there's more to where there's less. And, since everything (all matter) is made of energy, all things follow this behavior.

    For example, you can't heat your cup of coffe buy setting it out in the snow. Now, you might reason, "the temp. of the snow is not absolute zero, so there is some heat in it, this could go into my coffe". Entropy is there to say, "but it won't". Same way, you can form a tall collum of water by puling a glass upside-down out of a full sink. Break the seal and release the forces that hold the water in place, and it will fall untill it is a level surface again. Press the glass (right side up) down into the water, and you can make a dent. Remove the glass, and the water level of the entire sink will lower just to fill in that dent. If I may wax Biblical; "Every valley shall be exalted, and all the mountains and hills made low. The crooked shall be made strait, and the rough places plain."

    So when you take water from a water fall to make ellectricity, you are trapping some energy on its way from "up high" to "down low". But you can't be perfect; you can't catch ALL of that energy. So, if you used the ellectricity you get from the dam to operate a water-pump to try to put all the water back, you'll find that even if you have the most efficient dam and pump possible, you don't quite have enough juice to do it. You'll need a little more from somewhere (maybe another waterfall).

    Maybe you could use a bucket to put the rest back if it's a small enough amount. But only if you eat a good meal first, taking energy from the ecosystem and burning it as fuel for your muscles. Either way, energy is lost out into the cosmos. So, everything you do serves at least two purposes; 1)the task you intended to accomplish, and 2)heating outer space!
     
  14. May 13, 2003 #13

    enigma

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    Imagine you have a box with two compartments. The compartments are able to transfer heat, but at a slow rate. The box is also insulated from the outside world.

    If you put 'hot stuff' in one side and 'cold stuff' in the other side, eventually they will settle into an equal temperature 'warm stuff' on both sides. The entropy of the system has increased.

    If you then hook up a temperature seperating machine (like an AC/heat pump combo) you could seperate the hot and the cold into sides again. The only problem is that doing so would generate entropy in the environment (because the machine needs power to run).

    hopefully that makes some sense...
     
  15. May 14, 2003 #14
    Universal Entropy

    Hello students (Newton1, FZ+, jb, enigma) and 38 year old Lurch.

    Entropy, as a chemist sees it, does pertain to irreversibility of many processes that can be quantified as T[del]S.

    On the other hand, to some physicists, entropy is a measure of disorder in a system. When the obvious chaos in natural systems is considered without any balancing of the well-ordering aspect of that natural system, it is conceivable that universal entropy could be called ever-increasing. The epitome of chaos is the permanent and continuously forever-increasing light rays in the universe. The epitome of well-ordering is the natural event of the conversion of light energy into indestructible matter and antimatter. That includes "dark matter" that continues being alive with ever-cycling quantum orbitals.

    There is no argument here that universal entropy is not ever-increasing but that, without an accounting for well-ordering, quantification of universal entropy is not possible.
    Thanks for your audience. Cheers, Jim
     
  16. May 14, 2003 #15
    Increase of disorder is just s statistical (=arithmetical) law. You can't reverse it.

    Nor does arithmetic (statistics, or entropy) shows or indicates mystical "direction of time". (Indeed, reverse time, and system will still occupy all available states.)
     
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