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The physics behind the Le Chatelier principle ?

  1. Aug 13, 2011 #1
    Can anyone explain the physic behind the Le Chatelier principle ?

    I know the gas will expand to fill an area but why ?

    If I have a box full of air (just normal air) and I enlarge the box without allowing any more air in, the air will spread out to fill the larger box. What starts this movement and why would this be so ?

    What is the cause of the movement of gas from one area of higher pressure to the next of lower pressure until the pressure is equal ? I do understand that it happens but not "why" it happens - something must "tell" the molecules to move and spread out ?

    Is it some form of molecular dipole repulsion, or something else (that was just a guess by the way ;-)

    I have found lots of "laws" that say that this will happen, but none of these gives the reason - just the fact that it does.

    Or if anyone has a link to a webpage that gives the reason it would be very kind.

    Thank you
     
  2. jcsd
  3. Aug 13, 2011 #2
    it's not a dipole. just think of the molecules as small particles that bounces at each other. having more pressure means there are more molecules hitting each other, so they tend to spread.

    you can check this article in wikipedia http://en.wikipedia.org/wiki/Kinetic_theory

    i hope someone would come here and give a good explanation about this, i'm just starting my graduation and i'm no expert in this, but i think this link might help.
     
  4. Aug 13, 2011 #3

    atyy

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    Last edited: Aug 13, 2011
  5. Aug 13, 2011 #4
    So it’s the second law of thermodynamics ? ? ?
     
    Last edited: Aug 13, 2011
  6. Aug 13, 2011 #5
    yes, it's the entropy states. i wish i had a better knowledge about this... i know a little about the concepts but not enough to give you a nice explanation, but if you get the idea you can go and find out by yourself without being confused :)
     
  7. Aug 13, 2011 #6
    Thank you for that - it does explain it.


    This is not for "homework" as such so I hope it's alright to ask these basic questions here - although I am working at home.
    I left education 35 years ago and now (for some strange reason) my employer has introduced a research project requirement into the top promotion levels (I work for a ski holiday company). The only thing they ask is that the research topic is to do with work However tenuously and original/novel - so looked around about a year ago and have decided to write about "why snowflakes look like snowflakes" - not very original I know, but it's fun re-learning all the stuff I have forgot from my A level physics.
    Starting right from the make-up of H2O through to deposition within the pack – is quite complex but I’m getting there in the evening and at weekends (when the kids give me chance)
     
  8. Aug 14, 2011 #7
    I think have it. How does this sound ?

    There is no “rule” the forces a gas to expand, it’s just more likely. The 2nd Law of Thermodynamics can be explained (and used) in many ways. For our purposes I will say that “the entropy (disorder) in a system will either be constant or increase”. So, if we have a number of molecules moving around within a space, they will occasionally bump into each other and rebound in another direction, where they may meet yet another molecule and rebound again.

    Of course sometimes the molecules may bounce into the centre of the colligate (group of molecules) and at other times bounce away from it. The molecules that go towards the centre of the colligate are more likely to come into contact with another molecule than those that are moving away from the colligate (and have less chance of being bounced back into the centre), so the area of the colligate will increase (and the partial pressure will decrease) as the molecules become less and less likely to come into contact with each other.

    Due to this, if there are less molecules in one area (low pressure) than in other (high pressure,) the flow will always be from the high pressure into the low pressure area, until the both areas are in equilibrium.
     
    Last edited: Aug 14, 2011
  9. Aug 14, 2011 #8
    Sounds right, although I don't see a need to mention the word entropy, unless you also explain what entropy is (you kind of do, but not exactly, so I think you can best leave it out); what you do do, however, is explain why the 2nd law is true (which seems to be enough as I see it), i.e. why it is that particles spread out: it's simple probability theory (it might be adviseable to mention the word "probability", as that is the key thing; if you think about it, it is possible that the all the molecules, once spread out, come back together in a single corner [for an example: simply play the "video tape" of the expansion backwards; as the laws of Newton are time-reversible, this is also possible], the point is simply that that reversal is incredibly unlikely because there are many more ways to spread out than to come together.)

    And to nit-pick, if you are going to use the phrase "the entropy (disorder) in a system will either be constant or increase", the correcter version is:
    "the entropy (disorder) in an isolated system will either be constant or increase".
    That extra word is very important, because we continuously see order appear around us: a pump can, for example, pump all the particles into a small corner! But the isolated system "particles in box + pump" is increasing in disorder: to work the pump, you must use e.g. electricity, which comes from burning fossil fuel, and you can prove (and it's not hard to see) that burning fossil fuel increases entropy. (So you see, we're not experiencing an energy crisis but an entropy crisis.)

    Another example is life. We can heal our wounds (decrease entropy), because we increase the entropy around us (for example by eating); the net sum of entropy in the total system is never decreasing.

    I might be telling you things you already knew. Anyway your last post seemed to get it. A wonderful insight, isn't it? :) I get great pleasure from it.
     
  10. Aug 15, 2011 #9
    Thank you very much for your imput. Mr_Vodka

    Please feel free to nit-pick all you like ;-), I need it

    This is the revised text.

    There is no “rule” the forces a gas to expand, it’s just more likely. The 2nd Law of Thermodynamics can be explained (and used) in many ways. For our purposes I will say that “disorder in an isolated system will either be constant or increase” or “things will stay the same or get more muddled”.
    So, if we have a number of molecules moving around within a space, they will bump into each other and rebound in other directions, then meet yet another molecule and rebound again. Some (or even all) may bounce into the centre of the colligate (group of molecules)but, as there are simply more routes heading away from the centre, it is (much) more probable that they will bounce outwards, and, of course, any that head back towards the centre are more likely to come into contact with another molecule than those that are moving away, so the area of the colligate will increase (and the partial pressure will decrease) as the molecules spread out and are consequentially less likely to come into contact with others.

    Due to this, if there are less molecules in one area (low pressure) than in other (high pressure,) the flow will always be from the high pressure into the low pressure are, until the both areas have are in equilibrium.


    Here there is a graphic showing a time-line of a number of High/Low pressure segments with the molecules moving from one to the other

    As time proceeds, the dispersment of molecules, away from their neighbours, has the effect of moving a gas from the high pressure into the low pressure areas. They do not specifically move in a certain direction towards the low pressure area; this is the result of random scattering. This process will continue until both areas are equally filled
     
    Last edited: Aug 15, 2011
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