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What do you mean when we say there cant be E.F 'inside' a conductor?

  1. May 4, 2009 #1
    A metal is held by E.F...its an arrangement of electrons and kernels cause of which the metal sustains as a solid.


    Since Columbian forces are holding the metal together, there has to be an E.F inside the metal, or the fields of electrons and the kernels as holding each other to form the solid...its cause of their mutual field, a metal exists.
     
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  3. May 4, 2009 #2

    Born2bwire

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    In a metal, or any conductor, there are no gaps between the valence energy levels, these are levels that bind an electron to an atom or to a covalent bond between atoms that connect the lattice, and the conduction energy levels, levels where the electron is free to move about the bulk material. We say there are no gaps but this is technically incorrect, the energy levels are discretized and Pauli's Exclusion principle prevents two electrons from occupying the same state (though we can have multiple states at the same energy level and such). However, the gaps are so small that the thermal energy from normal environments is larger than the gaps. So the thermal energy allows the electrons to go from the valence bands to the conduction bands without any external input and putter about as if the energy levels were continuous.

    So in a metal, the electrons are free to move about, leaving behind positively charged ions (or we can talk about them as being holes but the rules for dealing with holes are a bit different than you would expect). In a normal environment, the Coulombic forces between these ions and the electrons sees to it that no local region obtains a net charge. Because if a small area gets a net negative charge, there is another area in the bulk with a positive charge and eventually the forces pull the electrons back making everything more or less neutral on an overall scale. However, if you apply an electric field, the electrons will move in the direction of the field due to the Lorentz force. However, the electric field from a negative charge points out in the radial direction. The field from a negative charge will oppose the applied electric field. Eventually, as the field pushes the electrons, they build up on the surfaces of the bulk because they cannot flow out of the material. They build up a net negative charge (and on the opposite sides a positive charge as those atoms give up their valence electrons to the applied field). The electric field between the electrons and ions opposes the applied field.

    As long as there is a net electric field, the charges will move about in reaction but eventually they will create their own electric field that cancels out the applied field. Once the applied field is cancelled out, everything becomes static as any net movement of charge creates a net field that moves charges back until the field is gone and no forces act on the charges. So it is simply a matter of a system seeking a minimum energy level. The electrons are not impeded from moving up into the conduction bands and moving about the bulk and so a conductor will always redistribute the charges in a manner that cancels out an applied electric field.

    Coulombic forces do not hold the metal together. The atoms of the metal bond with eachother via covalent bonds. In an effort to make a more stable outer shell (by either filling up the empty valence states with electrons or removing the extra valence electrons to empty the shell), the atoms create a hybrid orbital with neighboring atom(s). This hybrid orbital shares the needed electrons between the two of them. Coulombic forces are more at work with an ionic bond. This is where the atoms only need to strip or gain a few electrons, usually 1-3, to be more stable. Those with extra electrons only hold onto them weakly due to charge screening. This is where the inner electron clouds diminish the effective positive charge from the nucleus that is seen by the outer valence electrons. So in this case the extra electrons are stripped off and caught by other atoms who then become positive. Then, the ionic bond more closely resembles a coulombic attraction as the ions do not share their electrons like in covalent bonds.
     
    Last edited: May 4, 2009
  4. May 4, 2009 #3

    jtbell

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    On a microscopic level, there are certainly electric fields inside the metal. On a macroscopic level, these microscopic fields average out to zero in an electrostatic situation so that there is no net "long distance" force on conduction electrons, and no net motion of conduction electrons, i.e. no current.

    This is thinking in terms of a classical model of conductivity, not a quantum-mechanical one, of course.
     
  5. May 4, 2009 #4
    Equal charge distribution you mean.

    Yeah...exactly, so there are pressurised electrons at one end; since there's a force application on them, it manifests that there's a field which's causing the electrons to get squeezed, and cause the electrons are in the metal, the field is of course also in the metal (:tongue2:)

    A field can be said as 0 if there will be no force application on a test charge, in this case, there is a force application, that's why the electrons shift to the other side of the conductor.


    Sorry I'm bad at chemistry so............but as far are I remember, this was so............or was it the packing structure....anyway.....don't remember. :confused:

    :confused: :confused:

    Then what's the difference between metallic an covalent bond?
     
  6. May 4, 2009 #5

    Born2bwire

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    More or less, the charges are always in a state of movement, due to, at the very least, thermal motion. So the charge distribution more less is equal but there is always momentary perturbations as thermal motion or other effects cause charge to bunch up in one area but then Coulombic forces corrects this. On a macroscopic level, the metal is neutral and has uniform charge distribution.


    At steady state, the charge distribution creates its own field that cancels out the applied field. There is no forces at this point. However, the charge distribution is maintained by the fact that the applied field is still being applied. Just like in the equilibrium case where Coulombic forces ensure uniform charge distribution, whenever a net amount of charge deviates, then the secondary field no longers cancels the applied field creating a net field in the conductor. Once this net field occurs, this field applies a force which then redistributes the charges correctly. But again, the movement of charges is mainly due to thermal motion and on a macroscopic level this doesn't have much bearing.


    A covalent bond is a sharing of an electron or electrons between two atoms to form a molecule. When you have a lattice of molecules, the molecules are not attracted/repelled by each other by covalent bonds but rather Van Der Waal forces. But a metallic bond, if I recall correctly, is like a universal covalent bond. Where a covalent bond shares the electrons between two atoms, a metallic bond shares the valence electrons universally amongst all metal atoms. So the metal lattice is held together by a metallic bond and not Van Der Waal forces like with molecules. There can be lattices held by ionic bonds, like with sodium chloride, or covalent bonds, like silicon.
     
  7. May 5, 2009 #6
    doesn't current flow in a conductor because there is an electric field????
    please if any one can clear this doubt.
     
  8. May 5, 2009 #7

    Born2bwire

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    Only for the short time that it takes for the charges to configure themselves in such a manner that cancels out the electric field. If you have an AC voltage applied, the currents only travel on the surface of a perfect conductor and on a realistic metal the currents are still for all purposes confined to the surface of the conductor.
     
  9. May 5, 2009 #8
    It cannot be stated that if there's not motion, there's no field...suppose I'm holding a 1c charge in front of a 10000 c stationary charge, i.e the 1c charge is in a steady state, so it cannot be that the 1 c charge cancels the effect of the 10000 C charge and that if put a test charge in between them, it wont move.

    You basically mean to say in the above 10000 C charge scenario, there's no force on the test charge.

    We're gonna talk about the after earth scenario later...after resolving this i.e.

    And aaaa...lets forget chemistry for a while, cause I think that does not happen...I still remember the reason why metals are mailable and ductile and that was cause they were held together by a sea of electrons and so the change in geometry did not matter.

    Current flows in a conductor to stabilise the disbalanced field in the conductor, i.e as long as the current flows, there's not field.
     
  10. May 5, 2009 #9
    I think I'm getting it.

    It should be remembered that there's no boundary holding those electrons...the thing that's holding them are the positive ion's E.F...that means the stationary state of the electrons is cause the fields get canceled.

    Now I think I'm about to generate more doubts with this (i.e dynamics in a metal under the influence of a field).

    So what if the metal has a net charge?

    If a test charge is placed inside a charged metal, it will experience no force cause all of the positive ions will pull/repel it..............This one too solved.

    Now what if a charged metal is put in a field?...the metal will experience a force and start to move, since the stationary components of the metal are the positive ions, the force acts on the positive ions cause of the field.

    Since the nucleus are inside the metal, the field is inside the metal. :tongue2:
     
    Last edited: May 5, 2009
  11. May 5, 2009 #10

    jtbell

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    No, the current flows precisely because there is an electric field inside the conductor. The electric field exerts the force on the conduction electrons which causes them to move.

    The statement that there is no (macroscopic average) electric field inside a conductor applies only under electrostatic conditions: when the conduction electrons are stationary (except for random thermal motion). A current-carrying wire is not an electrostatic situation. The conduction electrons move because of an electric field inside the wire, which is associated with the potential difference between the ends of the wire.
     
  12. May 5, 2009 #11
    that solves my doubt.
    thank you very much.
    also can it be proven that electrostatic field very near the surface of a charged conductor is always perpendicular to it. I tried using Gauss' Law but of no use
     
  13. May 5, 2009 #12
    It's good that you are thinking this through dE logics and it seems that the feedback you are getting is giving you a greater understanding.I,however, am having a little trouble following your reasoning because of your references to the word "kernel".What exactly does this word mean in the context you are using it?Excuse my ignorance but I just googled and I am still none the wiser.
     
  14. May 5, 2009 #13
    Ok...I get it, thanks for telling! :approve:

    Kernel -

    I don't know...but by kernels I mean the positive ions of the metal...my book referred it as 'kernel'...I too searched for it after your notification...thanks for telling.



    Ok...in a metal influenced by a field, yes, there will be no field at the places where the electrons reside (i.e at certain places cause of the disturbance caused by the field) but, at other places there should be a field since the electrons moved to the other side, it means there exists a field inside a metal at other places and if a negative test charge is put at that place, it will move to one side, as with the electrons.
     
  15. May 5, 2009 #14

    jtbell

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    Is your book in German, by any chance? German "Kern" = English "nucleus".
     
  16. May 6, 2009 #15
    :biggrin:...no actually it was my XII grade book.

    I have the word in my notes I made at that time -

    "Sea electron theory-
    According to this theory-
    1.a metal has a no. Of free electrons, as metals loose there valance electrons as they have very less affinity to them(the ionisation energy is very less)
    2.These electrons move freely in the metals. The remaining nucleus and the shell below it gains a +ve charge and are named kernel.
    3.The kernels now occupy fixed positions called lattice sites, and surrounding them is a gas of electrons, the electrons can be also refered as sea, so this theory is also called electron sea theory. The electrons are said to be de localised.
    4.Now an electron is attracted by many kernels each of being of different charge, this electrostatic force binds the atoms of the metal together, this force is called cohesive force, the bond is called metallic bond."

    Old document...used MS office at that time. Stupid choice.
     
  17. May 6, 2009 #16
    A few ambiguities still remaining......
     
  18. May 7, 2009 #17

    Born2bwire

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    Which ones?
     
  19. May 7, 2009 #18
    Then -

     
  20. May 7, 2009 #19

    jtbell

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    Hmm, OK. Maybe it's British terminology. U.S. textbooks (at least the ones I've used) also never use "+ve" and "-ve", but instead spell the words out: "positive" and "negative."
     
  21. May 7, 2009 #20

    Born2bwire

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    The electric field of a charge is not a local effect, it goes on to infinity. So the field is not cancelled at the charge's location, it is cancelled throughout the entire volume of the conductor. The metal, as you put it, is a sea of electrons. All of the electrons do not move in response to the electric field. If there is a net field anywhere inside the conductor, then some form of charges will move in response to it until they setup their own secondary field that cancels the originally applied field. The whole process provides feedback for itself.
     
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