Microscopic fields inside conductor

Discussion in 'Physics' started by issacnewton, Feb 13, 2011.

  1. issacnewton

    Thread Starter New Member

    Feb 13, 2011
    3
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    Hi

    I have few questions about the E inside a conductor. When excess charge is put
    on a conductor , or when the conductor is placed in an external field, the charge accumulates on the surface of the conductor so that the electric field inside the conductor is zero. Now there are atoms inside the conductor and the electrons
    (bound electrons) move around the nucleus because of the electric field of the protons. So there is an electric field at the microscopic level. So why do we say that E = 0 inside a conductor ?
     
  2. fila

    Member

    Feb 14, 2011
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    5
    I will assume you are studying Electrostatics.
    Electric field E of some charge Q is defined as F/q where F is the force of charge Q exerted on some charge q. This definition is for macroscopic charge, I assume quantum physics explains the interaction on the microscopic level. So if you put charge on the conductor you expect it to distribute itself because of the Coulomb's forces between charges. The distribution depends on the shape of a conductor, and size of course. The distribution doesn't last very long. Maybe a few nanosecond (just guessing). Eventually the whole process stops and charges distribute themselves in such a way that the sum of the forces on each of them is zero. Vector sum of course. When the vector sum is zero there is no acceleration. If the acceleration is zero then there is no change in speed. If there is no change in speed than the speed is constant. In our case it is zero or we say the charge in not moving (macroscopically). So if the charge is NOT MOVING than the electric field must be ZERO. If it isn't zero then from the definition you have F = E*q and the force between charges isn't zero so that means that they are moving. But we stated before that the charge isn't moving so E must be zero. But have in mind this is ELECTROSTATICS. When we have current flowing then there is an electric field inside the wire. I hope this will help you. IT'S NOT EASY! :)
     
  3. issacnewton

    Thread Starter New Member

    Feb 13, 2011
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    Fila, thanks for the reply.
    But it still doesnt answer my question satisfactorily. Probably electric field in the interatomic
    space inside the conductor gets averaged to zero ( time average as well as spatial average). But we still have the field once we enter the atom. Otherwise electrons (bound electrons) will not go around the nucleus.
     
  4. fila

    Member

    Feb 14, 2011
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    5
    The electric field is just a concept developed over 150 years ago by Faraday and Maxwell. Back in those days they didn't know about the structure of an atom and things happening on the atomic level. So when somebody says that E = 0 inside a conductor that means that on a macroscopic scale there isn't any current flowing or the charge isn't moving in a specific direction. You can imagine an electron moving inside the conductor 24-7 around a specific point (like Earth around a Sun, although this is incorrect) but it will always stay in that localized region. But when you apply an external electric field all those electrons inside the conductor will start to move in one specific direction. They will have some average speed, for example v = 1 mm/s and we can measure that speed as current I. Electric field is just a concept that explains some phenomena. It can't explain everything. You know that the photoelectric effect can't be explained by Maxwell's equations. Electric field is the core concept in those equations and because of that it can't be applied on everything. I'm a student of electrical engineering so I was also interested in electric fields but this is the best way I can answer your question.
     
    Last edited: Feb 15, 2011
  5. issacnewton

    Thread Starter New Member

    Feb 13, 2011
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    0
    Fila

    I dont agree with everything you say. There is some explanation for this. The author of
    "Classical Electrodynamics", J.D.Jackson talks about this somewhere. The fields inside a conductor vanish because they average over time and space inside the conductor. If we go
    inside an atom, there is an electric field there. Jackson gives rigorous mathematical explanation of this using Maxwell's equations but I don't seem to recall it. We don't need quantum mechanics to answer this
     
  6. amilton542

    Active Member

    Nov 13, 2010
    494
    64
    A charge does Not feel its own electric field,if its positive it will be radially pointing outwards if its negative it will be radially pointing inwards,for E to equal zero, somewhere down the line forces have cancelled each other out,it depends on the charge configuration and symmetry arguments.Calculating electric fields only determines how charges move, what acceleration they will obtain which means how there kinetic energy wil change, if the electric fields are to complicated it will be easier to work with equipotentials because the change in kinetic energy depends on the change of potential when you go from one point to another. Listen "carefully" to what he has to say, this man is a genius.you MUST go through every session to understand whats going on. get the pen and paper ready

    http://www.youtube.com/watch?v=OsWDUqJQcpk

    http://www.youtube.com/watch?v=XaaP1bWFjDA

    http://www.youtube.com/watch?v=ldJhMDuOGxY
     
    Last edited: Feb 22, 2011
  7. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47

    The theory that there is no field is often based on the idea that the conductor is one homogenous thing without a fine structure. When looked at at an atomic level that breaks down and we see granularity. Really a many body problem ( without a general analytic solution ) and requiring quantum mechanics to boot. Yes at the microscopic level a conductor is swarming with fields and particles, but averaged over a macroscopic volume, and whrere resistance is low enough to be ignored there is no net field.
     
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