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Conductor Concept

  1. Feb 12, 2005 #1
    Consider a long conducting rod that is suspended by insulating strings. Assume that the rod is initially electrically neutral, and that it remains so. The rod is positioned along the x axis, and an external electric field that points in the positive x direction (to the right) can be applied to the rod and the surrounding region. The atoms in the rod are composed of positive nuclei (indicated by plus signs) and negative electrons (indicated by minus signs). Before application of the electric field, these atoms were distributed evenly throughout the rod.

    Imagine that the rightward current flows in the rod for a short time. As a result, the net charge on the right end is positive and left end is negative.

    The charge imbalance that results from this movement of charge will generate an additional electric field near the rod. In what direction will this field point?

    Points to the left and opposes the initial applied field?
    Last edited: Feb 12, 2005
  2. jcsd
  3. Feb 12, 2005 #2


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    That's right.
  4. Feb 16, 2005 #3
    Probably nothing related to this happens , thermal noise is quite large , much more so than any almost instantaneous differences in current, The field direction fleeting at the most could be anywhere -- nothing tells the electrons ( in your case ) where to go , except their own momentum , there are in random 3D motion at a fairly slow average velocity. The average field due to imbalance of motion is so small ( due to the large numbers) that on average it will never be seen ----- it's like asking what happens if ALL electrons move in the same direction --- well we do not ask this question until we reach almost absolute zero - then we get fantastic answers.
  5. Feb 16, 2005 #4


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    This is not true. In fact, the whole concept of Hall voltage is a clear illustration that one CAN maintain a potential difference across a conductor or semiconductor even at room temperature. Thermal motion doesn't always wash out such effects. So the original answer is correct.

  6. Feb 16, 2005 #5


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    How large are the thermal effects ?

    At room temperature, [tex]\epsilon_{th} \approx k_BT < 10^{-20} J [/tex]

    But if you apply even a modestly small potential of 1 volt between the ends of the long rod, you make the energy due to the field to be
    [tex]\epsilon_{field} \approx eV > 10^{-19} J [/tex]

    That's nearly 2 orders of magnitude bigger than the thermal effects. Surely, thermal noise can not dominate.
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