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Electric Field in Circuits/Conductors

  1. Oct 16, 2011 #1
    A little curiosity here. In my Electricity and Magnetism class, for the first half of the semester we were taught strictly: there can never be an electric field in a conductor. Alright, it makes sense the way it is explained, fair enough.

    Now we come to circuits, and are told that there is an electric field in the conducting wires that creates current, generated by the electromotive force. My question is, why is there an electric field in these wires? I thought it wasn't possible?
  2. jcsd
  3. Oct 16, 2011 #2

    Philip Wood

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    There cannot be an electric field in a conductor in electrostatic conditions. The reason is simple: if there were an electric field the free charges (which characterise a conductor) will drift in the direction of the force due to the field, so the conditions couldn't be electrostatic after all!

    When there is a current, i.e. a drifting of free charges, there must be an electric field, or the charges would quickly lose their drift velocity, because of resistive forces due to collisions with the lattice.
  4. Oct 16, 2011 #3
    I'm unfamiliar with the term "electrostatic conditions"... does this mean that charge carriers are at rest?
  5. Oct 16, 2011 #4


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    Sure I guess. It just means that it is a time-static problem (no currents). Magnetostatics has static currents (currents that don't change with time) so wouldn't be the same thing as time-static.
  6. Oct 16, 2011 #5
    Well I'm still confused then. Before there is a current, everything is at rest. It is an electrostatic condition then, so there is no electric field. How then, all of a sudden, is there magically a field, and the conductor is no longer electrostatic? Do the charge carriers start moving before the electric field is produced, or something?
  7. Oct 17, 2011 #6

    Philip Wood

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    Maybe I'm missing your point, but the free carriers start to drift when a field is applied, for example by connecting a battery across the conductor.
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