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Electromagnetic fields in a circuit?

  1. Nov 27, 2009 #1
    I'm having some trouble getting a good understanding of how the EM fields are working in a circuit. Essentially, Maxwell's equations and boundary conditions at a perfect conductor are confusing me.

    If, in a perfect conductor, the E and B fields are zero, then how does a voltage appear in a circuit? IE, if we attached an ideal DC voltage source to a loop of perfect conductor with a resistor in the middle, then if there is no E or B in the loop, is there a voltage at the resistor?

    Any references to understanding the fields when DC and AC are applied to a t-line or circuit would be very helpful.
  2. jcsd
  3. Nov 27, 2009 #2
    Even a perfectly conducting straight wire has external magnetic fields. So a straight wire has inductance. A coil has a lot more. In this link, here are formulas for a variety of coil geometries.
    In a coaxial transmission line, the center conductor has series inductance L per unit length and shunt capacitance C (to the outer conductor) per unit length. The characteristic impedance Z of a coaxial transmission line is
    Z = sqrt(L/C) ohms

    Bob S
  4. Nov 27, 2009 #3
    If you had a loop of a perfectly conducting wire looping around from one resister terminal to the other, ambient magnetic fields will induce a voltage across the resistor.

    Any current, ac or dc, in a circuit loop, will have a magnetic field in its center.

    A long straight perfectly conducting wire carrying a current will have a magnetic field surrounding the wire.

    Magnetic fields are anywhere there are currents.

    Magnetic fields exist between the inner and outer conductors of transmission lines.

    Read about transmission lines and the telegrapher's equation in


    Bob S
    Last edited: Nov 27, 2009
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