# Electromagnetic fields in a circuit?

1. Nov 27, 2009

### HydroGuy

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. Nov 27, 2009

### Bob S

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.
http://en.wikipedia.org/wiki/Inductor.
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

3. Nov 27, 2009

### Bob S

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.