# Current in two wires

1. May 24, 2013

### HAMJOOP

Wire A and B, which have the same cross-sectional area are connected in series. There is a p.d. V across the whole wire.

Suppose the two wires obey Ohm's law J = σE
Also, A and B have different conductivity.

Therefore electric field of A is different from B.
There should be a layer of charge between the junction of A and B.

So, when does the charge accumulation start ?
At the transient time ?

At that moment, current flows in wire A =/= current flows in wire B ?

2. May 24, 2013

### tech99

When the voltage is first applied at the end of the conductor, the electrons of the conductors support a longitudinal wave, which travels along the wire. It may be thought of as being a wave in the electron plasma, and it carries E and B fields along with it. When the wave reaches the junction and encounters different resistance, part of it is reflected back to the source. This explains the discontinuity at the junction. A steady current, caused by very slow electron drift, then continues to flow uniformly across the junction. All steady currents are first set up by a wave travelling along a conductor, and similarly when they are swirtched off.