# phsics

posted by
**lam**
.

Two materials, for example silver and copper, of same cross section can be welded into a composite wire. When a voltage is applied across this wire, a stable current i whose value is determined by Ohm¡¦s Law is formed. Since the wire has uniform cross section, the current densities in two different materials have to be the same. But their conductivities£m1 and£m2 of the two conductor types are different and thus the electric fields within each of the two conductors are different. According to Gauss¡¦s Law, the change from one electric field to another has to be generated by some net charge accumulated at their interface. Show that this amount of charge is equal to

i/(4£k) * [1/£m2- 1/£m1]

assuming£m2>£m1