= Solution
An ideal current-carrying wire consists of opposite charge populations, one associated with the stationary lattice and one with drifting carriers. They may cancel in the lattice frame while carrying a net current, as in part (c), up to exchanging the charge signs and current direction. In a carrier-rest description their <densities> no longer cancel because the two populations undergo different <length contractions>. The wire then has both <electric fields> and <magnetic fields>.
\b[A neutral steady current in one frame generally becomes a charged steady current in another.] This illustrates how <electric fields> and <magnetic fields> are parts of one relativistic field rather than separately invariant <forces>. Real resistive wires also have a driving longitudinal <electric field> and geometry-dependent surface charges; the infinite uniform filament model isolates the transverse fields of a neutral current and does not describe every field of a complete circuit.
Back to article page
