It may have occurred to you that since parallel currents attract, the current within a single wire should contract into a tiny concentrated stream along the axis. Yet in practice the current typically distributes itself quite uniformly over the wire. How do you account for this? If the positive charges (density p+) ge at rest, and the negative charges (density p –) move at speed v (and none of these depends on the distance from the axis), show that p – = – p+γ2, where γ ≡ l/√1 – (v/c)2 and c2 = 1/µ0ε0. If the wire as a whole is neutral, where is the compensating charge located? 16 [Notice that for typical velocities (see Prob. 5.19) the two charge densities are essentially unchanged by the current (since γ ≈ 1). In plasmas, however, where the positive charges are also free to move, this so-called pinch effect can be very significant.]