9.1.4—Use, for a current-carrying conductor, the expression I = Anvq

For charge carriers of number density n, cross-sectional area A, drift speed v and charge magnitude q, I=Anvq.

The formula counts carriers crossing the section each second; use the conductor’s actual area and the carrier type appropriate to the material.

A thinner wire with the same carrier density and drift speed carries less current because A is smaller.

Drift speed is not the same as the electromagnetic signal speed, and n is a volume number density, not total carrier count.