Question

A current passes through a wire of non-uniform cross-section. Which of the following quantities are independent of the cross-section?
(a) The charge crossing in a given time interval
(b) Drift speed
(c) Current density
(d) Free-electron density

Answer 1

(a) The charge crossing in a given time interval
(d) Free-electron density

Drift speed and current density are inversely proportional to the area of cross-section of a wire. Thus, they are dependent on the cross-section. The charge crossing in a given time interval is independent of the area of cross-section of the wire. Free electron density is the total number of free electrons per unit volume of the wire. The density of free electrons depends on the distribution of the free electrons throughout the volume of the wire. It does not depend on the cross-section of the wire.