Current density j gives the amount of charge flowing per second per
unit area normal to the flow,
j = nq vd
where n is the number density (number per unit volume) of charge
carriers each of charge q, and vd is the drift velocity of the charge
carriers For electrons q = – e If j is normal to a cross-sectional area
A and is constant over the area, the magnitude of the current I through
the area is nevd A 8