In most substances, the carriers of current are electrons; in some
cases, for example, ionic crystals and electrolytic liquids, positive and
negative ions carry the electric current 7 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