The time rate of flow of charge through any cross-section is called current. Therefore, if through a cross section, Δq charge passes in time Δt, the average electric current through that area
Regarding electric current following points are worth noting :
(a) Current is assumed to be a fundamental quantity in physics with unit ampere and dimension [A]. The CGS unit of current is emu of current and is called biot (Bi), i.e.,
(b) The current is same for all cross-sections of a conductor of non-uniform cross-section. Similar to water flow, charge flows faster where the conductor is smaller in cross-section and slower where the conductor is larger in cross-section,so that charge rate remains unchanged.
(c) Though conventionally a direction is associated with current (opposite to the motion of electrons) it is not a vector as the direction merely represents the sense of charge flow and not a true direction. Further current does not obey the law of parallelogram of vectors, i.e., if two currents i1 and i2 reach a point we always have i = i1 + i2 whatever be the angle between i1 and i2.
(d) By convention, the direction of current is taken to be that in which positive charge moves and opposite to the direction of flow of negative charge.
(e) As charge is conserved and current is the rate of flow of charge, the charge entering at one end per second of a conductor is equal to the charge leaving the other end per second.
(f) Current indifferent situations is due to motion of different charge carries. Current in conductors and vacuum tubes is due to motion of electrons, in electrolytes due to motion of both positive and negative ions, in discharge tube due to motion of positive ions and negative electrons and in semiconductors due to motion of electrons and holes.