Current in metals and electrolytes

At the sites of the crystal lattice of a metal there are positive ions, and between them free electrons detached from the atoms move randomly. When voltage is applied to the ends of the metal, the free electrons start an ordered motion under the electric field — this is electric current in metals. The collisions of the electrons with the ions resist their motion; this is why a conductor has resistance and heats up. The direction of current is taken as the direction of motion of positive charges, that is, from the “+” pole of the source to the “–” pole; the electrons actually move the opposite way, from “–” to “+”. The current is I = q/t, the charge passing through the cross-section in time t. The charge is found from the number of electrons and the electron charge e: q = N·e.