Current electricity

Generally, the free electrons are responsible for the flow of electric current. In a metallic conductor, on an average there will be one free electron available per atom. At any temperature, these free electrons move freely and collide with fixed atoms or ions inside the conductor. These collisions are inelastic in nature and causes the transfer of energy. The resultant velocity will be zero in any direction. But when potential difference is applied between the ends of a conductor, an electric field E will act on these free electrons. As a result, and electron moves with an average velocity in a direction opposite to the field. This is called drift velocity 'Vd' and is of the order of 10^-3m/s. Like that, all electrons will drift in the same direction. This drift is responsible for the flow of charge through the conductor. By convection, the direction of the current is the direction of positive charge motion.

In electrolytes or in gaseous conductors, the charge carries will be either positive or negative ions or both. But in case of semi conductors, the conduction is due to electrons and holes.