26.3: Current Density
The total amount of current flowing through one unit value of a cross-sectional area is referred to as current density. If the current flow is uniform, the amount of current flowing through a conductor is the same at all points along the conductor, even if the conductor area varies. The current density consists of the local magnitude and direction of the charge flow, which varies from point to point. Current density is measured in amperes per meter square, and direction is defined as the net flow of positive charges through the area. Also, for any given current, as the diameter of the wire in a circuit increases, the charge density decreases. The magnitude of the current density is the current divided by the area.
Thus, the current density can be determined as:
If q is positive, drift velocity is in the same direction as the electrical field. If q is negative, drift velocity is in the opposite direction to the electric field. Either way, the direction of the current density is in the direction of the electrical field. For example, in a sodium chloride solution, current can be carried by both positive sodium ions and negative chlorine ions; the total current is found by adding up the currents due to each kind of charged particle.