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A vector quantity equal in magnitude to the instantaneous rate of flow of electric charge, perpendicular to a chosen surface, per unit area per unit time. If a wire of cross-sectional area A carries a current I, the current density J is I/A. The units of J in the rationalized meter-kilogram-second system are amperes per square meter.
a vector quantity whose magnitude is equal to the electric charge flowing per unit time per unit area, where the area is perpendicular to the direction of motion of the charge. If the charge density (the charge per unit volume) is equal to ρ, then the current density is j = ρv, where v is the average velocity of the ordered flow of the charges. When the distribution of the current density is uniform over the cross section of the conductor, the current I is equal to jS, where S is the cross-sectional area of the conductor.