# Current Density (J)

Current density at any point inside a conductor is defined as a vector having magnitude equal to current per unit area surrounding that point. Remember area is normal to the direction of charge flow (or current passes) through that point.
• Current density at point P is given by

Fig. 3
• If the cross-sectional area is not normal to the current, but makes an angle θ with the direction of currentm then

di = JdA cos θ
• If current density is uniform for a normal cross-section , then J = i/A.
• Current density is a vector quantity. Its direction is same as that of . Its SI unit is amp/m2 and dimension [L–2A].
• In case of uniform flow of charge through a cross-section normal to it as i = nqvA J = i/A = nqv.

Fig. 4
• Current density relates with electric field as , where σ = conductivity, and ρ = resistivity or specific resistance of substance.