The ampere per meter squared, symbolized A/m 2 , is the International Unit of electric current density . A current density of 1 A/m 2 represents one ampere of electric current flowing through a material with a cross-sectional area of one square meter.
The ampere per meter squared is a small unit of current density. Suppose a wire has a cross-sectional area of one millimeter squared (1 mm 2 ). This is 0.000001 meter squared (10 -6 m 2 ). If the current density in this wire is 1 A/m 2 , then the wire carries 10 -6 A, or one microampere (1 µA), a tiny current. Suppose this same wire carries a current of one ampere (1 A), which is an entirely plausible scenario. Then the current density in the wire is 1,000,000 amperes per meter squared (10 6 A/m 2 ).
Sometimes, larger units of current density are specified. For example, one ampere per millimeter squared (A/mm 2 ) represents a current of 1 A flowing through a conductor with a cross-sectional area of 1 mm. This unit is equal to 1,000,000 (10 6 ) A/m 2 . One milliampere per millimeter squared (mA/mm 2 ) represents a current of 1 mA flowing through a conductor with a cross-sectional area of 1 mm. This unit is equal to 1,000 (10 3 ) A/m 2 .
Determination of current density is straightforward in direct-current ( DC ) and low-frequency alternating-current ( AC ) circuits, because the current is distributed uniformly throughout the cross section of a solid conductor. But at radio frequencies ( RF ), more current flows near the outer surface of a solid conductor than near its center. This is known as skin effect , and it dramatically reduces the conductivity of wires in RF applications as compared with DC and low-frequency AC circuits. At RF, current density is sometimes near zero near the center of a solid conductor, and quite high near the outer periphery. The average current density can nevertheless be calculated according to the following formula:
D = I / X
where D is the current density in amperes per meter squared, I is the current in amperes, and X is the cross-sectional area of the conductor in meters squared.