# ampere per meter squared (A/m 2)

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.

Also see ampere , meter squared , skin effect , and International System of Units ( SI ).

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