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In electromagnetism, current density is the amount of charge per unit time that flows through a unit area of a chosen cross section. [1] The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional area at a given point in space, its direction being that of the motion of the positive charges at this point.
Current density in round wire for various skin depths. Numbers shown on each curve are the ratio of skin depth to wire radius. The curve shown with the infinity sign is the zero frequency (DC) case. All curves are normalized so that the current density at the surface is the same.
For negative charges, the sign of the current density is opposite to the velocity of the charges. [2]: 749 In SI units, current density (symbol: j) is expressed in the SI base units of amperes per square metre. [4]: 22 In linear materials such as metals, and under low frequencies, the current density across the conductor surface is uniform.
Critical temperature T c, the temperature below which the wire becomes a superconductor; Critical current density J c, the maximum current a superconducting wire can carry per unit cross-sectional area (see images below for examples with 20 kA/cm 2). Superconducting wires/tapes/cables usually consist of two key features:
Since the E field is uniform in the direction of wire length, for a conductor having uniformly consistent resistivity ρ, the current density J will also be uniform in any cross-sectional area and oriented in the direction of wire length, so we may write: [40] =.
Silver, although it is the least resistive metal known, has a high density and performs similarly to copper by this measure, but is much more expensive. Calcium and the alkali metals have the best resistivity-density products, but are rarely used for conductors due to their high reactivity with water and oxygen (and lack of physical strength).
In practice, maximum current density can vary from 2.5 A/mm 2 for a wire isolated from free air to 6 A/mm 2 for a wire in free air. [citation needed] If the wire is carrying high frequency currents (above 10 kHz), the skin effect may affect the distribution of the current across the section by concentrating the current on the surface of the ...
The thermal current density is the flux per unit time of thermal energy across a unit area perpendicular to the flow. It is proportional to the temperature gradient. j q = − κ ∇ T {\displaystyle \mathbf {j} _{q}=-\kappa \nabla T} where κ {\displaystyle \kappa } is the thermal conductivity.