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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.
Drift current density due to the charge carriers such as free electrons and holes is the current passing through a square centimeter area perpendicular to the direction of flow. (i) Drift current density J n {\displaystyle J_{n}} , due to free electrons is given by:
The derivation of a useful equation for modelling the effect of eddy currents in a material starts with the differential, magnetostatic form of Ampère's Law, [6] providing an expression for the magnetizing field H surrounding a current density J: =.
The Poynting vector appears in Poynting's theorem (see that article for the derivation), an energy-conservation law: =, where J f is the current density of free charges and u is the electromagnetic energy density for linear, nondispersive materials, given by = (+), where
where ρ is the charge density, which can (and often does) depend on time and position, ε 0 is the electric constant, μ 0 is the magnetic constant, and J is the current per unit area, also a function of time and position.
Electric charge density: ρ Q: Electric charge per unit volume C/m 3: L −3 T I: intensive Electrical conductance: G: Measure for how easily current flows through a material siemens (S = Ω −1) L −2 M −1 T 3 I 2: scalar Electrical conductivity: σ: Measure of a material's ability to conduct an electric current S/m L −3 M −1 T 3 I 2 ...
The net electric current I is the surface integral of the electric current density J passing through Σ: =, where dS denotes the differential vector element of surface area S, normal to surface Σ. (Vector area is sometimes denoted by A rather than S , but this conflicts with the notation for magnetic vector potential ).
The diffusion current and drift current together are described by the drift–diffusion equation. [1] It is necessary to consider the part of diffusion current when describing many semiconductor devices. For example, the current near the depletion region of a p–n junction is dominated by the diffusion current. Inside the depletion region ...