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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.
Hence the probability current (density) is in SI units: = / = [(^ ^) | |] + () where S is the spin vector of the particle with corresponding spin magnetic moment μ S and spin quantum number s . It is doubtful if this formula is valid for particles with an interior structure.
Continuous charge distribution. The volume charge density ρ is the amount of charge per unit volume (cube), surface charge density σ is amount per unit surface area (circle) with outward unit normal nĚ‚, d is the dipole moment between two point charges, the volume density of these is the polarization density P.
Regardless of the driving force, the current density is found to be greatest at the conductor's surface, with a reduced magnitude deeper in the conductor. That decline in current density is known as the skin effect and the skin depth is a measure of the depth at which the current density falls to 1/e of its value near the surface. Over 98% of ...
The density of the linear momentum of the electromagnetic field is S/c 2 where S is the magnitude of the Poynting vector and c is the speed of light in free space. The radiation pressure exerted by an electromagnetic wave on the surface of a target is given by P r a d = S c . {\displaystyle P_{\mathrm {rad} }={\frac {\langle S\rangle }{\mathrm ...
The upper graph shows the current density as function of the overpotential η . The anodic and cathodic current densities are shown as j a and j c, respectively for α=α a =α c =0.5 and j 0 =1mAcm −2 (close to values for platinum and palladium). The lower graph shows the logarithmic plot for different values of α (Tafel plot).
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.
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 ).