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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.
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.
mass density usually simply called density kilogram per cubic meter (kg/m 3) volume charge density: coulomb per cubic meter (C/m 3) resistivity: ohm meter (Ω⋅m) sigma: summation operator area charge density: coulomb per square meter (C/m 2) electrical conductivity: siemens per meter (S/m) normal stress: pascal (Pa)
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.
Some quantities are known as several different names such as the magnetic B-field which is known as the magnetic flux density, the magnetic induction or simply as the magnetic field depending on the context. Similarly, surface tension can be denoted by either σ, γ or T. The table usually lists only one name and symbol that is most commonly used.
Symbol [1] Name of quantity Unit name Symbol Base units E energy: joule: J = C⋅V = W⋅s kg⋅m 2 ⋅s −2: Q electric charge: coulomb: C A⋅s I electric current: ampere
Symbol Quantity Expression in terms of SI base units coulomb per square metre: C/m 2: electric displacement field, polarization density: m −2 ⋅s⋅A coulomb per cubic metre: C/m 3: electric charge density: m −3 ⋅s⋅A ampere per square metre: A/m 2: electric current density: m −2 ⋅A siemens per metre: S/m electrical conductivity: m ...
In an ideal case, cross-section and physical composition of the examined material are uniform across the sample, and the electric field and current density are both parallel and constant everywhere. Many resistors and conductors do in fact have a uniform cross section with a uniform flow of electric current, and are made of a single material ...