Four-current

Electromagnetism
Electricity · Magnetism
Electrostatics Electric charge · Coulomb's law · Electric field · Electric flux · Gauss's law · Electric potential energy · Electric potential · Electrostatic induction · Electric dipole moment · Polarization density
Magnetostatics Ampère's law · Electric current · Magnetic field · Magnetization · Magnetic flux · Biot–Savart law · Magnetic dipole moment · Gauss's law for magnetism
Electrodynamics Lorentz force law · emf · Electromagnetic induction · Faraday’s law · Lenz's law · Displacement current · Maxwell's equations · EM field · Electromagnetic radiation · Liénard–Wiechert potential · Maxwell tensor · Eddy current
Electrical Network Electrical conduction · Electrical resistance · Capacitance · Inductance · Impedance · Resonant cavities · Waveguides
Covariant formulation Electromagnetic tensor · EM Stress-energy tensor · Four-current · Electromagnetic four-potential
Scientists Ampère · Coulomb · Faraday · Gauss · Heaviside · Henry · Hertz · Lorentz · Maxwell · Tesla · Volta · Weber · Ørsted
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In special and general relativity, the four-current is the Lorentz covariant four-vector that replaces the electromagnetic current density, or indeed any conventional charge current density. Its four components are given by:

$J^a = \left(c \rho, \mathbf{j} \right)$

where

c is the speed of light

ρ the charge density

j the conventional current density.

a labels the space-time dimensions

This can also be expressed in terms of four-velocity as ^{[1] [2]}

$J^a = \rho_0 u^a \,$

where

$\rho = \frac{\rho_0}{\sqrt{1-\frac{v^2}{c^2}}}$

In this article the metric η_μν = diag[1, − 1, − 1, − 1] is used.

Continuity equation

In special relativity, the statement of charge conservation (also called the continuity equation) is that the Lorentz invariant divergence of J is zero ^[3]:

$D \cdot J = \partial_a J^a = \frac{\partial \rho}{\partial t} + \nabla \cdot \mathbf{j} = 0$

where D is an operator called the four-gradient and given by (1/c ∂/∂t, ∇). The summation convention has been used, so that the space-time dimensions are implicitly summed over. i.e.

$\partial_a J^a = \sum_{i=0}^{3} \partial_i J^i$

Sometimes, the above relation is written as

$J^a{}_{,a}=0\,$

In general relativity, the continuity equation is written as:

$J^a{}_{;a}=0\,$

where the semi-colon represents a covariant derivative.

Other uses

This four-vector is related to four-potential as [ref. 1, p519]

$\Box A^a = \mu_0 J^a$

where $\Box$ is the D'Alembert operator, defined as

$\Box = \partial_{\mu} \partial^{\mu} = -\frac{{1}}{{c^2}} \frac{{\partial}}{{\partial t^2}} + \nabla^2$

Also, two of the Maxwell's Equations can be written in terms of the electromagnetic tensor as [ref. 1, p520]

$\partial_b F^{ab} = \mu_0 J^a$

Euclidean metric

Although Minkowski's space is not euclidean, some authors prefer to use a euclidean metric, where $\eta_{\mu\nu} = diag[1,1,1,1] \,$, and

$J^a = J_a \,$

Using this metric, the four-current is rewritten as

$J^a = \left(ic \rho, \mathbf{j} \right)$

where i is the imaginary unit.

General Relativity

In general relativity, the four-current is defined as the divergence of the electromagnetic displacement, defined as

$\mathcal{D}^{\mu \nu} \, = \, \frac{1}{\mu_{0}} \, g^{\mu \alpha} \, F_{\alpha \beta} \, g^{\beta \nu} \, \sqrt{-g} \,$

Thus,

$J^\mu = \partial_\nu \mathcal{D}^{\mu \nu}$

Physical interpretation

This four-vector unifies charge density (related to electricity) and current density (related to magnetism) in one electromagnetic entity. As studied in electrostatics, charges or distributions of charge moving only through time, thus having no velocity, have only charge density, while if they are moving through space too, thus having some velocity v, they have current density too. This means that charge density is related to time, while current density is related to space.

See also

• Noether's theorem
• Covariant formulation of classical electromagnetism

References

1. ^ Roald K. Wangsness, Electromagnetic Fields, 2nd edition (1986), p. 518, 519
2. ^ Melvin Schwartz, Principles of Electrodynamics, Dover edition (1987), p. 122, 123
3. ^ J. D. Jackson, Classical Electrodynamics, 3rd Edition (1999), p. 554