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# Magnetic potential

The magnetic potential provides a mathematical way to define a magnetic field in classical electromagnetism. It is analogous to the electric potential which defines the electric field in electrostatics. Like the electric potential, it is not directly observable - only the field it describes may be measured. There are two ways to define this potential - as a scalar and as a vector potential. (Note, however, that the magnetic vector potential is used much more often than the magnetic scalar potential.)

The magnetic vector potential is often called simply the magnetic potential, vector potential, or electromagnetic vector potential. If the magnetic vector potential is time-dependent, it also defines a contribution to the electric field.

## Magnetic vector potential

The magnetic vector potential $mathbf\left\{A\right\}$ is a three-dimensional vector field whose curl is the magnetic field, i.e.:

$mathbf\left\{B\right\} = nabla times mathbf\left\{A\right\}.$

Since the magnetic field is divergence-free (i.e. $nabla cdot mathbf\left\{B\right\} = 0$, called Gauss's law for magnetism), this guarantees that $mathbf\left\{A\right\}$ always exists (by Helmholtz's theorem).

Unlike the magnetic field, the electric field is derived from both the scalar and vector potentials:

$mathbf\left\{E\right\} = - nabla Phi - frac \left\{ partial mathbf\left\{A\right\} \right\} \left\{ partial t \right\}.$

Starting with the above definitions:

$nabla cdot mathbf\left\{B\right\} = nabla cdot \left(nabla times mathbf\left\{A\right\}\right) = 0$
$nabla times mathbf\left\{E\right\} = nabla times left\left(- nabla Phi - frac \left\{ partial mathbf\left\{A\right\} \right\} \left\{ partial t \right\} right\right) = - frac \left\{ partial \right\} \left\{ partial t \right\} \left(nabla times mathbf\left\{A\right\}\right) = - frac \left\{ partial mathbf\left\{B\right\} \right\} \left\{ partial t \right\}.$

Note that the divergence of a curl will always give zero. Conveniently, this solves the second and third of Maxwell's equations automatically, which is to say that a continuous magnetic vector potential field is guaranteed not to result in magnetic monopoles.

The vector potential $mathbf\left\{A\right\}$ is used extensively when studying the Lagrangian in classical mechanics (see Lagrangian#Special relativistic test particle with electromagnetism), and in quantum mechanics, such as the Schrödinger equation for charged particles or the Dirac equation. For example, one phenomenon whose analysis involves $mathbf\left\{A\right\}$ is the Aharonov-Bohm effect.

In the SI system, the units of A are volt seconds per metre.

### Gauge choices

It should be noted that the above definition does not define the magnetic vector potential uniquely because, by definition, we can arbitrarily add curl-free components to the magnetic potential without changing the observed magnetic field. Thus, there is a degree of freedom available when choosing $mathbf\left\{A\right\}$. This condition is known as gauge invariance.

## Magnetic scalar potential

The magnetic scalar potential is another useful tool in describing the magnetic field around a current source. It is only defined in regions of space in the absence of currents.

The magnetic scalar potential is defined by the equation:

$mathbf\left\{B\right\} = - mu_0 nabla mathbf\left\{psi\right\}.$

Applying Ampère's law to the above definition we get:

$mathbf\left\{J\right\} = frac\left\{1\right\}\left\{mu_0\right\} nabla times mathbf\left\{B\right\} = - nabla times nabla mathbf\left\{psi\right\} = 0.$

Solenoidality of the magnetic induction leads to Laplace's equation for potential:

$trianglemathbfpsi = 0.$

Since in any continuous field, the curl of a gradient is zero, this would suggest that magnetic scalar potential fields cannot support any sources. In fact, sources can be supported by applying discontinuities to the potential field (thus the same point can have two values for points along the disconuity). These discontinuities are also known as "cuts". When solving magnetostatics problems using magnetic scalar potential, the source currents must be applied at the discontinuity.

## Electromagnetic four-potential

In the context of special relativity, it is natural to join the magnetic vector potential together with the (scalar) electric potential into the electromagnetic potential, also called "four-potential".

One motivation for doing so is that the four-potential turns out to be a mathematical four-vector. Thus, using standard four-vector transformation rules, if the electric and magnetic potentials are known in one intertial reference frame, they can be simply calculated in any other inertial reference frame.

Another, related motivation is that the content of classical electromagnetism can be written in a concise and convenient form using the electromagnetic four potential, especially when the Lorenz gauge is used. In particular, in abstract index notation, the set of Maxwell's equations (in the Lorenz gauge) may be written (in Gaussian units) as follows:

$partial^mu A_mu = 0$
$Box^2 A_mu = frac\left\{4 pi\right\}\left\{c\right\} J_mu$

where $Box^2$ is the D'Alembertian and J is the four-current. The first equation is the Lorenz gauge condition while the second contains Maxwell's equations.

Yet another motivation for creating the electromagnetic four-potential is that it plays a very important role in quantum electrodynamics.

## References

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