Magnetic potential

From Wikipedia, the free encyclopedia

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 electomagnetic vector potential. If the magnetic vector potential is time-dependent, it also defines a contribution to the electric field.

1 Magnetic vector potential
- 1.1 Gauge choices
2 Magnetic scalar potential
3 Four dimensional potentials
4 References

[edit] Magnetic vector potential

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

$\mathbf{B} = \nabla \times \mathbf{A}$

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

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

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

Starting with the above definitions:

$\nabla \cdot \mathbf{B} = \nabla \cdot (\nabla \times \mathbf{A}) = 0$

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

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 $\mathbf{A}$ 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 $\mathbf{A}$ is the Aharonov-Bohm effect.

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

[edit] 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 $\mathbf{A}$ . This condition is known as gauge invariance.

[edit] 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:

$\mathbf{B} = - \mu_0 \nabla \mathbf{\psi}$

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

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

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

$\triangle\mathbf\psi = 0$

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.

[edit] Four dimensional potentials

Main article: Electromagnetic four-potential

In special relativity, the magnetic potential joins with the electric potential into the electromagnetic potential. This may be done by joining a scalar electric potential with a vector magnetic potential or by joining a scalar magnetic potential with a vector electric potential. Either way, the final result must have four dimensions. The former method is more popular because the scalar electric potential is widely familiar as voltage and because "the concept of vector electric potential is just too weird to exist in the same universe as decent common-sense folks."

In four dimensional notation, the Lorenz gauge may be written more concisely by using the D'Alembertian and the four-current, J:

$\Box^2 A_\mu = \frac{4 \pi}{c} J_\mu$

in Gaussian units. This equation can be expanded to yield Maxwell's equations, and by extension the rest of classical electrodynamics.

[edit] References

Ulaby, Fawwaz (2007). Fundamentals of Applied Electromagnetics, Fifth Edition. Pearson Prentice Hall, 226-228. 0-13-241326-4.
Jackson, John David (1998). Classical Electrodynamics, Third Edition. John Wiley & Sons.
Duffin, W.J. (1990). Electricity and Magnetism, Fourth Edition. McGraw-Hill.

Categories: Potential | Magnetism

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Magnetic potential

From Wikipedia, the free encyclopedia

Contents

[edit] Magnetic vector potential

[edit] Gauge choices

[edit] Magnetic scalar potential

[edit] Four dimensional potentials

[edit] References

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