Magnetic Field

Magnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field.

In view of special relativity, the electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic field. A pure electric field in one reference frame is observed as a combination of both an electric field and a magnetic field in a moving reference frame.

In quantum physics, the pure magnetic (and electric) fields are understood to be effects caused by virtual photons; in the language of the Standard Model the electromagnetic force in all of its manifestations is mediated by photons. Most often this microscopic description is not needed because the simpler classical theory covered in this article is sufficient; the difference is negligible under the low field energies of most circumstances.

The term magnetic field is used for two different vector fields, denoted B and H. There are many alternative names for both. To avoid confusion, this article uses B-field and H-field for these fields, and uses magnetic field where either or both fields apply.

The B-field can be defined in many equivalent ways based on the effects it has on its environment. For instance, a particle having an electric charge, q, and moving in a B-field with a velocity, v, experiences a force, F, called the Lorentz force . In SI units, the Lorentz force equation is

F = q ( V × B )

where × is the vector cross product. The B-field is measured in teslas in SI units and in gauss in cgs units.

An alternate working definition of the B-field can be given in terms of the torque on a magnetic dipole placed in a B-field:

τ = m_m × B

for a magnetic dipole moment m (in ampere-square meters).