Editing Magnetic field (section)

===Ampère's Law and Maxwell's correction===
{{Main|Ampère's circuital law}}

Similar to the way that a changing magnetic field generates an electric field, a changing electric field generates a magnetic field. This fact is known as ''Maxwell's correction to Ampère's law'' and is applied as an additive term to Ampere's law as given above. This additional term is proportional to the time rate of change of the electric flux and is similar to Faraday's law above but with a different and positive constant out front. (The electric flux through an area is proportional to the area times the perpendicular part of the electric field.)

The full law including the correction term is known as the Maxwell–Ampère equation. It is not commonly given in integral form because the effect is so small that it can typically be ignored in most cases where the integral form is used.

The Maxwell term ''is'' critically important in the creation and propagation of electromagnetic waves. Maxwell's correction to Ampère's Law together with Faraday's law of induction describes how mutually changing electric and magnetic fields interact to sustain each other and thus to form [[electromagnetic waves]], such as light: a changing electric field generates a changing magnetic field, which generates a changing electric field again. These, though, are usually described using the differential form of this equation given below.

<math display="block">
  \nabla \times \mathbf{B} = \mu_0\mathbf{J} + \mu_0 \varepsilon_0 \frac{\partial \mathbf{E}} {\partial t}
</math>

where {{math|'''J'''}} is the complete microscopic [[current density]], and {{mvar|ε{{sub|0}}}} is the [[vacuum permittivity]].

As discussed above, materials respond to an applied electric {{math|'''E'''}} field and an applied magnetic {{math|'''B'''}} field by producing their own internal "bound" charge and current distributions that contribute to {{math|'''E'''}} and {{math|'''B'''}} but are difficult to calculate. To circumvent this problem, {{math|'''H'''}} and {{math|'''D'''}} fields are used to re-factor Maxwell's equations in terms of the ''free current density'' {{math|'''J'''<sub>f</sub>}}:

<math display="block">\nabla \times \mathbf{H} = \mathbf{J}_\mathrm{f} + \frac{\partial \mathbf{D}} {\partial t}</math>

These equations are not any more general than the original equations (if the "bound" charges and currents in the material are known). They also must be supplemented by the relationship between {{math|'''B'''}} and {{math|'''H'''}} as well as that between {{math|'''E'''}} and {{math|'''D'''}}. On the other hand, for simple relationships between these quantities this form of Maxwell's equations can circumvent the need to calculate the bound charges and currents.