Editing Permittivity (section)

== Explanation ==
In [[electromagnetism]], the [[electric displacement field]] {{math|'''D'''}} represents the distribution of electric charges in a given medium resulting from the presence of an electric field {{math|'''E'''}}. This distribution includes charge migration and electric [[dipole]] reorientation. Its relation to permittivity in the very simple case of ''linear, homogeneous, [[isotropic]]'' materials with ''"instantaneous" response'' to changes in electric field is:

<math display="block"> \mathbf{D} = \varepsilon\ \mathbf{E} </math>

where the permittivity {{mvar|ε}} is a [[scalar (physics)|scalar]]. If the medium is [[anisotropic]], the permittivity is a second rank [[tensor]].

In general, permittivity is not a constant, as it can vary with the position in the medium, the frequency of the field applied, humidity, temperature, and other parameters. In a [[nonlinear optics|nonlinear medium]], the permittivity can depend on the strength of the electric field. Permittivity as a function of frequency can take on real or complex values.

In SI units, permittivity is measured in [[farads]] per meter (F/m or A<sup>2</sup>·s<sup>4</sup>·kg<sup>−1</sup>·m<sup>−3</sup>). The displacement field {{math|'''D'''}} is measured in units of [[coulomb]]s per [[square meter]] (C/m<sup>2</sup>), while the electric field {{math|'''E'''}} is measured in [[volt]]s per meter (V/m). {{math|'''D'''}} and {{math|'''E'''}} describe the interaction between charged objects. {{math|'''D'''}} is related to the ''charge densities'' associated with this interaction, while {{math|'''E'''}} is related to the ''forces'' and ''potential differences''.