Electrical polarization is the extent to which polar molecules have been induced in a dielectric, or in which permanent polar molecules have become aligned under the action of an external electric field. Electric polarization, P, is defined as the electric moment per unit volume. Thus,
P = dp/dv
dp = (σP dA)dConsequently, in keeping with the definition of polarization,
P = dp/dv = dp/(volume of tube of dielectric) = (σP dA)d/(dA)dor
P = σPThis presupposes, of course, that polarization throughout the dielectric is uniform, in which case the only unneutralized charges are those located at the surfaces of the dielectric. The polarization is then equal in magnitude to the charge per unit area on the surface of the dielectric.
For isotropic dielectrics (those whose electrical properties are identical in all directions), the polarization that occurs under the action of an electric field has the same direction as the field. Also, the degree of polarization is a function of the strength of the field. This dependence is expressed as
P = χeε0Ewhere χe, the electric susceptibility of the material, is a dimensionless quantity. For most isotropic dielectrics χe is constant except for very strong fields.
For anisotropic dielectrics, P and E are normally not parallel. Although their relationship is still expressed by the final equation above, the susceptibility is no longer a scalar, as for isotropic materials, but is instead a tensor.
Related category• ELECTRICITY AND MAGNETISM
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