Polarization (dielectric)

See also: Polarization

Polarization is a macroscopic physical size used in the study of the properties of the materials Diélectriques. It indicates the density of electric dipoles. Its unit in the international system is C/m². This concept was introduced by Faraday whereas he studied the behavior of the electrical insulator in electrostatic fields.

In dielectric perfect, there do not exist free electric charges. In particular, an electric field applied does not cause a Electric current. But the electric charges localized are likely to move at small distances or to vibrate under the influence of a Electric field: there is then appearance of a polarization.

Various types of polarization

From the microscopic point of view, several phenomena intervene under the effect of an electric field:

• the electronic polarization due to the displacement and the deformation of the electronic Cloud,
• the atomic polarization or ionic due to displacements of the atoms or the ions,

These phenomena are likely to create many electrostatic dipoles microscopic. Polarization is the macroscopic size corresponding to the sum, by unit of volume, of the dipole moments microscopic. Thus, if one notes $\backslash \; vec\; p\_1,\; \backslash \; vec\; p\_2,\dots ,\; \backslash \; vec\; p\_n$ N dipole moments present in a unit of volume of material, total polarization is:

$\backslash \; vec\; \{P\}\; =\; \backslash \; vec\; p\_1\; +\; \backslash \; vec\; p\_2\; +\dots \; +\; \backslash \; vec\; p\_n$

Electrically ordered materials

In certain materials, there exists an electric polarization in a spontaneous state, even in the absence of external electric field. Various signals, i.e. various arrangements of the electric dipoles in material, are then possible. The situation in the principle similar to materials is magnetically ordered. These is besides magnetic materials that was inherited the vocabulary indicating these various orders.

In a material ferroelectric, for example PbTiO₃, the electric dipoles in two close meshs are aligned in the same direction.

In a material antiferroelectric, for example PbZrO₃, the electric dipoles in two close meshs are aligned in opposite directions.

Polarization induced by a field, electric susceptibility

If polarization is due to an electric field $\backslash \; vec\; E$ applied to material, one writes with the first order that induced polarization is simply proportional to the electric field:

$\backslash \; vec\; P\; =\; \backslash \; epsilon\_0\; \backslash \; chi\; \backslash \; vec\; E$

where $\backslash \; epsilon\_0\; \backslash ,$ is the Permittivité of the vacuum and $\backslash \; chi\; \backslash ,$ is the electric Susceptibilité material. This relation is correct and sufficient for a material Isotrope and an electric field not too intense. It makes it possible to include/understand a great number of phenomena, like the Réfraction, the reflection and the Absorption of the light. For the materials Anisotropic S, the relation must be modified to include/understand the phenomenon of Biréfringence.

In the case of an intense electric field, the preceding approximation is not enough any more. The terms of higher order must be considered. It is the field of the non-linear Optique.

See too

Related articles

• Dielectric Polarizability

• Susceptibility

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