A material is dielectric if it does not contain a electric charges likely to move way Macroscopique. In other words, it is a medium which cannot lead the Electric current. For this reason, it is called sometimes electric Isolant . One counts among these mediums the Verre and of many plastic. For example, the electric cables are often protected from a plastic coating to prevent that the electric current can leave there.

In spite of the impossibility of the dielectric mediums of leading the current, they show many electric characteristics. Indeed the atoms which constitute material can present electrostatic dipoles which are likely to interact with a Electric field. This interaction results in the creation of a polarization connected to this electric field, the microscopic level, by a Polarisabilité, and at the macroscopic level, by the electric Susceptibilité.

Physical phenomena in the dielectric mediums

The electrons present in a dielectric medium cannot, by definition, to move at long distances. It can on the other hand present movements of very small amplitude to our scale, but which can be in the beginning many phenomena. These movements are often movements of Oscillation around the core: the electronic cloud can be deformed and thus create a electrostatic Dipôle. The same applies to the total displacement of the atoms within material (they also create dipoles).

Creation of a polarization

See also: Polarization (dielectric)

By subjecting material to a Electric field such dipoles can be created. If they existed already, that can have like effect of all to align them in the same direction. From a microscopic point of view, one can connect the Amplitude wave to the dipole created via the concept of Polarisabilité, which is a specific characteristic to each atom. It is however impossible to measure such microscopic sizes. One prefers to use a macroscopic size, the polarization, which is worth the sum of all the dipoles of material. This polarization thus comes from various physical effects:

• the electronic polarization , always presents, is due to displacement and with the deformation of each electronic cloud,
• the atomic polarization is due to displacements of the atoms,
• the polarization of orientation exists when dipoles already present all are aligned between them.

Polarization $\backslash \; vec\; P$ is often proportional to the electric field $\backslash \; vec\; E$ which created it (this case is known as linear ):

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

with $\backslash \; epsilon\_0\; \backslash ,$ the Permittivity of the vacuum and $\backslash \; chi\; \backslash ,$ the electric Susceptibility of the material, which is a Complex number. In the case of dielectric anisotropic, $\backslash \; chi\; \backslash ,$ is a Tenseur of row 2.

By generalizing that with phenomena where polarization is not proportional to the electric field, one reaches the field of the non-linear Optique.

Phenomena of refraction, reflection and absorption

The polarization created in the dielectric medium intervenes in phenomena bringing into play electromagnetic waves, like the Lumière, because they present an electric field. The Maxwell's equations then make it possible to show that the real part of $\backslash \; chi\; \backslash ,$ modifies the speed C of a light wave being propagated in material compared to the speed c₀ which it would have in the vacuum according to the relation:

$c\_0=c\; \backslash \; sqrt\; \{1+\; \backslash \; Re\; (\backslash \; chi)\}$.

That corresponds exactly to the definition of the Index of refraction N of a medium: $c\_0=c\; \backslash ;\; n$. That thus explains the phenomenon of Réfraction of the light. In addition, the imaginary part corresponds to a Absorption of the light by material. When the material is Anisotrope, the relations are not also simple, and one sees appearing the phenomenon of Biréfringence: two rays are refracted instead of only one.

The reflection can also be included/understood in this way. One can then show that, in the passing of the light through a Dioptre separating two different mediums, part of the wave is reflected, and the remainder is refracted. Calculation corresponding leads to the Coefficients of Fresnel which give the proportions of reflected light and refracted. If all the light is considered (total Réflexion), one can observe a Onde évanescente , i.e. a wave of very short range which appears on the other side of the diopter. One even can, while placing another diopter very close to the first, to recover this wave évanescente: it is the phenomenon of frustrated total Réflexion.

Sizes characteristic of the dielectric mediums

The dielectric materials are characterized in particular by:

• them dielectric Rigidity
• them dielectric ε
• their loss angle or tangent Permittivity delta

Some usual dielectric mediums

Solids

• the Glass, used to make Insulating S of lines high voltage;
• the ceramic , very much used for the materials HTB] of [[the electric station|electric stations]]; * majority of the plastics, in particular [[polyethylene]] in its réticulée form (XLPE) and [[polyvinyl Chloride|PVC]], both used for the cables; * it [[Polypropylene]], used in particular in [[condensing]] the S in [[High voltage has|HTA]] or [[High voltage B|HTB]]; * it [[mica]], which is hardly used more nowadays in [[electrotechnical industry]]; * it [[bakelite]], formerly very much used for [[switchgear]] the low tension; * it [[Teflon]], used for certain parts of [[the circuit breaker with high voltage|circuit breakers with high voltage]]. ===Gazeux=== *l' air *l' [[sulfur hexafluoride]] *l' [[nitrogenizes]] ===Liquides === *le [[pyralene]], formerly used in [[transformer]] the S, but which tends to disappear because of its risks *l' [[mineral oil]], which replaced it [[pyralene]] in [[transformer]] the S *l' pure water. If usual water is conducting, a perfectly pure water is a very good insulator. The difficulty in keeping a very pure water makes any use industrial difficult. == Utilizations of dielectric the == The dielectric ones being good electrical and thermal insulators, and are thus used to sheath the electric cables in order to avoid contacts with other cables or people as well as in the handles of the pans. The dielectric ones are useful in [[condensing]] S. In the case, very simple, of [[condensing plan]], one can bring closer the plates without risk to contact or breakdown. One thus inserts layers of dielectric in the industrial condensers, which makes it possible to increase it [[capacity]] by decreasing the obstruction. In addition, if one subjects it to a sufficiently powerful electric field, any substance will ionize and become conducting. The dielectric ones being rather difficult to ionize, the ambient air becomes conducting before them: one can employ them for condensers with high voltage. The majority of dielectric are also transparent in broad frequency bands, and are sometimes used to constitute an anti-reflecting layer, for example on certain models of lenses. {{Multi stringcourse|Physical gate|Gate electricity and electronics}} [[Category: Electrostatics]] [[bg: Диелектрик]] [[Ca: Dielèctric]] [[Cs: Dielektrikum]] [[da: Dielektrikum]] [[of: Dielektrikum]] [[in: Dielectric]] [[be: Dieléctrico]] [[it: Insulating elettrico]] [[ja: 誘電体]] [[lt: Dielektrikas]] [[nl: Diëlektricum]] [[pl: Dielektryk]] [[Pt: Dielétrico]] [[Ru: Диэлектрик]] [[sk: Dielektrikum]] [[SSL: Dielektrik]] [[the U.K.: Діелектрики]] [[VI: Chất cách điện]] [[zh: 介電質

  Random links:

  Parsec47 | Disorder of the thought | Today Ren | The released Cockroach | Roger Thabault