Dielectric

Categories: Electricity

Image:Dielectric.png
The electrons in the molecules shift toward the positively charged left plate. The molecules then create a leftward electric field that partially annuls the field created by the plates. (The air gap is shown for clarity; in a real capacitor, the dielectric is in direct contact with the plates.)

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Definition

A dielectric, or electrical insulator, is a substance that is highly resistant to flow of electric current.

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Applications

The use of a dielectric in a capacitor presents several advantages. The simplest of these is that the conducting plates can be placed very close to one another without risk of contact. Also, if subjected to a very high electric field, any substance will ionize and become a conductor. Dielectrics are more resistant to ionization than air, so a capacitor containing a dielectric can be subjected to a higher voltage. Layers of dielectric are commonly incorporated in manufactured capacitors to improve their performance above that of capacitors with only air or a vacuum between their plates, and the term dielectric refers to this application as well as the insulation used in power and RF cables.

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Dielectrics in Parallel-Plate Capacitors

Putting a dielectric material between the plates in a parallel plate capacitor causes an increase in the capacitance in proportion to k, the dielectric constant of the material:

<math>C = \frac{k \epsilon_0 A}{d} </math>
where <math>\epsilon_0</math> is the permittivity of free space, A is the area covered by the capacitors, and d is the distance between the plates.

This happens because an electric field polarizes the molecules of the dielectric, producing concentrations of charge on its surfaces that create an electric field opposed (antiparallel) to that of the capacitor. Thus, a given amount of charge produces a weaker field between the plates than it would without the dielectric, which reduces the electric potential. Considered in reverse, this argument means that, with a dielectric, a given electric potential causes the capacitor to accumulate a larger charge.

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Some practical dielectrics

Dielectric materials can be solid, liquid, or gaseous. Solid dielectrics are perhaps the most commonly used in electrical engineering and very many solids are very good insulators. Some examples include porcelain, glass, and plastics. Air and sulfur hexafluoride are the two most commonly used gaseous dielectrics.

  • Industrial coatings provide a dieltric barrier between the substrate and its environment.
  • Mineral oil is used extensively inside electrical transformers as a fluid dielectric and to assist in cooling, and various dielectric fluids are also used in high voltage capacitors.
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See also

de:Dielektrikum es:Dieléctrico it:Isolante elettrico nl:Diëlektricum ja:誘電体 pl:Dielektryk ru:Диэлектрик sl:Dielektrik