DoITPoMS

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Summary

  • Dielectrics are electrical insulators that support charge.
  • The properties of dielectrics are due to polarisation.
  • There are three main mechanisms by which polarisation arises on the microscopic scale: electronic (distortion of the electron cloud in an atom), ionic (movement of ions) and orientational (rotation of permanent dipoles).
  • A capacitor is a device that stores charge, usually with the aid of a dielectric material. Its capacitance is defined by Q = C V
  • The dielectric constant κ indicates the ability of the dielectric to polarise. It can be defined as the ratio of the dielectric’s permittivity to the permittivity of a vacuum.
  • Each of the polarisation mechanisms has a characteristic relaxation or resonance frequency. In an alternating field, at each of these (materials dependent) frequencies, the dielectric constant will sharply drop.
  • The dielectric constant is also affected by structure, as this affects the ability of the material to polarise.
  • Polar dielectrics show a decrease in the dielectric constant as temperature increases.
  • Dielectric loss is the absorption of energy by movement of charges in an alternating field, and is particularly high around the relaxation and resonance frequencies of the polarisation mechanisms.
  • Sufficiently high electric fields can cause a material to undergo dielectric breakdown and become conducting.

 

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