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18.12: Summary

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    • 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.

    Going further



    • Dielectrics, P. J. Harrop, 1972 (Butterworths)
      Contains a more mathematical treatment of dielectrics, as well as information on many other potential applications.
    • The Solid State, Second Edition, H. M. Rosenberg, 1978 (OUP)
      Chapter 13, “Dielectric properties”, provides a good overview of many of the subjects discussed here and contains the latter part of the derivation for the relationship between the dielectric constant and the refractive index.
    • Electronic and Magnetic Behaviour of Materials, A. Nussbaum, 1967 (Prentice-Hall) pp.70-77
      Provides a more detailed look at how the properties of dielectrics arise from their microscopic polarisation.

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