Dielectric Constant
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Introduction:
When the electric hysteresis of a dielectric is ignored and the dielectric properties are regarded as isotropic and linear, the polarization is directly proportional to the electric field strength, and the proportionality constant is independent of the direction of the field.This section mathematically explains the dielectric constant of dielectric in detail.
Mathematical Expression:
- We can write
- the polarization P would vary directly as the applied electric field E. For some dielectrics
- By substituting
- In eqs Ïµ is called the permittivity of the dielectric,
- Ïµo is the permittivity of free space,as approximately F/m, and
- Ïµr is called the dielectric constant or relative permittivity.
- The dielectric constant (or relative permittivity) Ïµr , is the ratio of the permittivity of the dielectric to that of free space
- It should also be noticed that er and χ_{e} are dimensionless whereas e and so are in farads/meter.
- Note that Ïµris always greater or equal to unity. For free space and nondielectric materials (such as metals) Ïµr = 1.
Dielectric strength:
- When the electric field in a dielectric is sufficiently large, it begins to pull electrons completely out of the molecules, and the dielectric becomes conducting.
- Dielectric breakdown occurs in all kinds of dielectric materials (gases, liquids, or solids) and depends on the nature of the material, temperature, humidity, and the amount of time that the field is applied.
- Dielectric breakdown is said to have occurred when a dielectric becomes conducting.
- The minimum value of the electric field at which dielectric breakdown occurs is called the dielectric strength of the dielectric material.
- The dielectric strength is the maximum electric field that a dielectric can tolerate or withstand without breakdown