Dielectric constant
The relative permittivity of a material is its (absolute) permittivity expressed as a ratio relative to the permittivity of vacuum.
Permittivity is a material property that affects the Coulomb force between two point charges in the material. Relative permittivity is the factor by which the electric field between the charges is decreased relative to vacuum.
Likewise, relative permittivity is the ratio of the capacitance of a capacitor using that material as a dielectric, compared to a similar capacitor that has vacuum as its dielectric. Relative permittivity is also commonly known as dielectric constant, a term deprecated in physics and engineering as well as in chemistry.
Relative permittivity is typically denoted as εr(ω) (sometimes κ or K) and is defined as
where ε(ω) is the complex frequency-dependent absolute permittivity of the material, and ε0 is the vacuum permittivity.
Relative permittivity is a dimensionless number that is in general complex-valued; its real and imaginary parts are denoted as:
The relative permittivity of a medium is related to its electric susceptibility, χe, as εr(ω) = 1 + χe.
In anisotropic media (such as non cubic crystals) the relative permittivity is a second rank tensor.
The relative permittivity of a material for a frequency of zero is known as its static relative permittivity.
The historical term for the relative permittivity is dielectric constant. It is still commonly used, but has been deprecated by standards organizations, because of its ambiguity, as some older authors used it for the absolute permittivity ε. The permittivity may be quoted either as a static property or as a frequency-dependent variant. It has also been used to refer to only the real component ε'r of the complex-valued relative permittivity.
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