In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter ε (epsilon), is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more in response to an applied electric field than a material with low permittivity, thereby storing more energy in the material. In electrostatics, the permittivity plays an important role in determining the capacitance of a capacitor.
In the simplest case, the electric displacement field D resulting from an applied electric field E is
More generally, the permittivity is a thermodynamic function of state. It can depend on the frequency, magnitude, and direction of the applied field. The SI unit for permittivity is farad per meter (F/m).
The permittivity is often represented by the relative permittivity εr which is the ratio of the absolute permittivity ε and the vacuum permittivity ε0
.
This dimensionless quantity is also often and ambiguously referred to as the permittivity. Another common term encountered for both absolute and relative permittivity is the dielectric constant which has been deprecated in physics and engineering as well as in chemistry.
By definition, a perfect vacuum has a relative permittivity of exactly 1 whereas at standard temperature and pressure, air has a relative permittivity of κair ≈ 1.0006.
Relative permittivity is directly related to electric susceptibility (χ) by
otherwise written as
The term "permittivity" was introduced in the 1880s by Oliver Heaviside to complement Thomson's (1872) "permeability". Formerly written as p, the designation with ε has been in common use since the 1950s.
Units
The SI unit for permittivity is farad per meter (F/m or F·m−1).
Explanation
In electromagnetism, the electric displacement field represents the distribution of electric charges in a given medium resulting from the presence of an electric field . This distribution includes charge migration and electric dipole reorientation. Its relation to permittivity in the very simple case of linear, homogeneous, isotropic materials with "instantaneous" response to changes in electric field is:
where the permittivity is a scalar. If the medium is anisotropic, the permittivity is a second rank tensor.
In general, permittivity is not a constant, as it can vary with the position in the medium, the frequency of the field applied, humidity, temperature, and other parameters. In a nonlinear medium, the permittivity can depend on the strength of the electric field. Permittivity as a function of frequency can take on real or complex values.
In SI units, permittivity is measured in farads per meter (F/m or A2·s4·kg−1·m−3). The displacement field is measured in units of coulombs per square meter (C/m2), while the electric field is measured in volts per meter (V/m). and describe the interaction between charged objects. is related to the charge densities associated with this interaction, while is related to the forces and potential differences.
Vacuum permittivity
The vacuum permittivity (also called permittivity of free space or the electric constant) is the ratio in free space. It also appears in the Coulomb force constant,
Its value is
where
is the speed of light in free space,{ { efn|Current practice of standards organizations such as NIST and BIPM is to use , rather than , to denote the speed of light in vacuum according to ISO 31. In the original Recommendation of 1983, the symbol was used for this purpose.<ref>{ { cite journal|url=http://physics.nist.gov/Pubs/SP330/sp330.pdf|website=NIST|title=Special Publication 330, Appendix 2|page=45|archive-url=https://web.archive.org/web/20160603215953/http://physics.nist.gov/Pubs/SP330/sp330.pdf|archive-date=2016-06-03 }}.</ref>}} is the vacuum permeability.
The constants and were both defined in SI units to have exact numerical values until the 2019 redefinition of the SI base units. Therefore, until that date, could be also stated exactly as a fraction, even if the result was irrational (because the fraction contained ). In contrast, the ampere was a measured quantity before 2019, but since then the ampere is now exactly defined and it is that is an experimentally measured quantity (with consequent uncertainty) and therefore so is the new 2019 definition of ( remains exactly defined before and since 2019).