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'Vacuum permittivity' is the 'electric constant' ε₀ (also known as the 'permittivity of free space', or by the term ''dielectric constant of vacuum''), which is a fundamental physical constant. The constant ε₀ connects mechanical quantities (time, length, mass) to the units for electrical charge, for example in Coulomb's law. Its exact value in SI units is
: F m^-1.^[1]
This value is a consequence of the relation ε₀ μ₀ ''c'' ² = 1 with the defined speed of light in vacuum ''c'' and with the defined magnetic constant μ₀.
The 'Coulomb force constant' or 'electrostatic constant' can thus be expressed as
: N·m²/C².
In other systems of electromagnetic units, it is common to have . This is the case in electrostatic cgs units, Gaussian units, Lorentz-Heaviside units, and some choices of natural units (while some other choices set ).

Contents

Terminology

Possible effects on ε of vacuum

See also

Footnotes

Terminology

The linear permittivity of a homogeneous material is usually given relative to that of vacuum, as a relative permittivity . (In an anisotropic material, the relative permittivity may be a tensor.)
Historically, the physical constant ε₀ has been known by many different names. Both "electric constant" and "vacuum permittivity" (or its variants, such as "permittivity of vacuum") are widespread. There is some evidence that standards organizations are moving towards "electric constant" as a uniform term for this quantity^[2], but "vacuum permittivity" and "permittivity of vacuum" continue to be listed as synonyms in official standards documents^[3][4] with no statement that the latter terminology is deprecated.
Another historical synonym was "dielectric constant of vacuum", as "dielectric constant" was sometimes used in the past for the absolute permittivity.^[5] However, in modern usage "dielectric constant" typically refers exclusively to a relative permittivity (where the relative permittivity of vacuum is 1 by definition),^[6] and even this usage is considered "obsolete" by some standards bodies in favor of "relative permittivity"^[7]. Hence, the term "dielectric constant of vacuum" for the absolute vacuum permittivity ε₀ (as opposed to the relative vacuum permittivity, 1) is likely to be considered obsolete by most modern authors, although occasional examples of continuing usage can be found.^[8]
As for notation, the constant can be denoted by either or $epsilon\_0$, using either of the common glyphs for the letter epsilon.

Possible effects on ε of vacuum

Therefore, although it is called the "permittivity of vacuum", the value of ε₀ (like the speed of light in SI units) is no longer tied to any experimental measurement; its value is precisely determined by the definition of the metre and other units. In principle, it is possible for the experimental ε of a perfect vacuum to vary slightly from ε₀ in unusual circumstances, due (e.g.) to quantum corrections to Maxwell's equations, although such deviations have not yet been measured. For example, the theory of quantum electrodynamics predicts that vacuum should exhibit nonlinear effects that will make it behave like a birefringent material with ε slightly greater than ε₀ for extremely strong electric fields.^[9][10]

See also

★ Permeability of free space

Footnotes

1. Electric constant CODATA
2. Fundamental Physical Constants National Physical Laboratory, UK
3. The International System of Units (SI) International Bureau of Weights and Measures
4.
5. Fundamental Electromagnetic Theory, , Ronold W. P., King, Dover, ,
6. Classical Electrodynamics, 3rd edition, , John David, Jackson, Wiley, ,
7. IEEE Standard Definitions of Terms for Radio Wave Propagation IEEE Standards Board
8. For example in this random patent.
9. Klein, James J. and B. P. Nigam, "Birefringence of the vacuum," ''Physical Review'' vol. '135', p. B1279-B1280 (1964).
10. Mourou, G. A., T. Tajima, and S. V. Bulanov, "Optics in the relativistic regime," ''Reviews of Modern Physics'' vol. '78' (no. 2), 309-371 (2006).