# Electric constant

*13 January 2011*.

The **electric constant** (also: *vacuum permittivity*), designated ε_{0}, is a physical constant appearing in equations relating electrical charge to mechanical quantities, for example in Coulomb's law. In scalar form, Coulomb's law can be given as:

- ,

where *F* is the magnitude of the force between two point charges *q* and *Q*, separated by a distance *r*.

Its value is given by

- ,

where *c* is the speed of light in vacuum and *μ*_{0} is the magnetic constant. In the SI system of units, *c* is defined and *μ*_{0} is a consequence of the definition of the ampere: μ_{0} = 4π × 10^{−7} N/A^{2}.
Consequently, ε_{0} is exact and expressed up to ten digits by:

## [edit] Terminology

Historically, the physical constant ε_{0} has had different names. One of these was *dielectric constant of vacuum*.^{[2]}
Although still in use,^{[3]} "dielectric constant" is now deemed obsolete.^{[4]}^{[5]}
In the 1987 IUPAP Red book this constant was called *permittivity of vacuum*.^{[6]}
Currently the nomenclature is *electric constant*.^{[1]}^{[7]} The vacuum permittivity ε = ε_{r} ε_{0} is equal to the electric constant ε_{0}.

## [edit] Footnotes

- ↑
^{1.0}^{1.1}CODATA. Electric constant.*2006 CODATA recommended values*. NIST. Retrieved on 2007-08-08. - ↑ King, Ronold W. P. (1963).
*Fundamental Electromagnetic Theory*. New York: Dover, p. 139. - ↑ for example in this random patent
- ↑ IEEE Standards Board (1997). IEEE Standard Definitions of Terms for Radio Wave Propagation p. 6.
- ↑ Braslavsky, S.E. (2007), "Glossary of terms used in photochemistry (IUPAC recommendations 2006)",
*Pure and Applied Chemistry***79**: p. 324., http://www.iupac.org/publications/pac/2007/pdf/7903x0293.pdf - ↑ SUNAMCO Commission (1987), "Recommended values of the fundamental physical constants",
*Symbols, Units, Nomenclature and Fundamental Constants in Physics*, pp. p.54, http://www-v2.sp.se/metrology/IUPAP_SUNAMCO/IUPAP%20SUNAMCO%20Commission_files/IUPAP_Red_book_1987/SUNAMCO%20Red%20book%201987/6_Recommended_fundamental_constants_iupap_sunamco_red_book_1987.pdf; (the IUPAP "Red book"). - ↑ National Physical Laboratory, UK (1998). Fundamental Physical Constants p. 2.

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