Electric constant

From Knowino
Jump to: navigation, search

The electric constant (also: vacuum permittivity), designated ε0, 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:

 F = \frac{1}{4 \pi \varepsilon_0} \ \frac{|q| |Q|}{r^2} ,

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

\varepsilon_0 = \frac{1}{\mu_0 c^2},

where c is the speed of light in vacuum and μ0 is the magnetic constant. In the SI system of units, c is defined and μ0 is a consequence of the definition of the ampere: μ0 = 4π × 10−7 N/A2. Consequently, ε0 is exact and expressed up to ten digits by:

\varepsilon_0 = \frac{10^7}{4\pi\,c^2} =  8.854\;187\;817... 10^{-12} F / m;[1]
\frac{1}{4 \pi \varepsilon_0} = 8.987\ 551\ 787... 10^9 N m²/C².

[edit] Terminology

Historically, the physical constant ε0 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 = εr ε0 is equal to the electric constant ε0.

[edit] Footnotes

Information.svg Some content on this page may previously have appeared on Citizendium.
Personal tools