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The Coulomb constant, the electric force constant, or the electrostatic constant (denoted ke, k or K) is a proportionality constant in electrodynamics equations. The value of this constant is dependent upon the medium that the charged objects are immersed in. In SI units, in the case of vacuum, it is equal to approximately 8987551787.3681764 N·m2·C−2 or 8.99×109 N·m2·C−2. It was named after the French physicist Charles-Augustin de Coulomb (1736–1806) who introduced Coulomb's law.
Value of the constant
The Coulomb constant is the constant of proportionality in Coulomb's law,
where êr is a unit vector in the r-direction and
Taking this integral for a sphere, radius r, around a point charge, we note that the electric field points radially outwards at all times and is normal to a differential surface element on the sphere, and is constant for all points equidistant from the point charge.
Noting that E = F/ for some test charge q,
In modern systems of units, the Coulomb constant ke has an exact numeric value, in Gaussian units ke = 1, in Lorentz–Heaviside units (also called rationalized) ke = 1/ and in SI ke = 1/, where the vacuum permittivity ε0 = 1/ ≈ 8.85418782×10−12 F⋅m−1, the speed of light in vacuum c is 299792458 m/s, the vacuum permeability μ0 is 4π×10−7 H⋅m−1, so that
The Coulomb constant is used in many electric equations, although it is sometimes expressed as the following product of the vacuum permittivity constant:
The Coulomb constant appears in many expressions including the following:
- Tomilin, K. (1999). "Fine-structure constant and dimension analysis". European Journal of Physics. 20 (5): L39–L40. Bibcode:1999EJPh...20L..39T. doi:10.1088/0143-0807/20/5/404.
- CODATA Value: electric constant. Physics.nist.gov. Retrieved on 2010-09-28.
- Coulomb's constant, HyperPhysics
- BIPM statement: Information for users about the proposed revision of the SI (PDF)
- "Decision CIPM/105-13 (October 2016)". The day is the 144th anniversary of the Metre Convention.