Electric constant

The electric constant (symbol: ε₀), also known as the vacuum permittivity or the permittivity of free space, is a physical constant that describes the force between two charged bodies in vacuum. The exact definition (and value) depends on the system of quantities used, but in the International System of Quantities (ISQ), the electric constant has a defined value of approximately 8.854 187 816 × 10⁻¹² F m⁻¹.

The electric constant is related to the magnetic constant μ₀ and the speed of light c₀ by the equation c20 = 1/ε₀μ₀. In SI units, the speed of light is defined (by the definition of the metre) as 299 792 458 m s⁻¹ and the magnetic constant is defined (by the definition of the ampere) as 4π × 10⁻⁷ N A⁻¹: hence, the electric constant is also fixed, and can be calculated to any desired precision (although the numerical value is a non-repeating decimal).