Vacuum permittivity
Vacuum permittivity, commonly denoted ε0, is the value of the absolute dielectric permittivity of classical vacuum. It may also be referred to as the permittivity of free space, the electric constant, or the distributed capacitance of the vacuum. It is an ideal (baseline) physical constant. Its CODATA value is:
  - ε0 = 8.8541878188(14)×10−12 F⋅m−1.‍ It is a measure of how dense of an electric field is "permitted" to form in response to electric charges and relates the units for electric charge to mechanical quantities such as length and force. For example, the force between two separated electric charges with spherical symmetry is given by Coulomb's law:
  - F C = 1 4 π ε 0 q 1 q 2 r 2 Here, q1 and q2 are the charges, r is the distance between their centres, and the value of the constant fraction 1 / is approximately 9×10⁹ N⋅m⁲⋅C−2. Likewise, ε0 appears in Maxwell's equations, which describe the properties of electric and magnetic fields and electromagnetic radiation, and relate them to their sources. In electrical engineering, ε0 itself is used as a unit to quantify the permittivity of various dielectric materials.
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