Vacuum permittivity

The physical constant ε0 (pronounced as 'epsilon nought' or 'epsilon one'), commonly called the vacuum permittivity, permittivity of free space or electric constant or the distributed capacitance of the vacuum, is an ideal, (baseline) physical constant, which is the value of the absolute dielectric permittivity of classical vacuum. It has the value(as of 2018 CODATA) The physical constant ε0 (pronounced as 'epsilon nought' or 'epsilon one'), commonly called the vacuum permittivity, permittivity of free space or electric constant or the distributed capacitance of the vacuum, is an ideal, (baseline) physical constant, which is the value of the absolute dielectric permittivity of classical vacuum. It has the value(as of 2018 CODATA) It is the capability of the vacuum to permit electric field lines. This constant relates the units for electric charge to mechanical quantities such as length and force. For example, the force between two separated electric charges (in the vacuum of classical electromagnetism) is given by Coulomb's law: The value of the constant fraction, 1 / 4 π ε 0 {displaystyle 1/4pi varepsilon _{0}} , is approximately 9 × 109 N⋅m2⋅C−2, q1 and q2 are the charges, and r is the distance between them. 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. The value of ε0 is defined by the formula where c is the defined value for the speed of light in classical vacuum in SI units, and μ0 is the parameter that international Standards Organizations call the 'magnetic constant' (commonly called vacuum permeability). Since μ0 has an approximate value 4π × 10−7 H/m, and c has the defined value 299792458 m⋅s−1, it follows that ε0 can be expressed numerically as The historical origins of the electric constant ε0, and its value, are explained in more detail below. The ampere was redefined by defining the elementary charge as an exact number of coulombs as from 20 May 2019, with the effect that the vacuum electric permeability no longer has an exactly determined value in SI units. The value of the electron charge becomes a numerically defined quantity, not measured, making μ0 a measured quantity. Consequently, ε0 will not be exact. As before, it will be defined by the equation ε0 = 1/(μ0c2), and is thus determined by the value of μ0, the magnetic vacuum permeability which in turn is determined by the experimentally determined dimensionless fine-structure constant α: with e being the elementary charge, h being the Planck constant, and c being the speed of light in vacuum, each with exactly defined values.The relative uncertainty in the value of ε0 is therefore the same as that for the dimensionless fine-structure constant, namely 1.5×10−10. Historically, the parameter ε0 has been known by many different names. The terms 'vacuum permittivity' or its variants, such as 'permittivity in/of vacuum', 'permittivity of empty space', or 'permittivity of free space' are widespread. Standards Organizations worldwide now use 'electric constant' as a uniform term for this quantity, and official standards documents have adopted the term (although they continue to list the older terms as synonyms). In the new SI system, the permittivity of vacuum will not be a constant anymore, but a measured quantity, related to the (measured) dimensionless fine structure constant.