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Vacuum permittivity, commonly denoted ε 0 (pronounced "epsilon nought" or "epsilon zero"), 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.
The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insulator measures the ability of the insulator to store electric energy in an electrical field.
The susceptibility is defined as the constant of proportionality (which may be a tensor) relating an electric field E to the induced dielectric polarization density P such that = , where ε o is the electric permittivity of free space.
The analogous quantity for a plane wave travelling through a dielectric medium is called the intrinsic impedance of the medium and designated η . Hence Z 0 is sometimes referred to as the intrinsic impedance of free space, [2] and given the symbol η 0. [3] It has numerous other synonyms, including: wave impedance of free space, [4] the vacuum ...
In free space the wave impedance of plane waves is: = (where ε 0 is the permittivity constant in free space and μ 0 is the permeability constant in free space). Now, since = = (by definition of the metre),
= is the dielectric constant of the material, which for non-magnetic materials is equal to n 2, where n is the refractive index; ε 0 is the permittivity of free space; N is the number density of the molecules (number per cubic meter);
The permeability of vacuum (also known as permeability of free space) is a physical constant, denoted μ 0. The SI units of μ are volt-seconds per ampere-meter, equivalently henry per meter. Typically μ would be a scalar, but for an anisotropic material, μ could be a second rank tensor.
For materials without polarization and magnetization, the constitutive relations are (by definition) [9]: 2 =, =, where ε 0 is the permittivity of free space and μ 0 the permeability of free space. Since there is no bound charge, the total and the free charge and current are equal.