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Concept
Impédance caractéristique du vide
Résumé
In electromagnetism, the impedance of free space, Z0, is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is,
where is the electric field strength and is the magnetic field strength. Its presently accepted value is
Where Ω is the ohm, the SI unit of electrical resistance. The impedance of free space (that is the wave impedance of a plane wave in free space) is equal to the product of the vacuum permeability μ0 and the speed of light in vacuum c0. Before 2019, the values of both these constants were taken to be exact (they were given in the definitions of the ampere and the metre respectively), and the value of the impedance of free space was therefore likewise taken to be exact. However, with the redefinition of the SI base units that came into force on 20 May 2019, the impedance of free space is subject to experimental measurement because only the speed of light in vacuum c0 retains an exactly defined value.
The analogous quantity for a plane wave travelling through a dielectric medium is called the intrinsic impedance of the medium, and designated η (eta). Hence Z0 is sometimes referred to as the intrinsic impedance of free space, and given the symbol η0. It has numerous other synonyms, including:
wave impedance of free space,
the vacuum impedance,
intrinsic impedance of vacuum,
characteristic impedance of vacuum,
wave resistance of free space.
From the above definition, and the plane wave solution to Maxwell's equations,
where
μ0 is the magnetic constant, also known as the permeability of free space ≈ 12.566e-7 Henries/meter,
ε0 is the electric constant, also known as the permittivity of free space ≈ 8.854e-12 Farads/meter,
c is the speed of light in free space.
e is the elementary charge,
α is the fine structure constant, and
h is Planck's constant.
The reciprocal of Z0 is sometimes referred to as the admittance of free space and represented by the symbol Y0.
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