Magnetic constant
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The magnetic constant μ0 (equal to the vacuum permeability, also known as the permeability of free space) is a universal physical constant, relating mechanical and electromagnetic units of measurement. In the International System of Units (SI), its value is exactly expressed by:
- <math>\mu_0 = 4 \pi\ \times \ 10^{-7}\ </math> N/A2 = 4π×10−7 H/m, or approximately 1.2566×10−6 H/m.[1]
This value is a consequence of the definition of the ampere in terms of forces between wires.[2] In vacuum, the magnetic constant is the ratio of the magnetic B-field (for calculating Lorentz force) to the H-field (calculated from currents):
- <math>\mathbf{B} = \mu_0 \ \mathbf{H}.</math>
The magnetic constant μ0 is related to the electric constant ε0 and to the speed of light in vacuum by c ² ε0 μ0 = 1.
[edit] Terminology
Historically, the constant μ0 has had different names. A now rather obsolete term is "magnetic permittivity of vacuum". In the 1987 IUPAP Red book this constant was called permeability of vacuum.[3] Currently the nomenclature in physics is magnetic constant.[1][4] The vacuum permeability μ = μrμ0 is equal to μ0.
[edit] Footnotes
- ^ a b Magnetic constant. 2006 CODATA recommended values. NIST. Retrieved on 2007-08-08.
- ^ Unit of electric current (ampere). Historical context of the SI. NIST. Retrieved on 2007-08-11.
- ^ SUNAMCO Commission (1987), "Recommended values of the fundamental physical constants", Symbols, Units, Nomenclature and Fundamental Constants in Physics, pp. p.54, <http://www-v2.sp.se/metrology/IUPAP_SUNAMCO/IUPAP%20SUNAMCO%20Commission_files/IUPAP_Red_book_1987/SUNAMCO%20Red%20book%201987/6_Recommended_fundamental_constants_iupap_sunamco_red_book_1987.pdf>; (the IUPAP "Red book").
- ^ National Physical Laboratory, UK (1998). Fundamental Physical Constants p. 2.
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