Magnetic constant: Difference between revisions
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:<math>\mathbf{B} = \mu_0 \ \mathbf{H}.</math> | :<math>\mathbf{B} = \mu_0 \ \mathbf{H}.</math> | ||
In SI units the magnetic constant ''μ''<sub>0</sub> is related to the [[electric constant]] ''ε''<sub>0</sub> and to the [[speed of light in vacuum | In SI units the magnetic constant ''μ''<sub>0</sub> is related to the [[electric constant]] ''ε''<sub>0</sub> and to the [[speed of light]] in vacuum by ''c'' ² ε<sub>0</sub> μ<sub>0</sub> = 1. | ||
In [[Gaussian units]] ''μ''<sub>0</sub> and ''ε''<sub>0</sub> do not enter the theory, only ''c'' appears in the equations. | |||
==Terminology== | ==Terminology== | ||
Revision as of 05:09, 11 July 2008
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:
This value is a consequence of the definition of the ampere in terms of forces between wires, see Ampère's equation.[2] In vacuum, the magnetic constant is the ratio of the magnetic B-field (entering the expression for the Lorentz force) to the magnetic H-field (the field inside a solenoid):
In SI units 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.
In Gaussian units μ0 and ε0 do not enter the theory, only c appears in the equations.
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, i.e., for the vacuum μr = 1.
Footnotes
- ↑ 1.0 1.1 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, at p.54; (the IUPAP "Red book").
- ↑ National Physical Laboratory, UK (1998). Fundamental Physical Constants p. 2.