Magnetic constant: Difference between revisions
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The '''magnetic constant''' ''μ''<sub>0</sub> (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: | The '''magnetic constant''' ''μ''<sub>0</sub> (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> [[newton|N]]/[[ampere|A]]<sup>2</sup> = 4π×10<sup>−7</sup> [[Henry (inductance)| | <!-- :<math>\mu_0 = 4 \pi\ \times \ 10^{-7}\ </math> --> | ||
:μ<sub>0</sub> = 4π × 10<sup>−7</sup> [[newton|N]]/[[ampere|A]]<sup>2</sup> = 4π×10<sup>−7</sup> [[Henry (inductance)|henry]]/[[metre]] (H/m) , or approximately 1.2566×10<sup>−6</sup> H/m.<ref name="NIST">{{cite web |url=http://physics.nist.gov/cgi-bin/cuu/Value?mu0 |title=Magnetic constant |accessdate=2007-08-08 |work=2006 [[CODATA]] recommended values |publisher=[[NIST]] }}</ref> | |||
This value is a consequence of the definition of the [[ampere]] in terms of forces between wires.<ref name="NIST amp hist">{{cite web |url=http://physics.nist.gov/cuu/Units/ampere.html |title=Unit of electric current (ampere) |accessdate=2007-08-11 |work=Historical context of the SI |publisher=[[NIST]] }}</ref> | This value is a consequence of the definition of the [[ampere]] in terms of forces between wires, see [[Ampere's_equation#Two straight, infinite, and parallel wires|Ampère's equation]].<ref name="NIST amp hist">{{cite web |url=http://physics.nist.gov/cuu/Units/ampere.html |title=Unit of electric current (ampere) |accessdate=2007-08-11 |work=Historical context of the SI |publisher=[[NIST]] }}</ref> | ||
In vacuum, the magnetic constant is the ratio of the | In vacuum, the magnetic constant is the ratio of the [[magnetic induction|magnetic '''B'''-field]] (entering the expression for the [[Lorentz force]]) to the [[magnetic field|magnetic '''H'''-field]] | ||
(the field inside a [[solenoid]]): | |||
:<math>\mathbf{B} = \mu_0 \ \mathbf{H}.</math> | :<math>\mathbf{B} = \mu_0 \ \mathbf{H}.</math> | ||
Revision as of 12:33, 7 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.
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.