Magnetic constant: Difference between revisions
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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.<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 magnetic [[ | In vacuum, the magnetic constant is the ratio of the magnetic [[magnetic induction|'''B'''-field]] (for calculating [[Lorentz force]]) to the [[magnetic field|'''H'''-field]] (calculated from currents): | ||
:<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]] by ''c'' ² ε<sub>0</sub> μ<sub>0</sub> = 1. | |||
==Terminology== | ==Terminology== | ||
Historically, the constant μ<sub>0</sub> 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''.<ref>{{citation|author=[[SUNAMCO Commission]]| chapter=Recommended values of the fundamental physical constants | url=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 |title=[http://www-v2.sp.se/metrology/IUPAP_SUNAMCO/IUPAP%20SUNAMCO%20Commission_files/IUPAP_Red_book_1987/SUNAMCO%20Red%20book%201987/index_red_book_iupap_sunamco_1987.htm Symbols, Units, Nomenclature and Fundamental Constants in Physics] |year=1987 | pages=p.54 }}; (the [[IUPAP]] "Red book").</ref> | Historically, the constant μ<sub>0</sub> 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''.<ref>{{citation|author=[[SUNAMCO Commission]]| chapter=Recommended values of the fundamental physical constants | url=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 |title=[http://www-v2.sp.se/metrology/IUPAP_SUNAMCO/IUPAP%20SUNAMCO%20Commission_files/IUPAP_Red_book_1987/SUNAMCO%20Red%20book%201987/index_red_book_iupap_sunamco_1987.htm Symbols, Units, Nomenclature and Fundamental Constants in Physics] |year=1987 | pages=p.54 }}; (the [[IUPAP]] "Red book").</ref> | ||
Currently the [[nomenclature]] in physics is ''magnetic constant''.<ref name="NIST"/><ref>{{cite web |author=[[National Physical Laboratory, UK]]|url=http://www.npl.co.uk/reference/fundamental_constants.pdf |title=Fundamental Physical Constants |year=1998 |pages=p. 2}}</ref> | Currently the [[nomenclature]] in physics is ''magnetic constant''.<ref name="NIST"/><ref>{{cite web |author=[[National Physical Laboratory, UK]]|url=http://www.npl.co.uk/reference/fundamental_constants.pdf |title=Fundamental Physical Constants |year=1998 |pages=p. 2}}</ref> | ||
The vacuum [[permeability (electromagnetism)|permeability]] μ = μ<sub>r</sub>μ<sub>0</sub> is equal to μ<sub>0</sub>. | The vacuum [[permeability (electromagnetism)|permeability]] μ = μ<sub>r</sub>μ<sub>0</sub> is equal to μ<sub>0</sub>, i.e., for the vacuum μ<sub>r</sub> = 1. | ||
== Footnotes == | == Footnotes == | ||
<references/> | <references/> | ||
Revision as of 12:08, 2 June 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.[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):
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.