Magnetic constant: Difference between revisions
imported>Paul Wormer No edit summary |
imported>John R. Brews (amplification of Gaussian units and source) |
||
| Line 11: | Line 11: | ||
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 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 | In [[Gaussian units]], the symbols ''μ''<sub>0</sub> and ''ε''<sub>0</sub> do not appear.<ref name=Wolf> | ||
{{cite book |title=Relativistic quantum chemistry: the fundamental theory of molecular science |author=Markus Reiher, Alexander Wolf |url=http://books.google.com/books?id=tpsHi7F16dMC&pg=PA7 |page=p. 7 |isbn=3527312927 |publisher=Wiley-VCH |year=2009}} | |||
</ref> Also, in Gaussian units, the speed of light is a measured, not a defined quantity. | |||
==Terminology== | ==Terminology== | ||
Revision as of 07:36, 29 November 2010
The magnetic constant μ0 (also known as vacuum permeability or 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, the symbols μ0 and ε0 do not appear.[3] Also, in Gaussian units, the speed of light is a measured, not a defined quantity.
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.[4] Currently the nomenclature in physics is magnetic constant.[1][5] The permeability μ ≡ μr μ0 is equal to μ0 for the vacuum, i.e., for the vacuum the relative permeability 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.
- ↑ Markus Reiher, Alexander Wolf (2009). Relativistic quantum chemistry: the fundamental theory of molecular science. Wiley-VCH. ISBN 3527312927.
- ↑ 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.