Difference between revisions of "Magnetic constant"
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The '''magnetic constant''' (symbol: '''''μ''<sub>0</sub>'''), also known as the '''vacuum permeability''' or the '''permeability of free space''', is a [[physical constant]] that relates [[force]] to [[magnetic field]] in [[vacuum]]. It also appears in Maxwell's equations, which describe the behaviour of [[electromagnetic wave]]s. The exact definition (and value) depends on the system of quantities used, but in the [[International System of Quantities]] (ISQ) it has a defined value of exactly {{nowrap|4π{{e|−7}} N A<sup>−1</sup>}} (approximately {{nowrap|12.566 370 614{{e|−7}} N A<sup>−1</sup>}}).<ref name="CODATA">{{CODATA 2006|url=http://physics.nist.gov/cgi-bin/cuu/Value?mu0}}.</ref> | The '''magnetic constant''' (symbol: '''''μ''<sub>0</sub>'''), also known as the '''vacuum permeability''' or the '''permeability of free space''', is a [[physical constant]] that relates [[force]] to [[magnetic field]] in [[vacuum]]. It also appears in Maxwell's equations, which describe the behaviour of [[electromagnetic wave]]s. The exact definition (and value) depends on the system of quantities used, but in the [[International System of Quantities]] (ISQ) it has a defined value of exactly {{nowrap|4π{{e|−7}} N A<sup>−1</sup>}} (approximately {{nowrap|12.566 370 614{{e|−7}} N A<sup>−1</sup>}}).<ref name="CODATA">{{CODATA 2006|url=http://physics.nist.gov/cgi-bin/cuu/Value?mu0}}.</ref> | ||
| − | ==References== | + | ==New SI== |
| + | Under the proposals to redefine the [[ampere]] as a fixed number of [[elementary charge]]s per second,<ref>{{citation | contribution = Recommendation E1 | title = Report of the 25th meeting (15–16 March 2007), Consultative Committee for Electricity and Magnetism (CCEM) | url = http://www.bipm.org/utils/common/pdf/CCEM25.pdf | publisher = International Bureau for Weights and Measures | location = Sèvres, France}}.</ref> the magnetic constant would no longer have an exact fixed value. Instead, it would be defined by the equation ''μ''<sub>0</sub> = 2''αh''/''e''<sup>2</sup>''c''<sub>0</sub>,<ref group="note">At present, this equation is used to calculate the [[elementary charge]].</ref> where ''e'' is the elementary charge, ''α'' is the [[fine structure constant]] and ''h'' is the [[Planck constant]]. The relative uncertainty in the value would be the same as that of the fine structure constant, currently 6.8{{e|−10}}.<ref name="CODATA"/> | ||
| + | |||
| + | ==Notes and references== | ||
| + | ===Notes=== | ||
| + | {{reflist|group=note}} | ||
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| + | ===References=== | ||
{{reflist}} | {{reflist}} | ||
Revision as of 19:16, 1 April 2010
The magnetic constant (symbol: μ0), also known as the vacuum permeability or the permeability of free space, is a physical constant that relates force to magnetic field in vacuum. It also appears in Maxwell's equations, which describe the behaviour of electromagnetic waves. The exact definition (and value) depends on the system of quantities used, but in the International System of Quantities (ISQ) it has a defined value of exactly 4π × 10−7 N A−1 (approximately 12.566 370 614 × 10−7 N A−1).[1]
New SI
Under the proposals to redefine the ampere as a fixed number of elementary charges per second,[2] the magnetic constant would no longer have an exact fixed value. Instead, it would be defined by the equation μ0 = 2αh/e2c0,[note 1] where e is the elementary charge, α is the fine structure constant and h is the Planck constant. The relative uncertainty in the value would be the same as that of the fine structure constant, currently 6.8 × 10−10.[1]
Notes and references
Notes
- ↑ At present, this equation is used to calculate the elementary charge.
References
- ↑ 1.0 1.1 Mohr, Peter J.; Taylor, Barry N.; Newell, David B. CODATA Recommended Values of the Fundamental Physical Constants: 2006. Rev. Mod. Phys. 2008, 80 (2), 633–730. doi:10.1103/RevModPhys.80.633, <http://physics.nist.gov/cuu/Constants/codata.pdf>. Direct link to value.
- ↑ Recommendation E1. In Report of the 25th meeting (15–16 March 2007), Consultative Committee for Electricity and Magnetism (CCEM); International Bureau for Weights and Measures: Sèvres, France, <http://www.bipm.org/utils/common/pdf/CCEM25.pdf>.
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