The constant μ0, called vacuum permeability, permeability of free space or magnetic constant, relates the unit for electric current, the ampere, to mechanical quantities such as length and force in the International System of Units.
Two thin, straight, stationary, parallel wires, a distance r apart in free space, each carrying a current I, will exert a force on each other. Ampère's force law states that the force per unit length is given by
.
The ampere is defined such that if the wires are 1 m apart and the current in each wire is 1 A, the force between the two wires is 2 × 10−7 N / m. Hence the value of μ0 is defined to be exactly
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Terminology
Historically, the constant μ0 has had different names. In the 1987 IUPAP Red book, for example, this constant was still called permeability of vacuum.[3] Another, now rather rare and obsolete, term is "magnetic permittivity of vacuum". See, for example, Servant et al.[4] The term "vacuum permeability" (and variations thereof, such as "permeability of free space") remains very widespread. However, Standards Organizations have recently moved to magnetic constant as the preferred name for μ0, although the older name continues to be listed as a synonym.[2]
The name "magnetic constant" is used by standards organizations in order to avoid use of the terms "permeability" and "vacuum", which have physical meanings. This change of preferred name has been made because μ0 is a defined value, and is not the result of experimental measurement (see below).
Systems of units and historical origin of value of μ0
In principle, there are several equation systems that could be used to set up a system of electrical quantities and units.[5] Since the late 1800's, the fundamental definitions of current units have been related to the definitions of mass, length and time units, using Ampère's force law. However, the precise way in which this has "officially" been done has changed many times, as measurement techniques and thinking on the topic developed. The overall history of the unit of electric current, and of the related question of how to define a set of equations for describing electromagnetic phenomena, is very complicated. Briefly, the basic reason why μ0 has the value it does is as follows.
Ampère's force law describes the experimentally-derived fact that, for two thin, straight, stationary, parallel wires, a distance r apart, in each of which a current I flows, the force per unit length, Fm, that one wire exerts upon the other in the vacuum of free space would be given by
-
.
Writing the constant of proportionality as km gives
-
.
The form of km needs to be chosen in order to set up a system of equations, and a value then needs to be allocated in order to define the unit of current.
In the old "electromagnetic (emu)" system of equations defined in the late 1800s, km was chosen to be a pure number, 2, distance was measured in centimetres, force was measured in the cgs unit dyne, and the currents defined by this equation were measured in the "electromagnetic unit (emu) of current" (also called the "abampere"). A practical unit to be used by electricians and engineers, the ampere, was then defined as equal to one tenth of the electromagnetic unit of current.
In another system, the "rationalized-metre-kilogram-second (rmks) system" (or alternatively the "metre-kilogram-second-ampere (mksa) system"), km is written as μ0/2π, where μ0 is a measurement-system constant called the "magnetic constant".[6] The value of μ0 was chosen such that the rmks unit of current is equal in size to the ampere in the emu system: μ0 is defined to be 4π × 10−7 N A−2.[7]
Historically, several different systems (including the two described above) were in use simultaneously. In particular, physicists and engineers used different systems, and physicists used three different systems for different parts of physics theory and a fourth different system (the engineers' system) for laboratory experiments. In the 1948, international decisions were made by standards organizations to adopt the rmks system, and its related set of electrical quantities and units, as the single main international system for describing electromagnetic phenomena in the International System of Units.
Obviously, Ampère's law as stated above describes a physical property of the world. However, the choices about the form of km and the value of μ0 are totally human decisions, taken by international bodies composed of representatives of the national standards organizations of all participating countries. The parameter μ0 is a measurement-system constant, not a physical constant that can be measured. It does not, in any meaningful sense, describe a physical property of the vacuum.[8] This is why the relevant Standards Organizations prefer the name "magnetic constant", rather than any name that carries the hidden and misleading implication that μ0 describes some physical property of the vacuum.
Significance in electromagnetism
The magnetic constant μ0 appears in Maxwell's equations, which describe the properties of electric and magnetic fields and electromagnetic radiation, and relate them to their sources. In particular, it appears in relationship to quantities such as permeability and magnetization density, such as the relationship that defines the magnetic H-field in terms of the magnetic B-field. In real media, this relationship has the form:
,
where M is the magnetization density. In free space, M = 0.
Maxwell's laws show that speed of light in a vacuum, c0 is related to the magnetic constant and the electric constant (vacuum permittivity), ε0, by the formula
References and Notes
- ^ "Magnetic constant". 2006 CODATA recommended values. NIST. http://physics.nist.gov/cgi-bin/cuu/Value?mu0. Retrieved 2007-08-08.
- ^ a b "CODATA Recommended Values of the Fundamental Physical Constants: 2006". Committee on Data for Science and Technology (CODATA): See Table 1. NIST. http://physics.nist.gov/cuu/Constants/codata.pdf.
- ^ SUNAMCO Commission (1987), "Recommended values of the fundamental physical constants", Symbols, Units, Nomenclature and Fundamental Constants in Physics, pp. 54, 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; (the IUPAP "Red book").
- ^ J R Lalanne, F Carmona & L Servant (1999). Optical spectroscopies of electronic absorption. (World Scientific series in contemporary chemical physics, vol. 17. ed.). Singapore;London: World Scientific. p. 10. ISBN 9810238614. http://books.google.com/books?id=7rWD-TdxKkMC&pg=PA10&lpg=PA10&dq=+%22magnetic+permittivity%22&source=web&ots=w62tNR56Yg&sig=YECrYWXFJWZS29tdZ_7Gr92LZ7M.
- ^ For an introduction to the subject of choices for independent units, see John David Jackson (1998). Classical electrodynamics (Third ed.). New York: Wiley. p. 154. ISBN 047130932X. http://worldcat.org/isbn/047130932X.
- ^ The decision to explicitly include the factor of 2π in km stems from the "rationalization" of the equations used to describe physical electromagnetic phenomena.
- ^ This choice defines the SI unit of current, the ampere: "Unit of electric current (ampere)". Historical context of the SI. NIST. http://physics.nist.gov/cuu/Units/ampere.html. Retrieved 2007-08-11.
- ^ The intrinsic properties of realizable vacuum, such as quantum vacuum, QCD vacuum, outer space or ultra-high vacuum are entirely separate from the definition of μ0.
See also
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