View source for Physics:Oersted

{{Short description|Unit of the auxiliary magnetic field H in the CGS system of units}}
{{Infobox unit
| name       = oersted
| image      =
| caption    =
| standard   = [[Physics:Gaussian units|Gaussian units]]
| quantity   = magnetic field strength
| symbol     = Oe
| namedafter = [[Biography:Hans Christian Ørsted|Hans Christian Ørsted]]
| extralabel  = Derivation
| extradata   = 1 dyn/Mx
| units1     = Gaussian base units
| inunits1   = 1&nbsp;cm<sup>−1/2</sup>⋅g<sup>1/2</sup>⋅s<sup>−1</sup>
| units2     = SI units
| inunits2   = (4π)<sup>−1</sup>{{e|3}}&nbsp;A/m ≈ {{convert|1e0|Oe|A/m|sigfig=7|disp=out}}
}}

The '''oersted''' (symbol '''Oe''') is the coherent derived unit of the [[Physics:Magnetic field#The H-field|auxiliary magnetic field '''H''']] in the [[Centimetre–gram–second system of units|centimetre–gram–second system of units]] (CGS).<ref>"as late as 1936 a subcommittee of the IEC [[Organization:International Electrotechnical Commission|International Electrotechnical Commission]] proposed the names 'maxwell', 'gauss' and 'oersted' for the cgs electromagnetic units of flux, induction and magnetic field strength, respectively". &mdash; John James Roche, ''The Mathematics of Measurement: A Critical History'', The Athlone Press, London, 1998, {{ISBN|0-485-11473-9}}, page 184 and John James Roche, [http://materias.df.uba.ar/ft1a2016c2/files/2016/09/B-vs-H.pdf "''B'' and ''H'', the intensity vectors of magnetism: A new approach to resolving a century-old controversy"], ''American Journal of Physics'', vol. 68, no. 5, 2000, doi: 10.1119/1.19459, p. 438; in both cases giving the reference as Claudio Egidi, editor, ''Giovanni Giorgi and his Contribution to Electrical Metrology: Proceedings of the meeting held in Turin (Italy) on 21 and 22, September 1988'', Politecnico di Torino, Turin (IT), 1990, {{ISBN|978-8885259003}}, pp. 53–56</ref> It is equivalent to 1 [[Physics:Dyne|dyne]] per [[Physics:Maxwell (unit)|maxwell]].

==Difference between CGS and SI systems==
In the CGS system, the unit of the H-field is the oersted and the unit of the B-field is the [[Physics:Gauss (unit)|gauss]]. In the SI system, the unit ampere per meter (A/m), which is equivalent to [[Newton (unit)|newton]] per [[Weber (unit)|weber]], is used for the H-field and the unit of [[Tesla (unit)|tesla]] is used for the B-field.<ref>Kaye, G. W. C, & Laby, T. H.: ''Table of Physical and Chemical Constants'', page 14. Longman, 1973.</ref>

==History==
The unit was established by the [[Organization:International Electrotechnical Commission|IEC]] in the 1930s <ref>{{Cite web |url=http://www.iec.ch/about/history/ |title=IEC history |access-date=2006-03-25 |archive-date=2019-05-21 |archive-url=https://web.archive.org/web/20190521213548/http://www.iec.ch/about/history/ |url-status=dead }}</ref> in honour of Danish physicist [[Biography:Hans Christian Ørsted|Hans Christian Ørsted]]. Ørsted discovered the connection between magnetism and electric current when a magnetic field produced by a current-carrying copper bar deflected a magnetised needle during a lecture demonstration.<ref>{{Cite web|url=https://www.famousscientists.org/hans-christian-oersted/|title=Hans Christian Oersted - Biography, Facts and Pictures|language=en-US|access-date=2020-03-31}}</ref>

==Definition==
[[File:TDK AM Pro cassette - wrapper detail with magneric specs.png|thumb|[[Engineering:Cassette tape|Cassette tape]] label with coercivity (a measure of the external magnetic flux required to magnetize the tape) measured in oersteds.]]
The oersted is defined as a [[Physics:Dyne|dyne]] per unit pole.<ref>Hirst, A. W. ''Electricity and Magnetism For Engineering Students''. Blackie & Son Limited, 1959, p.411</ref> The oersted is {{sfrac|1000|4π}} (≈{{convert|1.000000|Oe|A/m|disp=number}}) amperes per meter, in terms of SI units.<ref>[http://www-d0.fnal.gov/hardware/cal/lvps_info/engineering/mag_conv.htm Magnetic Conversion Factors<!-- Bot generated title -->]</ref><ref>[http://www.vitatech.net/emf_fundamentals.php4 EMF Fundamentals<!-- Bot generated title -->] {{webarchive|url=https://web.archive.org/web/20080407011207/http://www.vitatech.net/emf_fundamentals.php4 |date=2008-04-07}}</ref><ref>[http://everything2.com/index.pl?node_id=1145558 Everything2.com • Oersted]</ref><ref>[http://www.gordonengland.co.uk/conversion/sinon3.htm Derived CGS Units with Special Names<!-- Bot generated title -->]</ref>

The H-field strength inside a long [[Engineering:Solenoid|solenoid]] wound with 79.58 turns per meter of a wire carrying 1 [[Ampere|A]] is approximately 1 oersted. The preceding statement is exactly correct if the solenoid considered is infinite in length with the current evenly distributed over its surface.

The oersted is closely related to the gauss (G), the CGS unit of magnetic flux density. In a vacuum, if the magnetizing field strength is 1&nbsp;Oe, then the magnetic field density is 1&nbsp;G, whereas, in a medium having permeability {{math|''&mu;''<sub>r</sub>}} (relative to permeability of vacuum), their relation is:

:<math>B(\text{G}) = \mu_\text{r} H(\text{Oe})</math>

Because oersteds are used to measure magnetizing field strength, they are also related to the [[Physics:Magnetomotive force|magnetomotive force]] (mmf) of current in a single-winding wire-loop:

:<math>1\text{ Oe}= \frac{1000}{4 \pi} \text{A}/\text{m}</math><ref>{{cite book|last=BIPM|title=SI Brochure: The International System of Units (SI) [8th edition, 2006; updated in 2014]|year=2006|chapter=Table 9. Non-SI units associated with the CGS and the CGS-Gaussian system of units|chapter-url=http://www.bipm.org/en/publications/si-brochure/table9.html}}</ref>

===Stored energy===
{{main|Physics:Maximum energy product}}
The stored energy in a magnet, called ''magnet performance'' or ''maximum energy product''<ref>{{Cite web|url=https://www.duramag.com/techtalk/tech-briefs/what-is-maximum-energy-product-bhmax-and-how-does-it-correspond-to-magnet-grade/|title=What is Maximum Energy Product / BHmax and How Does It Correspond to Magnet Grade? {{!}} Dura Magnetics USA|language=en-US|access-date=2020-01-20}}</ref> (often abbreviated BH<sub>max</sub>), is typically measured in units of megagauss-oersteds (MG⋅Oe).

==See also==
* [[Centimetre–gram–second system of units]]
* Ampere's model of magnetization


==References==
{{Reflist}}

{{CGS units}}

[[Category:Centimetre–gram–second system of units]]
[[Category:Units of magnetic induction]]

{{Sourceattribution|Oersted}}