Editing Magnet (section)

=== Modelling magnets ===
[[File:VFPt cylindrical magnet thumb.svg|thumb|Field of a cylindrical bar magnet computed accurately]]
{{See also|Magnetic moment#Models|label 1=Two definitions of moment}}

Two different models exist for magnets: magnetic poles and atomic currents.

Although for many purposes it is convenient to think of a magnet as having distinct north and south magnetic poles, the concept of poles should not be taken literally: it is merely a way of referring to the two different ends of a magnet. The magnet does not have distinct north or south particles on opposing sides. If a bar magnet is broken into two pieces, in an attempt to separate the north and south poles, the result will be two bar magnets, ''each'' of which has both a north and south pole. However, a version of the magnetic-pole approach is used by professional magneticians to design permanent magnets.{{Citation needed|date=December 2011}}

In this approach, the [[divergence]] of the magnetization ∇·'''M''' inside a magnet is treated as a distribution of [[magnetic monopole]]s. This is a mathematical convenience and does not imply that there are actually monopoles in the magnet. If the magnetic-pole distribution is known, then the pole model gives the [[magnetic field]] '''H'''. Outside the magnet, the field '''B''' is proportional to '''H''', while inside the magnetization must be added to '''H'''. An extension of this method that allows for internal magnetic charges is used in theories of ferromagnetism.

Another model is the [[André-Marie Ampère|Ampère]] model, where all magnetization is due to the effect of microscopic, or atomic, circular [[bound current]]s, also called Ampèrian currents, throughout the material. For a uniformly magnetized cylindrical bar magnet, the net effect of the microscopic bound currents is to make the magnet behave as if there is a macroscopic sheet of [[electric current]] flowing around the surface, with local flow direction normal to the cylinder axis.<ref>{{ cite book |  title = Philosophy of the Mechanics of Nature, and the Source and Modes of Action of Natural Motive-Power |  author = Allen, Zachariah |  publisher = D. Appleton and Company |  year = 1852 |  page = [https://archive.org/details/bub_gb_EpUIAAAAIAAJ/page/n260 252] |  url = https://archive.org/details/bub_gb_EpUIAAAAIAAJ }}</ref> Microscopic currents in atoms inside the material are generally canceled by currents in neighboring atoms, so only the surface makes a net contribution; shaving off the outer layer of a magnet will ''not'' destroy its magnetic field, but will leave a new surface of uncancelled currents from the circular currents throughout the material.<ref>{{ cite book |  title = Electricity, Magnetism, and Light |  edition = 3rd |  author = Saslow, Wayne M. |  publisher = Academic Press |  year = 2002 |  isbn = 978-0-12-619455-5 |  page = 426 |  url = https://books.google.com/books?id=4liwlxqt9NIC&pg=PA426 |  url-status = live |  archive-url = https://web.archive.org/web/20140627092809/http://books.google.com/books?id=4liwlxqt9NIC&pg=PA426 |  archive-date = 2014-06-27 }}</ref> The [[right-hand rule]] tells which direction positively-charged current flows. However, current due to negatively-charged electricity is far more prevalent in practice.{{Citation needed|date=January 2018}}<ref>{{Cite web |date=2024-08-01 |title=Right Hand Rule |work=PASCO scientific |url=https://www.pasco.com/resources/articles/right-hand-rule }}</ref>