====== Magnetic polarity ======
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| //[[user/Stan Zurek]], Magnetic polarity, Encyclopedia Magnetica//, \\ @PAGEL@ |
**Magnetic polarity** - a concept in physics and [[electromagnetism]] applicable to [[magnetic field]], as produced by [[electric current]] and [[spin magnetic moment]].
Opposite poles attract (north-south), like poles repel (north-north, or south-south)
[[file/magnetic_poles_repel_and_attract_magnetica_jpg|{{magnetic_poles_repel_and_attract_magnetica.jpg}}]]
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It is possible to distinguish two types of [[magnetic pole|magnetic poles]], and by convention they are named "north" and "south", stemming from the use of [[magnetic compass]] which can indicate the location of the [[magnetic North Pole]] (or South Pole) of the [[Earth magnetic field|Earth's magnetic field]].
The concept of two magnetic poles is in some sense analogous to [[electric charge|electric charges]] (positive-negative) and chirality (right-left or clockwise-anticlockwise).[(Purcell)]
[[Magnetic monopoles]] do not exist and therefore the magnetic poles always occur at least in pairs north-south. Therefore, there is a basic magnetic configuration called [[magnetic dipole]] (two poles).
[[Electromagnet|Electromagnets]] with higher number of poles ([[quadrupole]] = 4, [[octupole]] = 8) are often used in [[particle accelerator|particle accelerators]], but they also have magnetically just two poles (north and south), simply positioned in an alternating way to produce the total number of poles, as required.
===== Polarity of magnetic forces =====
Like [[magnetic pole|magnetic poles]] repel (N-N or S-S), opposite poles attract each other (N-S). Magnetic poles can be permanent or induced, with changing polarity. For example, an [[iron]] nail ([[ferromagnetic]]) gets magnetised in the external magnetic field with such polarity that it always attracted to the magnetic pole which magnetises it.
Similarly, [[ferrimagnetic]], [[antiferromagnetic]] and [[paramagnetic]] materials are always attracted to the source of the field (if there are no additional effects, or without any prior exposure to magnetic field). On the other hand, [[diamagnetic]] materials are generally expelled from the field.
[[Magnetic force]] due to [[Magnetic pole|magnetic poles]], from left to right: like poles repel (position of the hanging magnet is deflected accordingly), opposite poles attract, nail ([[soft ferromagnet|soft ferromagnetic material]]) gets magnetised and is attracted to either pole of a magnet, the force on [[non-magnetic material|non-magnetic materials]] such as plastic and copper (without [[electric current]]) is typically negligible, [[electromagnetic coil]] with current can repel or attract the magnet (depending on the polarity of current), and the force on [[non-magnetic stainless steel]] (e.g. type 316) is negligibly small
[[file/bar_magnets_polarity_n_magnetica_jpg|{{bar_magnets_polarity_n_magnetica.jpg}}]]
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As in the previous image, but reversed polarity of the hanging magnet, so the direction of force is reversed when poles are involved (stationary magnet and electromagnetic coil), but the nail is still attracted
[[file/bar_magnets_polarity_s_magnetica_jpg|{{bar_magnets_polarity_s_magnetica.jpg}}]]
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===== Right-hand rule =====
By convention, it is agreed that electric current flows from the positive to the negative [[electrode]] of the energy source. If such current flows in a loop of wire, then looking from one side of the loop the current will appear to flow in a clockwise direction, and anticlockwise when looking from the opposite side. Therefore, from the viewpoint of the observer, the relative direction of the current reverses, and as a result different magnetic poles will be presented to the observer in the two situations.
Also by convention, it is agreed that from the position of the observer, if the current flows in the anticlockwise direction then the presented magnetic pole is "north", and for a clockwise current the magnetic pole is "south",[(Jiles>[[http://books.google.com/books?isbn=9780412798603|David C. Jiles, Introduction to Magnetism and Magnetic Materials, Second Edition, Chapman & Hall, CRC, 1998, ISBN 9780412798603]])][(Cullity)] as shown below in the illustration for magnetic field //B//.
The relation of the direction of current to the names of the poles can be remembered by using stylised letters, whose ends follow the circle - thus "N" denoting anticlockwise, and "S" - clockwise.
The magnetic field is always perpendicular and [[tangential]] to the electric current which produces it, and the directions follow the [[right-hand rule]]: if the thumb shows the direction of current then the curled fingers show the direction of magnetic field, and vice versa: if thumb shows the field then fingers indicate direction of current which produced it.
[[Magnetic poles]] (of [[flux density]] //B//) are created by a loop of electric current //I//: N - north (from above current is seen to flow anticlockwise), S - south (from below current is seen as clockwise)
[[file/magnetic_poles_convention_magnetica_jpg|{{magnetic_poles_convention_magnetica.jpg}}]]
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Orientation of magnetic field (shown here as [[/magnetic field strength]] //H// or [[flux density]] //B//) with respect to the [[electric current]] //I// follows the [[right-hand rule]]: if thumb shows the direction of current then curled fingers show the direction of magnetic field, and vice versa
[[file/right-hand_rule_both_magnetica_png|{{right-hand_rule_both_magnetica.png}}]]
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[[Compass|Compasses]] around a [[coil]] with electric current
[[file/right-hand_rule_compass_magnetica_jpg|{{right-hand_rule_compass_magnetica.jpg}}]]
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[[Energised electromagnetic coil]] (which is an [[electromagnet]] in the form of a [[solenoid]]) represents a [[magnetic dipole]], with the polarity of magnetic poles defined by the right-hand rule.
If a [[compass]] is placed inside such a coil, then the north-seeking end of the needle will be seen at the end from which the current flow appears anticlockwise, and the south-seeking end will be seen from where the current flow appears clockwise.
A compass anywhere outside of the coil will be oriented by following the rule that opposite poles attract, so an external compass will have its north-seeking needle pointing towards the south pole of the coil (with clockwise current).
Therefore, outside of the coil the vector of magnetic field points in the opposite direction to the field inside of the coil, but along closed contour of imaginary [[magnetic field lines]], which themselves have no beginning or end (because of the non-existence of [[magnetic monopoles]]).
By convention, magnetic field lines are said to emanate from the magnetic north pole (as illustrated by the tips of the arrows) and go to the magnetic south pole.
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===== Polarity of Earth's magnetic field =====
A magnetic compass is said to point to the magnetic north pole. The [[compass needle]] pointing towards the Earth's North is typically marked clearly with a colour or outline (as compared to the other side of the needle, which is marked with a different colour or left unmarked).
Historically, in English speaking countries it became customary to refer to the north-pointing end of the compass needle as "//**north-seeking pole**//" or "//**north pole**//" for short.[(Lockwood>[[https://books.google.co.uk/books?id=RYBRAAAAMAAJ|Thomas Dixon Lockwood, Electricity, magnetism, and electric telegraphy, D. Van Nostrand, New York, 1883]])][(Cullity>[[https://books.google.co.uk/books?isbn=9780471477419|B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, 2nd edition, Wiley, IEEE Press, 2009, ISBN 9780471477419]])][(Purcell>[[https://books.google.co.uk/books?isbn=1107014026|E.M. Purcell, D.J. Morin, Electricity and magnetism, 3rd edition, Cambridge University Press, 2013, ISBN 9781107014022]])]
[[Magnetic compass]] detects direction and polarity of [[Earth magnetic field|Earth's magnetic field]]; north-pointing end of the needle is marked with green colour
[[/file/compass_magnetica_jpg|{{/compass_magnetica.jpg}}]]
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However, it is that the opposite poles attract each other, so consequently actual [[Earth magnetic field|Earth's magnetic field]] must have its polarity such that at the Earth's geographic North Pole there is a [[magnetic south pole]].
[[Earth magnetic field|Earth's magnetic field]] has a magnetic south pole at its geographic North Pole
[[/file/earth_magnetic_field_schematic_magnetica_png|{{/earth_magnetic_field_schematic_magnetica.png}}]]
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For the same reason, a north-seeking end of the compass needle is always attracted to the "south pole" side of the current loop, as the nomenclature dictates, and as shown in this article in various photographs.
It is recognised that such a naming convention is somewhat confusing, but it was accepted that a change would create even more confusion[(Lockwood)] and therefore the names were left as originally introduced, namely that the north-seeking end of the compass needle represents magnetic north pole.
This naming convention has been in use since 19th century, used extensively in marine navigation.[(Lecky>[[https://books.google.co.uk/books?id=eDoDAAAAQAAJ|Captain S.T.S. Lecky, Wrinkles in practical navigation, George Philip & Son, London, 1881]])][(Lockwood)]
Distribution of magnetic field (local direction and amplitude) around an energised coil is equivalent to that of a similarly shaped bar magnet, with the same polarity and appropriate level of [[magnetisation]]. The images below show a photograph of a grid of small compasses placed around a coil and a magnet of similar size. The direction of field is the same everywhere around the structure.
The compasses are aligned along imaginary magnetic field lines, which are used widely for the purpose of illustration of the presence and distribution of magnetic field, with the spacing between the lines indicating the amplitude of [[magnetic flux]] (denser lines mean higher flux).
Magnetic field distribution around an energised coil indicated by a grid of small compasses; the polarity of current in the coil was set so that the magnetic south pole is at the top of the coil
[[/file/compasses_and_coil_magnetica_jpg|{{/compasses_and_coil_magnetica.jpg}}]]
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Magnetic field distribution around an bar magnet indicated by small compasses; magnetic south pole of the magnet is at the upper end and all "north-seeking" needles point to it
[[/file/earth_analogy_with_magnet_and_compasses_magnetica_jpg|{{/earth_analogy_with_magnet_and_compasses_magnetica.jpg}}]]
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Alignment of a compass needle in magnetic field follows [[magnetic field lines]] (illustrated here by [[finite element modelling]] simulation)
[[/file/alnico5_bar_magnet_magnetica_jpg|{{/alnico5_bar_magnet_magnetica.jpg}}]]
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===== Colour of poles =====
Some commercial manufacturers of permanent magnets mark the polarity by using colours. This practice is also related to the use of compasses, which had the north-pointing end of the needle marked with some colour.
In marine, aerospace and military application it was typical to mark the north-seeking end of the compass needle with a red colour, and by the naming convention described above, the red colour therefore represented magnetic north.[(Napier>[[https://books.google.co.uk/books?id=gDEDAAAAQAAJ|Robert Ashton Napier, Manual of navigation, James Maclehose, Glasgow, 1880]])][(Lecky)][(FAA>[[https://www.google.co.uk/books/edition/Instrument_Flying_Handbook/1ZSNZXTz6AgC|Instrument Flying Handbook, FAA-H-8083-15A, Federal Aviation Administration US, Government Printing Office, 2008, ISBN 9780160820922]])][(Coey>[[https://books.google.co.uk/books?isbn=0521816149|J.M.D. Coey, Magnetism and Magnetic Materials, Cambridge University Press, 2010, ISBN 9780521816144]])] It is easier to remember this nomenclature if one remembers that: //**R**ed = no**R**th//, and //bl**U**e = so**U**th//.
Before the colours were used, also a mechanical notch could be put in a magnet to denote the location of the north pole.[(Lockwood)]
Military compass: red colour is used to mark the north-seeking end of the needle
[[file/military_compass_jpg|{{military_compass.jpg}}]]
//by F.R. Bamber, [[http://creativecommons.org/licenses/by-sa/3.0/|CC-BY-SA-3.0]]//
Compass used for walking navigation: red colour used for north-end of the needle
[[file/walkers_compass_jpg|{{walkers_compass.jpg}}]]
//by A. Pingston, Public domain//
[[Bar magnet]]: red - north pole, other colour - south pole
[[file/bar_magnet_jpg|{{bar_magnet.jpg}}]]
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However, the colour convention of using red for north pole was not strictly observed[(Lecky)], and other colours are also used quite randomly.
The reverse colouring (red - south, blue - north) is often encountered in commercial magnets, and less strict disciplines such a unconventional medicine.[([[https://books.google.co.uk/books?isbn=9780914955429|Buryl Payne, Magnetic healing, Advanced techniques for the application of magnetic forces, Lotus Press, 1997]])]
===== See also =====
*[[Magnetic field lines]]
*[[Bar magnet]]
===== References =====
~~REFNOTES~~
{{tag> Magnetic_polarity Magnet Magnetic_pole Counter}}