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Magnetic polarity

Magnetic polarity - a concept in physics and electromagnetism applicable to magnetic field, as produced by electric current and spin magnetic moment.

Opposite pole attract (north-south), like poles repel (north-north, or south-south) magnetic_poles_repel_and_attract_magnetica.jpg

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

It is possible to distinguish two types of 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's magnetic field.

The concept of two magnetic poles is in some sense analogous to electric charges (positive-negative) and chirality (clockwise-anticlockwise).

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).

Electromagnets with higher number of poles (quadrupole = 4, octupole = 8) are often used in 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 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 poles, from left to right: like poles repel (position of the hanging magnet is deflected accordingly), opposite poles attract, nail (soft ferromagnetic material) gets magnetised and is attracted to either pole of a magnet, the force on 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 bar_magnets_polarity_n_magnetica.jpg

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

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 bar_magnets_polarity_s_magnetica.jpg

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

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”,1)2) 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) magnetic_poles_convention_magnetica.jpg

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

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

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

Energised electromagnetic coil and compasses right-hand_rule_compass_magnetica.jpg

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

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.3)4)5)

Magnetic compass detects direction and polarity of Earth's magnetic field; north-pointing end of the needle is marked with green colour compass_magnetica.jpg

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

However, it is that the opposite poles attract each other, so consequently actual Earth's magnetic field must have its polarity such that at the Earth's geographic North Pole there is a magnetic south pole.

Earth's magnetic field has a magnetic south pole at its geographic North Pole

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

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 confusion6) 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.7)8)

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 compasses_and_coil_magnetica.jpg

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

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 earth_analogy_with_magnet_and_compasses_magnetica.jpg

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

Alignment of a compass needle in magnetic field follows magnetic field lines (illustrated here by finite element modelling simulation) alnico5_bar_magnet_magnetica.jpg

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

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.9)10)11)12) It is easier to remember this nomenclature if one remembers that: Red = noRth, and blUe = soUth.

Before the colours were used, also a mechanical notch could be put in a magnet to denote the location of the north pole.13)

Military compass: red colour is used to mark the north-seeking end of the needle military_compass.jpg by F.R. Bamber, CC-BY-SA-3.0
Compass used for walking navigation: red colour used for north-end of the needle walkers_compass.jpg by A. Pingston, Public domain
Bar magnet: red - north pole, other colour - south pole Encyclopedia Magnetica

S. Zurek, Encyclopedia Magnetica, CC-BY-4.0

However, the colour convention of using red for north pole was not strictly observed14), 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.15)

See also


magnetic_polarity.txt · Last modified: 2021/02/20 23:21 by stan_zurek

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