Table of Contents
| Stan Zurek, Magnetic polarity, Encyclopedia Magnetica,
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.
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.
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”,2)3) 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.
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).
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.
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 confusion7) and therefore the names were left as originally introduced, namely that the north-seeking end of the compass needle represents magnetic north pole.
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).
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.10)11)12)13) 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.14)
However, the colour convention of using red for north pole was not strictly observed15), 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.16)