Magnetic fields

Magnetic fields are familiar in association with bar and horseshoe magnets, which have the power of attracting small pieces of iron. If a bar magnet is placed on a sheet of paper, on which lie tiny pieces of iron filings the direction of the magnetic forces can be directly observed by tapping the paper gently so that the filings arrange themselves along what are called lines of force of the magnet.

Substances can be classified into three groups, according to the way they behave when they are placed in a magnetic field. Most substances are what is called paramagnetic, that is, in an uneven magnetic field they tend to move to the region in which the field is strongest. In these substances the magnetic behavior is due to a lack of balance in the movements of the orbital electrons in the atoms of which they are composed. The imbalance may be due to the motion of unpaired orbital electrons, or to the spin of some electrons, or to a combination of both. Electrons in orbit round an atomic nucleus spin on their axes in much the same way as the planets spin on their axes in their orbits round the sun. This electron spin often has a dominant effect in magnetism.

Lack of balance makes atoms behave like tiny bar magnets, so that when a magnetic field is applied to a paramagnetic substance the atomic magnets try to align themselves with the field. It is the force rotating the individual magnets that tends to pull the substance, to the strongest part of a field. However, this effect is relatively weak in paramagnetic substances as the normal heat vibrations of the atoms override most of the magnetic force — the atomic magnets are only able to rotate to a limited extent.

In a few metals, such as iron, nickel, and cobalt, the spacing between the ions in crystals is such that individual ionic magnets line themselves up into groups all facing the same way — these groups are called domains. Because these domains are large compared to an atom, the normal thermal vibrations of the ions in the crystal lattice are not strong enough to override the magnetic forces. In an unmagnetized piece of iron, for example, the domains are orientated at random, but in an external magnetic field all the domains rotate so that their magnetic axes point in the same direction. These metals, which are called ferromagnetic, are therefore, easily magnetized and they make strong permanent magnets. However, if they are heated above a certain temperature, called the Curie point, the thermal Vibrations override the magnetic forces and the metals revert to be paramagnetic substances.

One might think that atoms that do not behave as tiny magnets, because they are without an electron imbalance, would be unaffected by a magnetic field. However, as the field itself has an effect on the orbital motions of the atomic electrons the effect is to turn the atoms into weak magnets that invariably oppose the direction of the field producing them. The forces so produced tend to make these substances move to the weakest part of an uneven magnetic field. Substances, such as copper and silver, that behave in this way are called "diamagnetic. In fact all substances are diamagnetic, but usually the diamagnetism is masked either by paramagnetism or ferromagnetism.

A freely pivoted ferromagnetic bar or needle will always adopt a definite position with respect to the Earth, such that one end points in the general direction on the North Pole and the other in the direction of the South Pole. This behavior, which is of course the basis of the magnetic compass, occurs because the direction of the lines of force of the magnet strives to coincide with the direction of the lines of force of the Earth's magnetic field. For convenience it is usual to refer to the north-seeking end of a magnet as its north pole, and its south-seeking end as its south pole. Like magnetic poles repel each other and unlike magnetic poles attract each other, behaving in exactly the same manner as electric charges.