Magnets

A magnet is an object that has a small magnetic field. It can be in the form of a permanent magnet or an electromagnet. Permanent magnets do not rely upon outside influences to generate their field. They occur naturally in some rocks, but can also be manufactured. more...

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Electromagnets rely upon electric current to generate a magnetic field - when the current increases, so does the field.

Magnets are attracted to, or repelled by, other materials. A material that is strongly attracted to a magnet is said to have a high permeability. Examples of materials with very high permeability include iron and steel. Liquid oxygen is an example of something with a low permeability, and it is only weakly attracted to a magnetic field. Water has such a low permeability that it is actually slightly repelled by magnetic fields. Everything has a measurable permeability: people, gases, and even the vacuum of outer space.

The SI unit of magnetic field strength is the tesla, and the SI unit of total magnetic flux is the weber. 1 weber = 1 tesla flowing through 1 square meter, and is a very large amount of magnetic flux.

Physical origin of magnetism

Permanent magnets

Normal pieces of matter are composed of particles such as protons, neutrons, and electrons; and all of these have the fundamental property of quantum mechanical spin. Spin gives each one of these particles an associated magnetic field. Because of this, and the fact that the average microscopic piece of matter contains huge numbers of these particles, it would be expected that all matter would be magnetic. Even antimatter would have magnetic characteristics. However, everyday experience shows that many materials do not.

Within each atom and molecule, the spin of each of these particles is highly ordered as a result of the Pauli Exclusion Principle. However, there is no long-range ordering of these spins between atoms and molecules. Without long-range ordering, there is no net magnetic field because the magnetic moment of each one of the particles is canceled by the magnetic moment of other particles.

Permanent magnets are special in that long-range ordering does exist. The highest degree of ordering exists within magnetic domains. These domains can be likened to microscopic neighborhoods in which there is a strong reinforcing interaction between particles, and as a result, a great deal of order. The greater the degree of ordering within and between domains, the greater the resulting field will be.

Long-range ordering (and the resulting strong net magnetic field) is one of the hallmarks of a ferromagnetic material.

Electronic generation of magnetism

Electrons play the primary role in generating a magnetic field. Within an atom, electrons can exist either individually or in pairs within any given orbital. When they are paired, the individuals in that pair always have opposite spin—one up, one down. The fact that the spins have opposite orientation means that the two cancel one another. If all electrons are paired, no net magnetic field will be generated.

Read more at Wikipedia.org