MAGNETS

A magnet is a material or object that produces a magnetic field. and attracts or repels other magnets. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, cobalt, etc.
A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. The overall strength of a magnet is measured by its magnetic moment or, alternatively, the total magnetic flux it produces. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.


Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron, which can be magnetized but do not tend to stay magnetized, and magnetically "hard" materials, which do. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). Often, the coil is wrapped around a core of "soft" ferromagnetic material such as mild steel, which greatly enhances the magnetic field produced by the coil. Permanent magnets are made from "hard" ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a strong magnetic field during manufacture to align their internal microcrystalline structure, making them very hard to demagnetize. The local strength of magnetism in a material is measured by its magnetization.

An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops. These include the elements iron, nickel and cobalt and their alloys, some alloys of rare-earth metals, and some naturally occurring minerals such as lodestone. An everyday example is a refrigerator magnet used to hold notes on a refrigerator doors

Discovery


Lodestones, suspended so they could turn, were the first magnetic compasses. The earliest known surviving descriptions of magnets and their properties are from Anatolia, India, and China around 2500 years ago. The properties of lodestones and their affinity for iron were written of by Pliny the Elder in his encyclopedia Naturalis Historia. 

By the 12th to 13th centuries AD, magnetic compasses were used in navigation in China, Europe, the Arabian Peninsula and elsewhere. Ancient people learned about magnetism from lodestones (or magnetite) which are naturally magnetized pieces of iron ore. The word magnet was adopted in Middle English from Latin magnetum "lodestone", ultimately from Greek μαγνῆτις [λίθος] (magnētis [lithos]) meaning "[stone] from Magnesia",] a place in Anatolia where lodestones were found (today Manisa in modern-day Turkey).

Magnetic Moment



A magnet may also be subject to a force driving it in one direction or another, according to the positions and orientations of the magnet and source. In SI units, the magnetic moment is specified in terms of A·m2 (amperes times meters squared).

A magnet both produces its own magnetic field and responds to magnetic fields. For a bar magnet, the direction of the magnetic moment points from the magnet's south pole to its north pole,[9] and the magnitude relates to how strong and how far apart these poles are. A magnet's magnetic moment (also called magnetic dipole moment and usually denoted μ) is a vector that characterizes the magnet's overall magnetic properties. In addition, when the magnet is put into an external magnetic field, produced by a different source, it is subject to a torque tending to orient the magnetic moment parallel to the field.[10] The amount of this torque is proportional both to the magnetic moment and the external field. The strength of the magnetic field it produces is at any given point proportional to the magnitude of its magnetic moment. If the field is uniform in space, the magnet is subject to no net force, although it is subject to a torque.

A wire in the shape of a circle with area A and carrying current I has a magnetic moment of magnitude equal to IA.


Magnetic Materials



Most materials, however, produce a magnetic field in response to an applied magnetic field – a phenomenon known as magnetism. Compared to paramagnetic and ferromagnetic substances, diamagnetic substances, such as carbon, copper, water, and plastic, are even more weakly repelled by a magnet. Superconductors repel magnetic fields from their interior and are strongly diamagnetic.
There are various other types of magnetism, such as spin glass, superparamagnetism, superdiamagnetism, and metamagnetism. Although force on a diamagnetic object from an ordinary magnet is far too weak to be felt, using extremely strong superconducting magnets, diamagnetic objects such as pieces of lead and even mice[19] can be levitated, so they float in mid-air. Several forms of magnetic behavior have been observed in different materials, including:

Ferromagnetic and ferrimagnetic materials are the ones normally thought of as magnetic; they are attracted to a magnet strongly enough that the attraction can be felt. Magnetic ferrofluids, although they are made of tiny ferromagnetic particles suspended in liquid, are sometimes considered paramagnetic since they cannot be magnetized.
Diamagnetic means repelled by both poles. These materials are the only ones that can retain magnetization and become magnets; a common example is a traditional refrigerator magnet. Only certain classes of materials can do this. The term magnet is typically reserved for objects that produce their own persistent magnetic field even in the absence of an applied magnetic field. This attraction is hundreds of thousands of times weaker than that of ferromagnetic materials, so it can only be detected by using sensitive instruments or using extremely strong magnets. The difference between ferro- and ferrimagnetic materials is related to their microscopic structure, as explained in Magnetism.
Paramagnetic substances, such as platinum, aluminum, and oxygen, are weakly attracted to either pole of a magnet. All substances not possessing one of the other types of magnetism are diamagnetic; this includes most substances. There are several types of magnetism, and all materials exhibit at least one of them.

The overall magnetic behavior of a material can vary widely, depending on the structure of the material, particularly on its electron configuration. The permeability of diamagnetic materials is less than the permeability of a vacuum. Ferrimagnetic materials, which include ferrites and the oldest magnetic materials magnetite and lodestone, are similar to but weaker than ferromagnetics.















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