Magnet

magnet is a material or object that produces a magnetic field. The name derives from the Greek μαγνήτης λίθος (magnétes líthos), that is “Magnesia stone,“ from the name of a place in Asia Minor, known since ancient times for the huge deposits of magnetite.

The magnet, made in the shape of a horseshoe, has the two magnetic poles close together. This shape creates a strong magnetic field between the poles, allowing the magnet to pick up a heavy piece of iron.

The magnetization, induced by an external magnetic field, can be permanent (permanent magnets), if it persists when the external field ceases, or temporary (temporary magnets), if it ceases when the magnetizing field ceases. Typical temporary magnets are, for example, the cores of electromagnets, which are magnetized under the action of the magnetizing field generated by an electric current and then demagnetize when that ceases. Among the permanent magnets are the so-called natural magnets, consisting of fragments of magnetite, a mineral that often has a conspicuous magnetization of its own; natural magnets, of no practical interest today, were for a long time the only magnets available; currently permanent magnets are obtained by temporarily subjecting ferro- and ferrimagnetic substances, ie materials with high residual induction and high coercive field, such as special ferrous alloys and special ferrites, to an intense magnetic field.

The magnetization takes place in uniform magnetic fields created by electromagnets, generally shaped in such a way as to form a magnetic circuit with the pieces to be magnetized, possibly without air gap. The intensity of the magnetizing field is such as to bring the piece into saturation; the latter, extracted from the electromagnet, can be considered as part of a magnetic circuit, the other part of which is made up of air. In this case, the magnet retains a residual induction proportional to the residual induction responsible for the rigid magnetization magnetic hysteresis cycle of the material it is made of; hence the convenience of using materials with high residual induction.

The magnetic induction is less than the residual induction due to the action of the so-called demagnetizing field, which arises in the material.

A magnet is in a very delicate state of equilibrium, which can be disturbed by mechanical, thermal and magnetic stresses, representing as many causes for which the ferromagnetic domains can change their particular structure which corresponds to the magnetization. It is necessary to avoid exposure of one m. to heat sources: magnetization, in fact, irreversibly decreases with increasing temperature and even cancels itself out if a certain temperature is exceeded (Curie temperature, of the order of a few hundred °C) characteristic of the material used.

A magnet, whatever its shape, does not have a uniform distribution of magnetism: this thickens, so to speak, in particular zones, called polar regions or poles of the magnet, separated from each other by other zones, called neutral zones, in which the magnetism detectable from the outside is sensibly nil. In a magnet with a simple shape, homogeneous and magnetized with a uniform field, the poles are two, of equal intensity and opposite polarity (one North, the other South), located at the ends; but in m. complicated in shape and magnetized with non-uniform fields, the poles can also be more than two, and of different intensity.

Permanent magnet

A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. The materials that can be magnetized are also those strongly attracted to a magnet, and are called ferromagnetic (or ferrimagnetic); these include iron, nickel, cobalt, some rare earth alloys and some natural minerals such as magnetite. Even if ferromagnetic (and ferrimagnetic) materials are the only ones attracted by a magnet so intensely as to be commonly considered “magnetic“, all substances weakly respond to a magnetic field, through one of the numerous types of magnetism.