Magnet

A magnet is an object manufactured in a magnetic material hard, i.e. whose remanent field and the coercive excitation are large (see below). That gives him particular properties, like exerting an attraction force on all ferromagnetic material.

Applications of the magnets

• Any bar magnet is naturally directed in the Northern direction - southern according to the lines of the Terrestrial magnetic field, for little that one leaves him a free axis of rotation of all forced. This property is used in the manufacture of the Boussole S.
• the magnets are very much used for the realization of machines to D.C. current or synchronous machines.
• the existence of magnetic field in the absence of current is made profitable for the realization of Capteur S, for example of the sensors of proximity.
• the magnets are also used in the design of dipolar sources in order to produce plasmas microwave. It is necessary however that this one makes it possible to check the conditions of coupling RCE (Electronic Cyclotronic Resonance) that is to say 0.0875 Tesla for a turning electric field of 2.45 GHz. In general the magnets used are in Samarium Cobalt.

Characteristics of the magnets

The magnets almost systematically contain atoms of at least one of the chemical element S following: iron, cobalt or nickel, or of the family of the Lanthanide S (rare earths). The natural magnets are Iron II mixed oxides and Iron III of the family of the Ferrite S (oxide mixed of a divalent metal and Iron III). They are hard magnetic materials (with broad hysteresis loop).

• the remanent field is the existing magnetic field in material in the absence of current.

• the coercive excitation of demagnetization is the excitation (magnetic field created by currents circulating around material) which it is necessary to produce to demagnetize this material.
• the temperature of Curie: temperature for which the material loses its magnetization, in a reversible way.

Calculation of the force of contact exerted by a magnet

If one knows the intensity of the Magnetic field $\backslash \; scriptstyle\; \{B\}$ (in Teslas) produced by the magnet on his surface, one can calculate a good approximation of the force necessary to separate it of an iron surface. It is imagined that the force $\backslash \; scriptstyle\; \{F\}$ took off the magnet of a distance $\backslash \; textstyle\; \{\backslash \; varepsilon\}$ of the iron surface. The distance $\backslash \; textstyle\; \{\backslash \; varepsilon\}$ is very small so that one can accept that in all the volume located between the magnet and iron the magnetic field is equal to $\backslash \; scriptstyle\; \{B\}$. The work made by the force $\backslash \; scriptstyle\; \{F\}$ is

$W\; =\; F\; \backslash \; varepsilon\; \backslash ,$

This work was transformed into energy of the magnetic field in the volume created between the magnet and iron. The density of energy per unit of volume due to the magnetic field is:

$\backslash \; rho\; =\; \{\backslash \; frac\; \{1\}\; \{2\}\}\; \{B^2\; \backslash \; over\; \backslash \; driven\}\; \backslash ,$ J. m ^-3

Here $\backslash \; scriptstyle\; \{\backslash \; driven\}\; \backslash ,$ is the permeability of the air, almost equal to that of the vacuum: $\backslash \; mu\_0\; =\; \backslash \; scriptstyle\; 4\; \backslash \; pi\; \backslash ,\; \backslash ,\; 10^\; \{-\; 7\}\; \backslash ,$ H. m ^-1.

The volume of the space created between the magnet and iron is equal to $\backslash \; scriptstyle\; S\; \backslash \; varepsilon$ where $\backslash \; scriptstyle\; S$ is the surface of the magnet which was stuck to iron. Made work was transformed into energy:

$F\; \backslash \; varepsilon\; =\; \{1\; \backslash \; over\; 2\}\; \{S\; \backslash \; varepsilon\; B^2\; \backslash \; over\; \backslash \; driven\}\; \backslash ,$

One deduces the value from the force of contact:

$F\; =\; \{1\; \backslash \; over\; 2\}\; \{B^2\; S\; \backslash \; over\; \backslash \; driven\}\; \backslash ,$

For a magnet of 2,54 cm (1 inch) of diameter and producing a field equal to 1 Tesla in the Magnetic circuit formed with the metal part with the contact of which it is, the force obtained is of 205 newton S, i.e. approximately equal to the weight of 21 kilograms.

See too

• Magnetism
• Magnetic field

Simple: Magnet

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