There is a large variety of elements and compounds on earth. Some new elements alloys and compounds have been synthesized in the laboratory. Faraday classified these substances on the basis of their magnetic properties into the following three categories:
Paramagnetic substances and
Their main characteristics are discussed below.
Diamagnetic substances are those in which the individual atoms/ molecules/ ions do not posses any net magnetic moment on their own.
Some of the other important properties of diamagnetic substances are:
When suspended in a uniform magnetic field they set their longest axis at right angles to the direction of the field the shorts axis is along the direction of the field.
When placed in a non-uniform magnetic field these substances have a tendency to move from stronger parts of the field to the weaker parts a watch glass placed on two pole pieces of an electromagnet held close to each other. The depression in the middle is due to stronger field at the centre if the distance between the poles is increased. The effect is reversed as in the case the field is stronger at the poles and weaker at the centre.
The level of a diamagnetic liquid in U tube is depressed instead of rising when subjected to a magnetic field it confirms movement of diamagnetic from stronger parts of the field to weaker parts. The property implies the whereas a magnet attracts metals like iron it would repeal a diamagnetic substance.
When a specimen of a diamagnetic material is placed in a magnetizing field the field lines prefer not to pass through the specimen it implies that the magnetic field lines are repelled or expelled and the field inside the material is reduced. This reduction is slight, being one part in 10.
Relative magnetic permeability of diamagnetic substances is always less then unity.
μ = b / B0 and B < B0
∴ μr < 1
From the relation μr = 1 + Xm as μr <M1. Xm is negative. Hence susuceptaibitly of diamagnetic substances is negative.
Susceptibility of diamagnetics does boot charge with temperature. Bismuth at low temperature is an exception to this general property.
Explanation of diamagnetism
Diamagnetism can be understood as follows
Electrons in an atom are revolving around t5he nucleus in some orbit. The orbiting electrons are equivalent to current carrying loops and thus possess orbital magnetic moment M in addition to probate motion every electron has a spin motion around its axis. Therefore another dipole magnetic moment called spin magnetic moment M is also associated with electron.
In a diamagnetic substance M and M cancel each other for every atom so that the atom has no net magnetic dipole moment.
In a diamagnetic substance M and M cancel each tougher the every atom so that the atom has no net magnetic dipole moment.
When an external magnetic dials is applied those electrons having magnetic moment in the same direction slow down and those having magnetic moment in opposite direction speed u therefore the substance develops direction speed up therefore the substance develops net magnetic moment in a direction opposite two that f the supplied field. This accounts for all the properties of diamagnetic substances as stated above.
Examples of some diamagnetic materials are bismuth antimony copper lead silicon gold quartz mercury water alcohol air nitrogen hydrogen etc. infect diamagnetism is present in all the substances. However the effect in many cases is too weak.
Superconductors are the most exotic diamagnetic materials. These are metals cooled two very low temperatures which have zero resistance perfect conductivity and perfect diamagnetism. The magnetic field lines are completely expelled from superconductors χm = - 1 and μr = 0 in case of superconductors. A superconductor repels a magnet and in turn is repelled by the magnet.
The phenomenon of perfect diamagnetism in superconductors is called messier effect. Superconducting magnets have been used for running magnetically levitated superfast trains.
Paramagnetic substances: paramagnetic substances are those in which each individual atom molecule ion has a net non zero magnetic moment of its own some of the other important properties of paramagnetic substance are:
When suspended in a uniform magnetic field they rotate so as to bring their longest axis along the direction of the field.
When placed in a non- uniform magnetic field, they tend to move from weaker parts of the field to the stronger parts. It implies that paramagnetic substances get weakly attracted to a magnet. For example consider a paramagnetic powder or liquid put in a watch glass placed on two pole pieces of an electromagnet held close to each other (≈ 2 mm or so).if the distance between the poles is increased, the effect is reversed as in that case the field is stronger at the poles and weaker at the centre.
When a magnetic field is applied to the level of a paramagnetic liquid in one limb of a U – tube. It confirms that paramagnetic substances move form weaker to stronger parts of the magnetic field.
When a specimen of a paramagnetic substance is placed in a magnetizing field, the magnetic field lines prefer to pass through the specimen rather than through air. Thus magnetic induction B inside the sample is more than the magnetic induction Bo outside the sample. The enhancement is slight, being one part in 10.
Relative magnetic permeability of paramagnetic substances is always more than unity.
As μr = B / B0 and B > B0
∴ μr > 1.
From the SI relation μr = 1 + ym as μr > 1 therefore ym must be positive. Hence susceptibility of paramagnetic substances is positive.
Susceptibility of paramagnetic substances varies inversely as the temperature of the substance χ m ∝ 1 / T they lose their magnetic character with rise in temperature.
Explanation of par magnetism
The individual atoms or molecules or ions of a paramagnetic material possess some permanent magnetic dipole moment of their own. However, due to ceaseless random thermal motion of the atoms the dipoles are randomly oriented and no net magnetization is seen a paramagnetic material does not behave as a magnet in the absence of an external magnetic field.
When an external magnetic field is applied at low temperatures, the individual atomic dipole moments are made to align along the field. That is why the specimen gets magnetized weakly along the field. This is par magnetism.
When we raise the temperature of paramagnetic material the atomic dipoles acquire some kinetic energy. They try to disorient. That is why magnetic susceptibility of paramagnetic materials decreases with rise in temperature.
Some examples of paramagnetic materials are aluminum sodium calcium platinum chromium manganese oxygen at STP and copper chloride etc.
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