Q.1 Write concept of magnetism. (AKTU. 2007 - 08)
Ans. According
to the Weber and Ewing each atom of a magnetic substance is a tiny magnet with
two poles, north and south. In the natural state of the material the atomic
magnets are randomly oriented so that the north pole of one atomic magnet
cancel the effect of the south pole of the other and make zero net-magnetic
effect. According to the modern concept the magnetic effect materials are due
to atomic magnetic dipoles in the materials. These dipoles result from tiny
current loops of electron in atomic orbits. Each revolving electrons in atomic
orbit is equivalent to a tiny current loop which is capable of producing a
magnetic field and thus atom has a magnetic dipole moment. Each electron in an
atom has an orbital magnetic moment due to its orbital motion and spin magnetic
moment due to its spin motion. The magnetic moments of the atoms by themselves
are distributed in space in a statistically random way so that the net moments
of the moment is zero. But in some cases the resultant magnetic moment of an
atom is not zero. Such atoms has a magnetic dipole with a permanent magnetic
moment. Hence magnetic properties of a magnetic material depends whether it has
permanent dipole moment.
Q.2. Describe the magnetisation in magnetic
material.
Ans. Magnetisation:-
When a magnetic material is
placed in a magnetic field, it gets magnetised and the tiny current loops in
the medium aligned parallel to the field. Thus there is a net magnetic moment
in the direction of the field in any small volume of the material medium. The
magnetic moment of the unit volume of the medium is defined as the
magnetisation vector I or the intensity of magnetisation or simply
magnetisation. Therefore,
The M.K.S. unit of magnetisation, I is ampre/metre
(or Am-1).
Q.3. Briefly describe the para, dia and
ferromagnetic materials.
Ans. Classification and distinction of
materials into para-, dia- and ferromagnetic materials:-
It is
commonly observed that when a magnetic material is brought near a pole of a bar
magnet, it is either attracted weakly or strongly towards it or repelled by it.
On the basis of this behaviour, magnetic materials, in the broad sense, are
classified into three categories, viz. paramagnetie, diamagnetic and
ferromagnetic materials. Further they are classified into two more categories
as, anti-ferromagnetic and ferrimagnetic. This classification is on the basis
of the direction of alignment of their atomic magnetic moments. A magnetic
field in the material exerts a force on the moving electrons and perturbs their
motion, thus the effect of magnetic field on the motion of an electron is to
induce an additional current in the atom.
Diamagnetic Materials:-
In certain
materials, which do not have any permanent magnetic dipole moments, the dipoles
are induced when they are placed in a strong magnetic field. The magnetic field
produced by these induced dipoles is such that it opposes the field that
produce it (Lenz’s law). Hence, diamagnetic materials become feebly magnetised
in a direction opposite to the direction of the field in which they are placed.
The resultant field in such materials is therefore less than the applied field.
Thus the phenomenon of reduction of resultant magnetic field in the material
medium by magnetisation is called diamagnetism and the materials which exhibit
this property are called diamagnetic materials. The relative permeabilities of
diamagnetic materials are slightly less than unity and their magnetic
susceptibilities are negative, of the order of . The materials which possess
diamagnetism are dismuth, zinc, copper, silver, gold, lead, water, mercury,
sodium chloride (NaCl), nitrogen, hydrogen, etc. In fact, the magnetic moments
are induced in all materials whenever they are placed in a magnetic field. Thus
all materials possess the property of diamagnetism. However, in materials which
have permanent dipole moments, magnetic effect due to induced dipole moments is
much smaller than the effect of paramagnetism or ferromagnetism, that is why
the material does not show diamagnetism.
Generally in diamagnetic materials, the outermost
orbit is completely filled, pairing off all the electrons. Hence, there is no
net magnetic moment due to permanent dipoles. The magnetic field in which they
are placed does not align the atomic dipoles but modifies the motion of the
electrons in orbits. The electrons moving in the direction of the applied field
are slowed down while the others are accelerated (Lenz’s law) and thus orbiting
electrons acquire an effective magnetic moment which is opposite to the applied
field. The other characteristics of diamagnetic materials are as follows:
1. When a bar of diamagnetic material is suspended freely
in magnetic poles, the bar turns until its axis becomes perpendicular to the
field direction and the poles developed at the ends of the bar are same as the
nearer magnetic poles figure.
2. When a diamagnetic material is placed in a non-uniform
magnetic field, it tends to move from stronger to the weaker parts of the
field.
3. The susceptibility of a diamagnetic material is
independent of both temperature and magnetising field.
4. It is observed that when a diamagnetic gas is allowed
to ascend in the field it gets spread across the field.
