Q.8 Briefly explain the uses of hysteresis
curve.
Ans. Uses of Hysteresis Curve: -
The hysteresis curves of different ferromagnetic materials
provide information about retentivity, coercivity, susceptibility, permeability
and
energy
loss per cycle. The magnetic materials are used now a days in electronic
products, automobiles, communication equipments and data processing devices.
Based upon the area of the hysteresis loop, the magnetic materials are
classified in two categories:
1. Soft
magnetic materials and
2. Hard
magnetic materials.
The hysteresis curve for these
materials is shown in Fig. 1.
Soft
Magnetic Materials: -
Soft magnetic materials have high
permeability and low coercive force. They are easily magnetised and
demagnetised. Examples of soft magnetic materials are silica-steel (Fe-97 and
Si-3), Sandust (Al-5, Si-10, Fe-85), magnetic mild steel (Fe, c < 0.1%),
permalloy (Ni-45, Fe-55), etc.
The soft magnetic
materials have the following characteristics:
1. Thin
hysteresis loop. 2. High permeability.
3. Low
coercivity. 4. High susceptibility.
5. Low
hysteresis loss. 6. Low eddy current loss.
Applications
: They are selected for
applications on the basis of B-H loop or I-H loop characteristics.
Some of the applications are
described as under:
1. Pure
iron is frequently used as the magnetic core for d.c. current applications.
2. Permalloy
is used for making recording head and magnetic shields.
3. Silicon
steel is used as transformer cores at low frequencies.
4. The
magnetic mild steel is used for relays, reed switches and pole-pieces of
electromagnets.
5. Sendust
is used as recording head material.
6. Iron-nickel
alloy is used for audio-frequency applications.
Hard
Magnetic Materials: -
Hard magnetic materials have the following characteristics:
1. High
permeability.
2. Large
hysteresis loop.
3. High
coercivity.
4. High
hysteresis loss.
5. High
residual magnetism.
Hard magnetic material may be
achieved by heating ferromagnetic materials to a high temperature and then
suddenly quenching them. Examples of some hard magnetic materials are: Carbon
steel, alloy steel, tungusten-steel alloy, platinum-cobalt alloy and invar.
They are used as permanent magnetic materials.
Applications:
-
1. Hard
magnetic materials are used as permanent magnets in instrument like
galvanometers, ammeter, voltmeters, flux-meters, wattmeters, etc. They also
constitute the part of generators and motors.
2. They
are used in devices like telephones, tape-recorders, and loudspeakers.
3. They
are used in mechanical appliances like tool holders, conveyors, mineral
separators, screw drivers, mixters and magnetic bearings, etc.
4. They
are used in toys, stickers and small motors, etc.
Q.9 The dielectric constant of helium at 0oC and 1
atmospheric pressure is 1.000074. Find the dipole moment induced in helium atom
when the gas is in an electric field of intensity 100 volt/m. Number of atoms
per unit volume of helium gas are 2.68 ´ 1027.(AKTU. 2008-09)
Ans. Given, er =
1.000074
E = 100 Volt/m
N = 2.68 ´ 1027 atom/unit volume
Dipole
moment,
= 2.44 ´ 10-41 Fm volt.
Q.12. Show that susceptibility of diamagnetic
material is negative and independent of temperature. (AKTU. 2015 - 16)
Ans. The
susceptibility of a diamagnetic material is negative.
Magnetic susceptibility is
given by 
The magnetization M in the
diamagnetic material is directed opposite to the direction of the applied
magnetic field, H and hence susceptibility is negative.
Hence M = - M0
is negative and small (of the order of -10-8)]
Dimagnetic
susceptibilityis independent of
temperature and external magnetic field intensity. This is because the
electronic structure of the atoms of the material is indenpendent of
temperature.
is independent of
temperature and external magnetic field intensity. This is because the
electronic structure of the atoms of the material is indenpendent of
temperature. 