DIELECTRIC POLARIZATION, DIELECTRIC LOSS, MAGNETIC MATERIALS DIA, PARA FERROMAGNETIC CURIE TEMP, BH CURVE SOFT HARD MAGNETS

1. BY PRASHANT KUMAR
ASST. PROFESSOR
MITS GWALIOR

2.  DIELECTRIC MATERIALS:
 What are dielectrics ?
 A dielectric (or dielectric material) is an electrical insulator that can
be polarized by an applied electric field.
 When a dielectric is placed in an electric field, electric charges do not flow
through the material as they do in an electrical conductor , but only slightly shift
from their average equilibrium positions causing dielectric polarization.
 Because of dielectric polarization, positive charges are displaced toward the field
and negative charges shift in the opposite direction. This creates an internal
electric field that reduces the overall field within the dielectric itself

3.  Basically, there are four mechanisms of polarization (1.Electronic or Atomic
Polarization 2. Ionic Polarization 3. Dipolar or Orientation Polarization 4. Space
charge polarization
Polarization is separation of opposite charges in external electric field)
 Electronic or Atomic Polarization
 This involves the separation of the centre of the electron cloud around an atom
with respect to the centre of its nucleus under the application of electric field

4.  Ionic Polarization
 This happens in solids with ionic bonding which automatically have dipoles but
which get cancelled due to symmetry of the crystals. Here, external field leads to
small displacement of ions from their equilibrium positions and hence inducing a
net dipole moment .

5.  Dipolar or Orientation Polarization
 This is primarily due to orientation of molecular dipoles in the direction of applied
field which would otherwise be randomly distributed due to thermal
randomization.

6.  Interface or Space Charge Polarization
 This involves limited movement of charges resulting in alignment of charge
dipoles under applied field. This usually happens at the grain boundaries or any
other interface such as electrode-material interface. (eg multiphase material)
 Ions diffuse over considerable distance and redistribution occurs in presence of
external field.

11. ORIGIN OF
MAGNETIS
MOMENT
ANGULAR
MOMENTUM OF
ELECTRON (L)
ELECTRON
SPIN
NUCLEAR SPIN
( very week effect)
ANGULA
R
DIPOLE
MOMEN
T
SPIN MAGNETIC
MOMENT S
e
S
m



2
L
e
L
m




12.  The spin of the electron produces a magnetic field with a direction dependent on
the quantum number S .

14. Comparison of Dia, Para and Ferro Magnetic materials:
DIA PARA FERRO
1. Diamagnetic
substances are those
substances which are
feebly repelled by a
magnet.
Eg. Antimony, Bismuth,
Copper, Gold, Silver,
Quartz, Mercury, Alcohol,
water, Hydrogen, Air,
Argon, etc.
Paramagnetic substances
are those substances
which are feebly attracted
by a magnet.
Eg. Aluminium, Chromium,
Alkali and Alkaline earth
metals, Platinum, Oxygen,
etc.
Ferromagnetic substances
are those substances
which are strongly
attracted by a magnet.
Eg. Iron, Cobalt, Nickel,
Gadolinium, Dysprosium,
etc.
2. When placed in magnetic
field, the lines of force tend
to avoid the substance.
The lines of force prefer to
pass through the substance
rather than air.
The lines of force tend to
crowd into the specimen.
N S
S N S N

15. 2. When placed in non-
uniform magnetic field, it
moves from stronger to
weaker field (feeble
repulsion).
When placed in non-
uniform magnetic field, it
moves from weaker to
stronger field (feeble
attraction).
When placed in non-
uniform magnetic field, it
moves from weaker to
stronger field (strong
attraction).
3. When a diamagnetic
rod is freely suspended in
a uniform magnetic field, it
aligns itself in a direction
perpendicular to the field.
When a paramagnetic rod
is freely suspended in a
uniform magnetic field, it
aligns itself in a direction
parallel to the field.
When a paramagnetic rod
is freely suspended in a
uniform magnetic field, it
aligns itself in a direction
parallel to the field very
quickly.
SN SN SN

16. 4. If diamagnetic liquid
taken in a watch glass is
placed in uniform
magnetic field, it collects
away from the centre
when the magnetic poles
are closer and collects at
the centre when the
magnetic poles are
farther.
If paramagnetic liquid
taken in a watch glass is
placed in uniform
magnetic field, it collects
at the centre when the
magnetic poles are closer
and collects away from
the centre when the
magnetic poles are
farther.
If ferromagnetic liquid
taken in a watch glass is
placed in uniform
magnetic field, it collects
at the centre when the
magnetic poles are closer
and collects away from
the centre when the
magnetic poles are
farther.

17. 5. When a diamagnetic
substance is placed in a
magnetic field, it is
weakly magnetised in the
direction opposite to the
inducing field.
When a paramagnetic
substance is placed in a
magnetic field, it is
weakly magnetised in the
direction of the inducing
field.
When a ferromagnetic
substance is placed in a
magnetic field, it is
strongly magnetised in
the direction of the
inducing field.
6. Induced Dipole
Moment (M) is a small
– ve value.
Induced Dipole Moment
(M) is a small + ve value.
Induced Dipole Moment
(M) is a large + ve value.
8. Magnetic permeability
μ is always less than
unity.
Magnetic permeability μ
is more than unity.
Magnetic permeability μ
is large i.e. much more
than unity.
7. Intensity of
Magnetisation (I) has a
small – ve value.
Intensity of Magnetisation
(I) has a small + ve value.
Intensity of Magnetisation
(I) has a large + ve value.

18. 9. Magnetic susceptibility
cm has a small – ve value.
Magnetic susceptibility cm
has a small + ve value.
Magnetic susceptibility cm
has a large + ve value.
10. They do not obey
Curie’s Law. i.e. their
properties do not change
with temperature.
They obey Curie’s Law.
They lose their magnetic
properties with rise in
temperature.
They obey Curie’s Law. At
a certain temperature
called Curie Point, they
lose ferromagnetic
properties and behave
like paramagnetic
substances.(Follows
modified Curie’s Weiss
Law)Curie’s Law:
Magnetic susceptibility of a material varies inversely
with the absolute temperature.
I α H / T or I / H α 1 / T
Xm α 1 / T
Xm = C / T (where C is Curie constant)
Curie temperature for iron is 1000 K, for cobalt 1400 K
and for nickel 600 K.
I
H / T

19. As temperature
increases above the curie
temp (Tc) due to higher
thermal energy many
domains align randomly
to diminish the net
dipole moment resulting
in change in magnetic
behaviour from
ferromagnetic to
paramagnetic behaviour

21.  FERROMAGNETIC MATERIAL IS MAGNETISED USING TWO MECHANISMS
DOMAIN
GROWTH/DOMAIN
WALL
MOMENT==domain that
are parallel or nearly
parallel to external m.
field will Grow in size at
the cost of other domain
ROTATION OF DOMAIN
MAGNETIC MOMENT==
magnetic moment of domain
can rotate in direction of
applied field

22. Hysteresis means “remaining” in Greek, an effect remains after its cause has
disappeared. Hysteresis, a term coined by Sir James Alfred Ewing in 1881, a
Scottish physicist and engineer (1855-1935), defined it as: When there are two
physical quantities M and N such that cyclic variations of N cause cyclic
variations of M, then if the changes of M lag behind those of N, we may say that
there is hysteresis in the relation of M to N".

23.  STEPS
 An initially unmagnetized material is subjected to a cycle of magnetization. The
values of Magnetic flux density B and the magnetizing field H are calculated at every
stage and a closed loop is obtained on plotting a graph between M and H as shown in
the figure.
 The point ‘O’ represents the initial unmagnetized condition of the material. As the
applied field is increased, the magnetization increases to the saturation point ‘A’ along
‘OA’.
 As the applied field is reduced, the loop follows the path ‘AB’. ‘OB’ represents the
magnetic flux density remaining in the material when the applied field is reduced to
zero. This is called the residual magnetism or remanence. The property of retaining
some magnetism on removing the magnetic field is called retentivity.
 OC represents the magnetizing field to be applied in the opposite direction to remove
residual magnetism. This is called coercive field and the property is called coercivity.
 When the field is further increased in the reverse direction the material reaches
negative saturation point ‘D’.
 When the field is increased in positive direction, the curve follows path ‘DEF’.

26.  Retentivity - It is a material's ability to retain a certain amount of residual magnetic
field when the magnetizing force is removed after achieving saturation. (The value
of B at point c on the hysteresis curve.)
 Residual Magnetism - the magnetic flux density that remains in a material when the
magnetizing force is zero. Note that residual magnetism and retentivity are the same
when the material has been magnetized to the saturation point. However, the level of
residual magnetism may be lower than the retentivity value when the magnetizing
force did not reach the saturation level.
 Coercive Force - The amount of reverse magnetic field which must be applied to a
magnetic material to make the magnetic flux density return to zero. (The value of H at
point d on the hysteresis curve.)
 Coercivity: the resistance of a magnetic material to changes in magnetization,
equivalent to the field intensity necessary to demagnetize the fully magnetized
material.
 Permeability - A property of a material that describes the ease with which a magnetic
flux density is established in the component.

27. PARAMETERS HARD MAGNET SOFT MAGNET
HYSTRESIS LOSS HIGH LOW
Coercivity HIGH LOW
Retentivity HIGH LOW
Ease of magnetization and
demagnetization
Very difficult easier
Magnetic Permeablity Very small Large
Magnetic Susceptiblity Very small Large
Application Permanent magnet Electro magnet