engineering

2. TOPIC :- ELECTRICAL POLARIZATION MECHANISMS
MADE BY :-
MALAY FALDU(150090119005)
NAVED FRUITWALA(150990119006)
UTKARSH GANDHI(150990119007)
DIGVIJAYSINH GOHIL(150990119008)
GUIDED BY :-
SANJAY SIR
SUBJECT :- PHYSICS

3. ELECTRICAL POLARISATION
MECHANISM
 There are four different types of mechanisms through
which electrical polarization can occur in dielectric
materials when it is subjected to an external electric field.
1. Electronic Polarization
2. Ionic Polarization
3. Orientation Polarization
4. Space charge or Interfacial Polarization

4. Electronic Polarization
When an EF is applied to an atom, +ve ly charged
nucleus displaces in the direction of field and ẽ could
in opposite direction. This kind of displacement will
produce an electric dipole with in the atom.
i.e, dipole moment is proportional to the magnitude
of field strength and is given by,
μe = αeE
where ‘αe’ is called electronic Polarizability constant.

5. Expression for Electronic
Polarization
 Consider a atom in an EF of intensity ‘E’ since the
nucleus (+Ze) and electron cloud (-Ze) of the atom
have opposite charges and acted upon by Lorentz
force (FL).
 Subsequently nucleus moves in the direction of field
and electron cloud in opposite direction.
 When electron cloud and nucleus get shifted from their
normal positions, an attractive force b/w them is
created and the separation continuous until columbic
force FC is balanced with Lorentz force FL, Finally a
new equilibriums state is established.

6.  Fig(2) represents displacement of nucleus
and electron cloud and we assume that the
-ve charge in the cloud uniformly distributed
over a sphere of radius R and the spherical
shape does not change for convenience.

7. sphere.in thechargetotaltherepresentsZe-
3
4 3
R
Ze




 
(1)-----
..
.
.
3
4
.q
isx''radiusofspherein thechargeve-theThus
3
3
3
3
4
3
3
4
3
e
x
R
ze
x
R
ze
x








  (2)-----
4
.
4
1.
.
4
1
F
Now
3
0
22
3
3
2
0
2
0
c
R
xez
ze
R
xze
xx
qq pe







 

Let σ be the charge density of the sphere

8. • Force experienced by displaced nucleus in EF of Strength E is
FL = Eq = ZeE -----(3)
 Hence electronic Polaris ability is directly proportional
to cube of the radius of the atom.
ee
cL
zex
R
zex
E
R
zex
R
xez
FF



momentdipole
E
4
4
(4)-----ZeE
4
3
0
3
0
3
0
22










3
04 Re  

9. Ionic polarization
 The dielectric material having ionic bonds such as Nacl, show
ionic polarization. Polarization in such ionic crystals arises on
account of the ions displaced from their equilibrium positions by
the force of the applied electric field.
 The induced dipole moment due to ionic polarization is
proportional to the applied electric field, i.e.
 For the most of the materials, the ionic polarizability is very less
than the electronic polarizability,
E
E
ii
i





10. When a EF is applied to the molecule, the positive ions
displaced by X1 to the negative side electric field and negative
ions displaced by X2 to the positive side of field.
The displacement between ions causes an increase or
decrease in distance between the atoms, depending on the
location of the ion pairs.
The resultant dipole moment µ = e ( X1 + X2)..
Restoring force constant depend upon the mass of the ion
and natural frequency and is given by,

11. Where ‘M’ mass of anion and ‘m’ is mass of cat ion.
It is independent of temperature.
 
 Mm
ionic
ionic
Mmionic
w
e
E
w
Ee
xx
11
2
0
2
11
2
0
2
21
or
)e(





 Mm
w
eE
xx
wm
eE
x
xwmeEF
11
2
0
21
2
0
2
0
.
or
.




12. Orientation Polarization
Orientation polarization occurs in polar substances. These
substances exhibit dipole moment even in the absence of external
electric field. Due to random orientations of dipoles or molecules, the
net dipole moment is zero.
When such materials are subjected to an external electric field, the
permanent molecular dipoles rotate about their axis of symmetry to
align with the applied field.
In electronic and ionic polarization processes, the force due to the
externally applied field is balanced by elastic binding forces, but no
such forces exist for orientation polarization. At thermal equilibrium
with no external electric field, the permanent dipoles contribute no net

13. With the application of external electric field, dipole
alignment is largely offset by thermal agitation. The
orientation polarization is strongly temperature dependent; it
decreases with increase in the temperature.
Expression for orientation polarization
kT
orie
o
3
2

 

14. Space Charge Polarization
Sometimes due to the application of electric field to the
dielectric material, charges accumulate at the electrodes or at
the interface due to sudden change in conductivity shown in
Fig. (a) and (b). Under the influence of applied electric field, the
ions are diffused over appreciable distance, due to which
redistribution of charges in the dielectric medium takes place.
The tendency of redistribution of charges in the dielectric
medium in the presence of an external electric field is known as
space charge polarization.

15. Total Polarization
Although the dielectric materials are classified in different
groups based in the their mode of polarization, if a material
can experience all forms of polarization, then its total
polarizability can be given as the sum of electronic, ionic, and
orientation polarizability, i.e.
This is called Langevin – Debye equation for Total
Polarizability in dielectrics.
Here the contribution due to space charge polarization is not
considered because it is almost negligible in most common
dielectrics. In the above equations, first two terms on the right hand
side are the functions of molecular structure of the dielectric material
which are usually independent of temperature. Due to this reason,
they are also known as deformation polarisability.
 
kTw
e
R ori
mMooriionicelec
3
4
2
11
2
0
2
3 
 

16. THANK YOU …