Dielectrics

1. Dielectrics
Unit – 3

2. Dielectrics are the materials having electric dipole moment permantly.
Dipole: A dipole is an entity in which equal positive and negative
charges are separated by a small distance..
DIPOLE moment (µele ):The product of magnitude of either of the
charges and separation distance b/w them is called Dipole moment.
µe = q . x  coul – m
All dielectrics are electrical insulators and they are mainly used to store
electrical energy.
Ex: Mica, glass, plastic, water & polar molecules…
X
q -q
Introduction

3. Dielectric Constant
Dielectric Constant is the ratio between the permittivity of
the medium to the permittivity of free space.
its value changes widely from material to material
For vacuum =1
For all other dielectric it is Ɛr >1.
So, we can write Ɛr=1+χ , χ is susceptibility
The characteristics of a dielectric material are determined
by the dielectric constant and it has no units.
0


 
r

4. Electric field
 The region surrounded by charged body is always under
stress because of electrostatic charge . If a small charge q or
a charged body is placed in this region ,then the charge q or
a charged body will experienced a force according Coulomb
s law . This stressed region around charged body is called
electric field .
 Electric field at a point is defined as the force that acts on a
unit positive charge placed at that point thus E.
 According to coulomb law when two point charges Q1 and
Q2 are separated by a distance r, the force of attraction or
repulsion between two charges is given by

5. Electric field

6. Electric Polarization
The process of producing electric dipoles by an electric field is
called polarization in dielectrics.
Polarizability:
The induced dipole moment per unit electric field is called
Polarizability.
The induced dipole moment is proportional to the intensity of
the electric field.
constant
lity
polarizabi







E
E

7. Electric flux ɸ
 (1) Number of lines of force that pass through a surface placed
in the vector field .
 (2) As the product of surface area and the components of the
electric field normal to the surface
 a unit charge is supposed to emanate one flux.
 In the case of an isolated charge q coulomb the flux is
q=ɸ
 Is independent of of nature of medium.

8. Polarization vector
 Definition: induced dipole moment per unit
volume of dielectric medium.
 P is vector quantity and its direction is along
the direction of applied field .If m is the
average induced dipole moment per unit
molecule and N is the number of molecule per
unit volume then polarization is given by
P=Nµ

9. V=At ,where A is the area of slab
Thus ,the polarization is also defined as induced surface charge
per unit area
But q/A is the induced charge density .so magnitude of
polarization is equal to the induced charge density.

10. Electric flux Density (D):
Electric flux density is defined as charge per unit area and it has
same units of dielectric polarization.
Electric flux density D at a point in a free space or air in terms of
Electric field strength is
At the same point in a medium is given by
As the polarization measures the additional flux density arising
from the presence of material as compared to free space
(1)
-
-
E
D 0
0 

(3)
-
-
P
E
D
i.e, 0 

 
(2)
-
-
E
D 


11. P
.
)
1
(
P
E
)
-
.
(
(or)
P
E
)
-
(
P
E
E
0
0
0
r
0
0






E
r 








Using equations 2 & 3 we get

12. Electric susceptibility
The polarization vector P is proportional to the total electric
flux density and direction of electric field.
Therefore the polarization vector can be written as,
In a large number of dielectrics it is Found that
polarization is directly Proportional to the
external applied Field E.
Def. The ratio of polarization to net electric field as
Given by the induced charge on the surface of
Dielectric is called susceptibility. 1
)
1
(
0
0
0
0






r
e
r
e
e
E
E
E
P
E
P










13. Relation between polarization P, susceptibity
χ and dielectric constant Ɛr
 Lets us consider a parallel plate capacitor between which an
electric field Ɛ0.
 If σ is the surface charge density then gauss law.
E=σ/Ɛ0 (1)
if a dielectric slab is placed between the plates of capacitors , then
due to polarization, charges appear on the two faces of the slab
and establish another field E1 within the dielectric . This field
will be in a direction opposite to that of E0
Resultant value E=E0-E1 (2)
If σs is the surface charge density on the slab then from (1)
E1= σs / Ɛ0 (3)
From (1)(2)(3)

15. POLARISATION OF DIELECRICS
 When we are applying external electric field, it causes the
electron cloud to move away. Thus the centroides of the
positive and negative charges now no longer coincides and as a
result of that an electric dipole is induced in the atom. Thus,
atom is said to be polarized.
 Polarization : the process of creating or inducing dipoles in a
dielectric medium by an external field.
 On the basis on that dielectrics are the material that have
either permanent diploes or induced in the presence of
external electric field .
 They are classified into two categories (1) Non polar (2)polar

16. Various polarization processes:
When the specimen is placed inside a d.c.
electric field, polarization is due to four
types of processes….
1.Electronic polarization
2.Ionic polarization
3.Orientation polarization
4.Space charge polarization

17. Electronic Polarization
When an EF is applied to an atom, +vely charged
nucleus displaces in the direction of field and ẽ move 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
E
E
e
e
e
or





where ‘αe’ is called electronic Polarizability constant

18. It increases with increase of volume of the atom.
This kind of polarization is mostly exhibited in Monatomic
gases.
10
____ 2
-40
m
F
e 



He Ne Ar Kr Xe
0.18 0.35 1.46 2.18 3.54
It occurs only at optical frequencies (1015Hz)
It is independent of temperature.

19. Ionic polarization
 The ionic polarization occurs, when atoms form
molecules and it is mainly due to a relative displacement
of the atomic components of the molecule in the
presence of an electric field.
 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 resultant dipole moment µ = q ( X1 + X2)..

20. Electric field
+
+
+
+
+
+
+
+
_
_
_
_
_
_
_
_
1
x 2
x
anion
cat ion

21. Restoring force constant depend upon the mass of the ion
and natural frequency and is given by
 
M
m
w
eE
x
x
w
m
eE
x
x
w
m
eE
F
1
1
2
0
2
1
2
0
2
0
.
or
.








22. Where ‘M’ mass of anion and ‘m’ is mass of cat ion
 
 
M
m
ionic
ionic
M
m
ionic
w
e
E
w
E
e
x
x
1
1
2
0
2
1
1
2
0
2
2
1
or
)
e(











This polarization occurs at frequency 1013 Hz (IR).
It is a slower process compared to electronic polarization.
It is independent of temperature.

23. Orientation Polarization
It is also called dipolar or molecular polarization. The
molecules such as H2 , N2,O2,Cl2 ,CH4,CCl4 etc., does not carry
any dipole because centre of positive charge and centre of
negative charge coincides. On the other hand molecules like
CH3Cl, H2O,HCl, ethyl acetate ( polar molecules) carries
dipoles even in the absence of electric field.
How ever the net dipole moment is negligibly small since all
the molecular dipoles are oriented randomly when there is no
EF. In the presence of the electric field these all dipoles orient
them selves in the direction of field as a result the net dipole
moment becomes enormous.

24.  It occurs at a frequency 106 Hz to 1010Hz.
 It is slow process compare to ionic
polarization.
 It greatly depends on temperature.

25.   kT
w
e
R
kT
E
N
kT
E
N
N
P
ori
m
M
o
ori
ionic
elec
orie
o
o
orie
orie
o
3
4
3
.
.
3
.
.
.
2
1
1
2
0
2
3
2
2
























Expression for orientation polarization
This is called Langevin – Debye equation for total Polaris ability in
dielectrics.

26. FERRO ELECTRICITY
Ferroelectricity is a characteristic of certain materials that have a
spontaneous electric polarization that can be reversed by the application
of an external electric field.
All ferroelectrics are pyroelectric, with the additional property that their
natural electrical polarization is reversible.
Examples of Ferroelectric Materials Include:
Lead titanate, (PbTiO3)
Lead zirconate titanate (PZT)
Lead lanthanum zirconate titanate (PLZT)

31. PIEZO ELECTRICITY
Piezoelectric Effect is the ability of certain materials to generate an
electric charge in response to applied mechanical stress.
When piezoelectric material is placed under mechanical stress,
a shifting of the positive and negative charge centers in the material
takes place, which then results in an external electrical field.
When reversed, an outer electrical field either stretches or compresses the
piezoelectric material.

36. PYRO ELECTRICITY
Pyroelectricity is the property of a polar crystal to produce electrical energy when it is
subjected to a change of thermal energy.
It is possible also to define the pyroelectric effect as the ability of crystals to generate
electricity when they are dynamically heated or cooled;
a pyroelectric becomes polarized positively or negatively in proportional to a change in
temperature.

37. Examples of Pyroelectric Material
:Cobalt phthalocyanine
Lithium tantalitePolyvinyl fluorides
Gallium nitrideTourmaline

38. MAGNETIC MATERIALS

46. Magnetostriction is a property of ferromagnetic materials which causes them to expand
or contract in response to a magnetic field.
This effect allows magnetostrictive materials to convert electromagnetic energy into
mechanical energy.
As a magnetic field is applied to the material, its molecular dipoles and magnetic field
boundaries rotate to align with the field. This causes the material to strain and elongate
The interior of a ferromagnetic material is divided into magnetic “domains.”
MAGNETO STRICTION

47. The boundaries separating these regions (in bulk materials) are
called Bloch walls.
Bloch walls are made up of a thin layer of molecules in which their
molecular dipoles are gradually rotated to align with their adjacent
domains.

48. MAGNETO RESISTANCE
The resistance of some of the metal and the semiconductor material varies in the
presence of the magnetic field, this effect is called the magnetoresistance.
The element which has these effects is known as the magnetoresistor.
In other words, the magnetoresistor is a type of resistor whose resistance varies
with the magnetic field.
It works on the principle of electrodynamics, which states that the force
acting on the current place in the magnetic field changes their direction.

49. Giant Magnetoresistance (GMR) – In this effect, the resistance of the magneto
Resistor becomes small when their ferromagnetic layers are parallel to each other.
The resistance becomes very high for the antiparallel alignment of the layer
Extraordinary Magnetoresistance –
In this effect, the resistance of the metal is high in the absence of magnetic field and
low in the presence of a field.
Tunnel Magneto Resistor – The current will flow from the one ferromagnetic
electrode through the insulator.
The magnitude of current flows through the tunnel depends on the direction of
magnetization.

50. magnetic bubble. : a tiny movable magnetized cylindrical volume
in a thin magnetic material that along with other like volumes can
be used to represent a bit of information
Garnet is a type of magnetic material is normally used in bubble
memories.
Bubble memory is a type of non-volatile computer memory that
uses a thin film of a magnetic material to hold small magnetized
areas, known as bubbles or domains, each storing one bit of data.
Yttrium iron garnet (YIG) is a ferrite material with excellent
magnetic and magneto-electric properties that make it the best
magnetic material for high frequency applications. Thus, the
magnetic moment of the garnet is a result of the surplus iron ion
from d-site.

51. MAGNETIC FIELD SENSORS
A magnetic sensor usually refers to a sensor that converts the magnitude and variations
of a magnetic field into electric signals.
Magnetic fields, as exemplified by the magnetic field of the earth (earth magnetism) or
magnets are familiar yet invisible phenomena. Magnetic sensors that convert invisible
magnetic fields into electric signals and into visible effects have long been the subject
of research.

54. Materials that manifest both ferroelectricity and
ferromagnetism.
Materials that show both spontaneous electric dipole moment
and magnetic dipole moment.
Could be used in multistate data storage or novel memory
media which allows simultaneou sreading and writing of
data. Unfortunately very few multiferroic materials.
E.g.BiFeO3,Pb(Fe1 2 Nb1 2 )O3,YbMnO3,etc.