EP

1. Course: B.Tech
Subject: Engineering Physics
Unit: I
Chapter: 2
1

2. Dielectrics are the materials having electric dipole moment
permanently.
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  coulmb.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

4. The relative permittivity(εr) is often known as
dielectric const. of medium it can given by,
εr=ε/ε0
Dielectric constant is ratio of permittivity of medium
to permittivity of free space.
The value of capacitance of capacitor is given by,
C0=εrε0A/d
By this eqn we can say that high εr increases
capacity of capacitor.
Dielectric const. of medium

5. Polar and Nonpolarized Molecules
Non-polar Molecules : The Dielectric material in which
there is no permanent dipole existence in absence of an
external field is …..
O=O N N Cl-Cl F-F Br-Br I-I
2 – Compounds made of molecules which are symmetrically shaped
carbon tetra fluoride CF4
propane
C3H8
methane CH4
carbon tetra fluoride CCl4,
carbon dioxide
O=C=O

6. Polar Molecules The Dielectric material in which there is
permanent dipole existence even in absence of an
external field is …..
HCl
hydrogen chloride
carbon monoxide
C O
2 – molecules with O, N, or OH at one end – asymmetrical
e.g.; CH2Cl2,CH3Cl
water
H2O
unbounded electron pairs
bend the molecule
ammonia
nitrogen trihydride
NH3
alcohols
methanol
CH3OH

7. Learning Check
Identify each of the following molecules as
1) polar or 2) nonpolarized. Explain.
A. PBr3
B. HBr
C. Br2
D. SiBr4
7

8. Solution
Identify each of the following molecules as
1) polar or 2) nonpolarized. Explain.
A. PBr3 1) pyramidal; dipoles don’t cancel; polar
B. HBr 1) linear; one polar bond (dipole); polar
C. Br2 2) linear; nonpolarized bond; nonpolarized
D. SiBr4 2) tetrahedral; dipoles cancel; no polar
8

9. Polarization of Dielectrics
As shown in fig. when an electric field is
applied to dielectric material their
negative & positive charges tend to align
in equilibrium position.
2

10.  They produce electric dipole inside the material.
 This phenomenon is known as Polarization.
 It can be represented by,
P=polarization
μ= dipole moment
V=Volume
 Unit=Cm-2
 Now dipole moment depends upon applied electric
field.
 α polarizability of material.
P
V


E
P E
P E
 

 




11. Gauss’s Law In Dielectrics
++
++
++
++
--
--
--
--
E
--
--
--
--
--
--
--
--
--
--
--
--
++ --
++ --
++ --
++ --
++ --
++ --
++ --
++ --
++ --
+q
-
q
-
q
+
q
-
q
+
q
++
++
++
++
++
++
++
++
++
++
++
E

12.  In absence of
dielectric
 In presence of
dielectric
0
0
0
0
0
.E ds q
q
E A
q
E
A






0
0 0
0 0
. '
'
'
E ds q q
q q
EA
q q
E
A A
 
 
 

 
 

13.  V=Ed
 So
 Now
0 0
d
E V
k
E V
 
0
0
0 0
0 0 0
'
'
,
1
, ' (1 )
E q
E
k kA
q q
E
A A
q q q
So
kA A A
then q q
k
 
 
 
 

 
  

14. 0
0
So, . '
1
(1 )
q
.
E ds q q
q q
k
k
k E ds q
 
  






This relation true is for parallel plate capacitor
Which is Gauss’s law for dielectrics

15. Three Electric vectors
 The resultant dielectric field is given by,
Where,
E=Electric field
D=Flux Density or
Displacement vector
P=Polarization
0 0
0 0
0
0
'
'
,
,
, D
p
q q
E
A A
q
now P
A
q P
E
A
q
E P
A
q
now D
A
So E P
 
 
  
  

 
 

 



16. 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
0
0
0
0
( 1)
1
e
e
r
e r
P E
P
E
E
E




 
 


 

 

17. Relation between εr & ‫א‬
 Displacement vector,
0
0
0
r 0 0
0
D E P
Now,P=
( - ) E P
(or) ( . - ) E P
( 1) . P
Where,( 1)
r
r
E
E
 
 

 
 


 
  
 
 

18. Types of
polarization
1. Electron polarization
2. Ionic polarization
3. Orientation polarization
4. Space charge
polarization

19. Electronic polarization
 When no external field is applied nucleus of
atom is like in fig. (a)
 When external field is applied, displacement
in opposite direction is observed between
nucleus & electrons due to this dipole
moment is induced.
 This type of polarization is called Electronic
polarization.
 Ex. Germanium, Silicon, Diamond etc…
19
+
-
+
-
-
Electric Field
(b)

20. Ionic Polarization
 Some materials like ionic crystal does not
possess permanent dipole moment.
 Fig. (a) shows natural arrangement of ionic
crystal. When Ele. Field is applied on this type
of material displacement of ions is observed.
 Due to an external electric field a positive &
negative ion displaces in the direction opposite
to each other due to which distance between
them is reduced & ionic polarization is
generated.
 Ionic polarization is observed in materials like
NaCl, KBr, KCl etc…

21.  Let us consider simple example of NaCl
crystal.
 As shown in fig. when crystal is placed in an
external electric field Na+ ion displaces in
one direction & Cl- ion goes in opposite
direction.

22. Orientation polarization
 Some molecules like H2O, HCl having permanent dipole
moment p0.
 In the absence of a field, individual dipoles are arranged in
random way, so net average dipole moment in a unit volume
is zero as shown in fig. (b).
 A dipole such as HCl placed in a field experiences a torque
that tries to rotate it to align p0 with the field E.
4

23.  In the presence of an applied field, the
dipoles try to rotate to align parallel to each
other in direction of electric field fig (d).
 This type of polarization is Orientation
polarization.
 This type of polarization occurs only in polar
substances like H2O, CH3Cl when they are
placed in external field.

24. Space charge polarization(Interfacial polarization)
 A crystal with equal number of mobile positive ions
and fixed negative ions.
 In the absence of a field, there is no net separation
between all the positive charges and all the
negative charges.
5

25.  In the presence of an applied field, the
mobile positive ions migrate toward the
negative charges and positive charges in the
dielectric.
 The dielectric therefore exhibits Space charge
or interfacial polarization.

26. Energy stored in dielectric field
.
?
.
.
dW F dr
F
dW qE dr
dW E dp
p p
P
lA V




 
0
0
0
2
0
2
0
( 1) .
. .( 1) .
. .( 1) .
1
( 1)E
2
1
( 1)E
2
?
r
r
r
r
r
p PV
dW EVdP
P E
dW E V dE
dW E V dE
W V
W
V
U


 
 
 
 
 

 
 
 
 
 
 

27. References:
Engineering physics By Dr. M N Avadhnulu, S
Chand publication
Engineering physics by K Rajgopalan
http://web.mit.edu/viz/EM/visualizations/coursenot
es/modules/guide05.pdf

28. Image references links
1. http://www.physics.sjsu.edu/becker/physics51/capac
itors.htm
2. https://encrypted-
tbn0.gstatic.com/images?q=tbn:ANd9GcSJZq2cY-
80x2c9Jzlqs-
ebnfPHmznS_SZXueW9fAQFTf9xUzAg4A
3. http://image.slidesharecdn.com/dielectrics-
140801000936-phpapp02/95/dielectrics-24-
638.jpg?cb=1406869836
4. http://www.pixentral.com/show.php?picture=1SH99q
e2FTmMv6M4lmJrV2iNZ8tR
5. http://www.pixentral.com/pics/1LMvZyZGCYX3JZkg
Q2rEQMX876K89v1.png