3. 5 Dielectrics in alternating L6-7 29-31
fields.
Frequency dependence
6 Ferro and piezo L8 32-33
electricity.
3
4. INTRODUCTION
LECTURE-1
Dielectrics represent a class of materials which,
although insulators, exhibit a number of effects
when placed in an electric field.
A good example is their effect on capacitors.
A capacitor has capacitance C0 when the space
between its two conductors is a vacuum, filling
this space with a dielectric increases the
capacitance to a new value Cm. The ratio
Cm/C0=εr is known as the relative permittivity
of the dielectric. 4
5. When the atoms or molecules of a
dielectric are placed in an external electric
field, the nuclei are pushed with the field
resulting in an increased positive charge on
one side while the electron clouds are
pulled against it resulting in an increased
negative charge on the other side.
5
6. This process is known as polarization and a
dielectric material in such a state is said to be
polarized. There are two principal methods by
which a dielectric can be polarized: stretching
and rotation.
Stretching an atom or molecule results in an
induced dipole moment added to every atom or
molecule.
6
7. Polarizability
It can be defined as induced dipole
moment per unit electric field.
i.e. µ= αE
Where α is the proportionality
constant called Polarizability.
7
8. Polarization vector
The dipole moment per unit volume of
the dielectric material is called
polarization vector.
If µ is the average dipole moment per
molecule and N is the number of
molecules per unit volume then the
Polarization vector P=N µ
8
9. Electric flux density (D)
The flux density or electric displacement
D at a point in a material is given by D=єr
є0E.
Where E is the electric field strength, є0 is
the dielectric constant and єr is relative
permitivity of the material.
The 3 vectors D,E and P are related by the
equation D= є0 E+P
9
10. Electric susceptibility(אּe)
The polarization vector can be
written as P= אּє0 אּeE
Where the constant אּe is the
electric susceptibility.
(= אּє -1).
e r
10
11. Dielectric constant (єr ) Lecture- 2
Dielectric constant (є) is the ratio
r
between the permitivity of the
medium and the permitivity of free
space.
i.e є = є/ є .
r 0
Є has no units.
r
1 11
12. Electric Polarization Lecture- 3
If a material contains polar molecules, they
will generally be in random orientations
when no electric field is applied.
An applied electric field will polarize the
material by orienting the dipole moments
of polar molecules.
This decreases the effective electric field
between the plates and will increase the
capacitance of the parallel plate structure.
12
13. The process of producing electric
dipoles which are oriented along
the field direction is called
Polarization in dielectrics.
P=NαE.
13
15. Electronic polarization
Electronic polarization represents the
distortion of the electron distribution or
motion about the nuclei in an electric field.
The positive charge in the nucleus and the
center of the negative charges from the
electron "cloud" will thus experience forces
in different direction and will become
separated. We have the idealized situation
shown in the image below.
15
17. Electronic polarization
The separation distance d will have
a finite value because the
separating force of the external
field is exactly balanced by the
attractive force between the centers
of charge at the distance d.
17
18. Ionic Polarization
In the absence of electric field,
The polarization of a given volume,
however, is exactly zero because for every
dipole moment there is a neighboring one
with exactly the same magnitude, but
opposite sign.
18
20. When field is applied
In an electric field, the ions feel forces in
opposite directions. For a field acting as shown,
the lattice distorts a little bit
The Na+ ions moved a bit to the right, the Cl–
ions to the left.
The dipole moments between adjacent NaCl -
pairs in field direction are now different and
there is a net dipole moment in a finite volume
now.
20
21. The Na+ ions moved a bit to the
right, the Cl– ions to the left
21
23. Orientational polarization.
The polarization arising due to the
allignment of already existing but
randomly oriented dipoles in the polar
substance is called the Orientational or
dipolar polarization.
It is denoted by α .
o
23
24. It depends on temperature T
It decreases with T.
α (T)=µ 2/3K T.
o m B
24
26. Electric dipoles in Electric field
MOLECULAR DIPOLES ORIENTED IN FIELD DIRECTION.
E
ELECTRIC FIELD IS APPLIED
26
27. Internal fields in solids. Lecture- 4
The total electric field at the site of the
atom within the dielectric is called the local
field or the internal field.
It is also called the Lorentz field.
We have P=NαE .
i
E =[ є (є -1)E]/N α.
i 0 r
E =E+(ГP/ є ).
i 0
27
28. Claussius-Mosotti relation Lecture-5
It gives the relation between the microscopic
polarizability and the macroscopic dielectric
constant.
Clasius Mossotti equation is given by (єr
-1)/( єr +2)= N α/3 є0 .
28
29. Dielectrics in alternating fields
Lecture-6
According to Maxwell’s theory of wave
propagation V=√1/ єµ.
C= √1/ є µ
0 0.
Hence C/V=n=√ єr µr.
If the materials are non magnetic, µr=1
29
30. n=√ єr( or) єr =n2.
Then the Clasius Mossotti relation
becomes (n2-1)/( n2 +2)= N α/3 є0 .
This is known as Lorentz-Lorentz relation.
30
31. Lecture-7
In case of the alternating fields, we write E=E
(t) and P=P (t) to indicate that both E and P
vary with time t.
There will be some time lag between the
response P (t) and the cause E (t).
If the applied field E (t) is oscillatory, then P (t)
is also oscillatory.
If E (t) is given by E (t)=E0coswt, then P
(t)=P0cos(wt+δ).
31
32. The ferroelectricity Lecture- 8
Some dielectrics become spontaneously
polarized when their temperature is equal to
critical temperature.
This phenomena is called the ferroelectricity.It is
not because of it is possessed by the ferrous
materials but because its origin and
characteristics are same as those of ferro
magnetism.
The critical temperature of the polar dielectrics
is called the ferroelectric curie temperature. 32
33. PIEZOELECTRICITY
When crystals are subjected to electric field, their
geometrical dimensions are altered. This
phenomenon is called electrostriction.
If crystals are subjected to mechanical stress,
electrical charges will be induced on the surfaces
of the crystals. This phenomenon is called
piezoelectricity.
When an electric stress (voltage) is applied, the
material becomes strained. This phenomenon is
known as inverse piezoelectric effect. 33
34. UNIT INDEX
UNIT-I
S.No. Module Lecture PPT Slide No.
No.
1 Introduction L9 4-9
2 Magnetic permeability, L10 10-11
Magnetization
3 Origin of magnetic L11 12-14
moment.
4 Classification of L12 15-22
magnetic materials. 34
35. 5 Hysteresis curve, L13 23
6 Soft & Hard Magnetic L14 24
Materials
35
36. Lecture-9
Introduction
Magnetic materials play a prominent role in
modern technology.
They are widely used in industrial electronics
and computer industry.
The traditional methods of information storage
and retrieval are rapidly replaced by magnetic
storage.
36
37. The magnetism of materials is mainly a
consequence of interactions of uncompensated
magnetic moments of constituent atoms and
molecules.
Basing on the response of materials in external
magnetic field, and on the alignment of
magnetic moments in the materials, they are
classified into five types.
37
38. Magnetic Polestrength
Magnetic poles always occurs in pairs.
Magnetic Polestrength (m) : It is scalar
quantity
.It is independent of the shape of the
magnet.
.It depends on the state of
magnetisation.
SI unit is – Am.
38
39. Magnetic field strength(B)
Magnetic field : The space around a
magnet where its influence is felt is called
magnetic field.
Magnetic induction field strength (B):
Magnetic induction at a point is the force
experienced by a unit north pole at that
point.
B is a vector.
39
40. Intensity of magnetic field (H)
It is defined as the field that induces
magnetism in a magnetic material.
H is measured in Ampere/metre
When a medium is exposed to magnetic
field of intensity H it causes an induction B
in the medium.
40
41. Magnetic flux(Φ).
Magnetic flux(Φ): It is the total
number of lines of induction passing
normal to the cross section.
S I unit : weber.
Magnetic flux (Φ)
µo.m
: Φ is a scalar.
41
42. Magnetic permeability. Lecture-10
Magnetic permeability: It is defined as
the ability of a medium to allow the
magnetic lines of force to pass through
it.
B = μo (H+M) = μo (H + χ m H)
B =μo μr H.
Where μ =1+χm. Which is called
r
relative permeability. 42
43. Intensity of magnetisation.
Intensity of magnetization : It is the
magnetic moment per unit volume or
pole strength per unit area.
I=M/V = (2l.m)/(2l.a)
a= area of crossection.
It is measured in ampere/metre.
43
44. Magnetic moment Lecture-11
It is a product of Magnetic length and
pole strength of a magnet .
Magnetic moment M=2l.m
S.I unit of Magnetic moment is
=Am2.
(or) N-m3/wb.
44
45. Magnetic susceptibility.
Magnetic susceptibility is defined as
the ratio of intensity of magnetization
(I) to intensity of magnetizing field.
Magnetic susceptibility(χ):
χ = I/H.
χ has no units.
45
46. Relative permeability.
Relative permeability of material is
expressed as the ratio of permeability
of the material to the permeability of
free space.
Thus μr =μ/μo.
(or)
μ=μrμo.
46
47. Magnetic materials
Lecture-12
These are the substances, which upon
which being introduced into the external
magnetic field, change so that they
themselves become sources of an
additional magnetic field.
And they are classified into 5 groups.
1Diamagnetic. 4.Antiferromagnetic
2.Paramagnetic. 5.Ferrimagnetic
47
48. Diamagnetic materials
The materials which when placed in magnetic
field acquire feeble magnetism in the direction
opposite to that of field are known as
Diamagnetic substances.
Diamagnetic materials exhibit negative magnetic
susceptibility.
The magnetization in diamagnetic materials is
directed in opposite direction of the field
applied.
48
49. The relative permeability of a diamagnetic substance is
slightly less than unity.
μr< 1; which implies that substances are repelled by a
magnetic field.
The magnetic susceptibility of diamagnetic materials is
practically independent of temperature.
Examples: Hydrogen, air, water, gold silver.
49
50. Paramagnetic materials
These are the substances which when
placed in magnetic field acquire feeble
magnetism in the direction of magnetic
field.
Examples: copper chloride, chromium,
platinum.
The magnetic susceptibility of
paramagnetic substances is positive as the
magnetization coincides the magnetic field.
50
51. Ferromagnetic materials
Large magnetization occurs in thedirection of
the field.
The relative permeability is very high (several
thousands).
When placed in magnetic field, it attracts the
magnetic lines of force very strongly.
Permanent and electromagnets are made using
ferromagnetic materials.
Examples:ZnFe2O4, CuFe2O4, Zn-CuFeO4 &
51
52. Antiferromagnetic materials
They show very little external
magnetism.
Magnetic susceptebility is
positive and small.
The magnetic dipole moments
of adjacent atoms are
antiparallel. 52
53. Due to antiparallel magnetic dipole
moments, the magnetic effect of antiferro
magnetic material is zero, but possess
magnetism due to temperature dependent
disruption of the magnetic moment
alignment.
The susceptibility increases with
temperature upto TN (Neil temperature).
Above Neil temperature, susceptibility
decreases with increasing temperature. 53
54. Ferrimagnetic materials
Magnetic dipole moments of adjacent
moloecules or atoms are antiparallel and
unequal in magnitude. It results in a net
magnetisation in the material.
Magnetic susceptibility is large and positive.
Above Curie temperature, thermal dnergy
randimizes the individual magnetic moments
and the material becomes paramagnetic.
Examples: copper, zinc, cadmium, iron, cobalt,
nickel, etc. 54
55. Hysteresis Lecture-13
When a magnetic field is applied on a
ferromagnetic material then magnetization
takes place. This magnetizatio9n always lags
behind the applied magnetic field. This
phenomenon is known as hysteresis of a
ferromagnetic material.
55
56. Magnetic materials are classified into
soft materials and hard materials.
Lecture-14
Soft magnetic Hard magnetic
materials are easily materials retain
magnetised and magnetism on a
permanent basis, and
demagnetised, and
are used in producing
therefore used in ac permanent magnets .
applications. These materials play
an important role in
information storage
devices. 56
