SHORT QUESTIONS AND ANSWERS
UNIT I
CONDUCTING MATERIALS
1.
What are conducting materials?
When an electric potential difference is applied across a material and if it
conducts electricity, then that material is called as a conducting material. These materials
generally allow heat energy to flow through it. Metals are the good conductors of heat
and electricity.
2.
What is electron theory of solids?
The electron theory of solids explains the structure and properties of solids
through their electronic structure.
3.
List out the three main theories developed for metals?
(i)
(ii)
(iii)
4.
What are the merits of classical free electron theory?
(i)
(ii)
(iii)
(iv)
5.
Classical Free Electron Theory (Drude and Lorentz free electron theory)
Quantum Free Electron theory (Sommerfeld Quantum theory)
Brillouin Zone theory (Band theory)
It verifies Ohm’s law
It explains the electrical & thermal conductivity of metals
It proves Wiedemann-Franz law
The optical property of metals can be explained using this theory.
Mention any three drawbacks of classical free electron theory.
(i)
The Lorentz number by classical theory does not have good agreement with the
experimental value.
(ii) By classical theory k/σT = constant for all temperatures. But by quantum
theory k/σT  a constant for all temperatures.
(iii) This theory cannot explain the Black Body Radiation, Compton effect, photoelectric effect, paramagnetism, ferromagnetism etc.
6.
Define drift velocity.
The average velocity acquired by the free electron in a particular direction after
a steady state is reached on the application of an electric field is called drift velocity (νd).
7.
Define mobility of electrons.
The drift velocity acquired by the free electrons per unit electric field (E)
applied to it is called mobility of electrons.
μ 
8.
νd
E
Define mean free life time (τc).
The average time taken by a free electron between two successive collisions
is called mean free life time or collision time (τc).
9.
Define Relation time (τ).
Relaxation time is defined as the time taken for the drift velocity of electron
to decay to (1/e) of its initial value.
10. Define mean free path of an electron.
The average distance travelled by an electron between two successive
collisions in the presence of applied field is known as mean free path of an electron (λ).
λ  νd τc
11. Define electrical conductivity.
Electrical conductivity is defined as the current density per unit electric field.
In terms of electron mobility (μ), electrical conductivity, σ = neμ.
also,
σ 
ne 2 τ
m
12. Define thermal conductivity.
Thermal conductivity of a material is defined as the amount of heat flowing
per unit time through the material having unit area of cross section and maintaining at
unit temperature gradient.
k 
θ
dT/d x 
13. State Wiedmann-Franz law.
Wiedmann-Franz law states that the ratio of thermal conductivity (K) to
electrical conductivity (σ) is directly proportional to the absolute temperature of the
metal.
14. What is Lorentz number?
According to Wiedmann-Franz law,
K
αT
σ
K
 LT
σ
Where L is called as Lorentz-number which is a constant.
2
3k 
L   B   1.12  10 8 wΩ.k 2 .
2 e 
15. What is Fermi-Dirac distribution function.?
The Fermi-Dirac distribution function F(E) gives the probability of an
electron to occupy a given energy level ‘E’ at absolute temperature.
F(E) 
1
1 e
(E  E f )/kT
where F(E) = Fermi function
E = Energy of the level whose occupancy by electron is considered.
Ef = Fermi energy
k = Boltzmann’s constant
T = absolute temperature
16. Define Fermi level.
Fermi level is defined as the reference energy level which separates filled
and vacant energy levels at absolute zero. The Fermi level is the highest occupied energy
level at absolute zero temperature.
17. Define Fermi energy.
The energy of a quantum state corresponding to the Fermi level at absolute
zero is known as Fermi energy. It is the maximum energy that an electron can possess at
absolute zero temperature.
18. Define density of energy states.
Density of energy states is defined as the number of available energy
states per unit volume in an energy interval (dE).
Z(E)dE 
Number of energy states in dE
Volume
19. Give the expression for density of energy states in an energy interval dE.
Z(E)dE 
4π
(2m) 3/2 E 1/2dE
3
h
20. What is work function?
The amount of kinetic energy required o absolute zero temperature to move
an electron from the outer orbit at absolute zero temperature is called work function.
UNIT II
SEMICONDUCTING MATERIALS
1. Enumerate some of the properties of a semiconductors?






2.
They are formed by covalent bonds.
At 0K conduction band is empty and they behave as insulators.
When the temperature is raised their conducting increases.
When impurities are added their conductivity increases.
They have negative temperature coefficient of resistance.
The charge carriers in semiconductors are electrons and holes. They take
part in current conduction.
What are the different types of semiconductors?
Based on the purity, semiconductors are classified into two types such as,
a. Intrinsic Semiconductors (pure) and
b. Extrinsic Semiconductors (impure)
3. What is an intrinsic semiconductor ? Give examples.
Chemically pure semiconductor are known as intrinsic semiconductors. In these
semiconductors. The electrons and holes can be created only by thermal agitation. As there
are no impurities the number of free electrons must be equal to the number of holes. E.g.
Germanium, Silicon etc.
4. Define carrier concentration in intrinsic semiconductor or density of charge
carriers.
Carrier concentration is the number of electrons inn the conduction band per unit
volume (‘n’) or the number of holes in the valence band per (p) per unit volume of the
semiconducting material. It is also known as density of charge carriers.
5. Write the expression for the concentration of electrons in the conduction band of an
intrinsic semiconductor.
 2 me *  kT 
n  2 

h2


3/ 2
. e ( EF  EC ) / kT
6. Write the expression for the concentration of holes in the valence band of an
intrinsic semiconductor.
 2  mh * kT 
P  2 

h2


3/ 2
e ( EV  EF ) / kT
7.
Write the expression for intrinsic carrier concentration.
ni
8.
 2 kT 
 2  2 
 h 
3/ 2
(me * mh*) 3 / 4 e  Eg / 2 kT
What is meant by doping?
The process of adding impurities to a pure semiconductor is known as doping.
9.
What is p-type semiconductor?
p-type semiconductor is obtained by doping an intrinsic semiconductor with any
one of the trivalent impurity elements such as boron, gallium and indium, etc.
10.
What is n-type semiconductor?
n-type semiconductor is obtained by doping an intrinsic semiconductor with any
one of the pentavalent (5 electrons in valence band) impurity atoms like arsenic, antimony,
phosphores etc.
11. What are donors and acceptors?
Donors are the doped pentavalent impurity atoms like P, as Sb silicon or
germanium donating an electron from its atom to silicon or germanium crystal. The
acceptors are the doped trivalent impurity atoms like Al, Ga and In in pair semiconductor
(eg. Si or Ge) accepting an electron from each silicon or germanium
atom.
12. Explain the variation of Fermi level with temperature in p-type semiconductor.
At T = 0K
EF

Ea  EV
2
At T = 0K, the Fermi energy level lies exactly in the midway between valence
band and acceptor level.
13. Explain the variation of Fermi level with impurity concentration in n-type
semiconductor.
At T = 0K,
EF

E d  EC
2
The above equation shows that at 0K, the Fermi energy level lies exactly in the
midway between conduction band and donor level.
14. Define Hall effect and Hall voltage.
When a conductor (metal or semiconductor) carrying current (I) is placed in a
perpendicular magnetic field (B) an electric field is produced inside the conductor in a
direction normal to both the current and the magnetic field. This phenomenon is called
Hall Effect and the voltage thus generated is known as Hall Voltage.
15. Mention the application of Hall effect.
(i) Hall effect is used in the
(ii) Determination of Semiconductor Type
(iii) Calculation of carrier concentration
(iv) Determination of mobility
(v) Measurement of magnetic flux density
(vi) Hall effect multiplier .
16. Mention the importance of Hall effect.
It helps to determine
17.

The type of semiconductor

The sign of majority charge carriers

The majority charge carriers concentration

The mobility

Drift velocity
What are semiconductors?
Semiconductors are materials having electrical conductivity greater than that of an
insulator but lower than that of a conductor.
18. Define impurity range in n-type semiconductor.
This range is due to the transfer o electrons from donar energy levels to conduction
band. Here the electron concentration in the conduction band steadily increases due to
ionization of donar atoms.
19. Discuss the variation of Fermi level with temperature for p-type semiconductors.
When the temperature is increased for p-type semiconductor, some of the
electrons in the valence band goes to acceptor energy levels and hence the Fermi level is
shifted in upward direction.
20.Discuss the variation of Fermi level with temperature for n-type semiconductors.
When the temperature is increased for p-type semiconductor, some of the
electrons are shifted from donar energy level to the conduction band and hence the
Fermi level is shifted down.
UNIT III
MAGNETIC AND SUPERCONDUCTING MATERIALS
1. What is anti-ferromagnetism?
In anti-ferromagnetism, electron spin of neighbouring atoms are aligned antiparallel. Anti-ferromagnetic susceptibility is small and positive and it depends greatly on
temperature.
2. What are ferrites and mention its types.
Ferrites are modified structure of iron with no carbon in which the adjacent
magnetic moments are of unequal magnitudes aligned in anti-parallel direction. It’s
general formula is given by X2+ Fe2 3+ O42-

Types: Normally there are two types of structure present in the ferrites.
1.
Regular spinel 2. Inverse spinel.
3. What is Bohr magneton?
When the atom is placed in a magnetic field, the orbital magnetic moment of the
electrons is quantized. A qantum of magnetic moment of an atomic system is known as
eh 

Bohr magneton  μ B 
.
4π m 

4. What is magnetic levitation?
The magnetic levitation is based on diamagnetic property of a superconductor
which is the rejection of magnetic flux lines. A superconductor can be suspended in air
against the repulsive force from a permanent magnet. This magnetic levitation effect can
be used for high speed transportation without frictional less.
5. What is meant by magnetic materials? Give examples.
Magnetic materials are the materials which can be easily magnetised by keeping it
in an external magnetic field.
Examples : Iron, Manganese, Ferrites, Nickel, Cobalt etc.
6. Define magnetic lines of force.
It is defined as the continuous curve in a magnetic field which travels externally in
the magnet from North pole to South pole.
7. Define magnetic flux.
The total number of magnetic lines of force passing normally through a closed area
is called magnetic flux. It is measured in Weber.
8. Define magnetic flux density.
The magnetic flux density (B) is defined as the number of magnetic lines of force
passing through a unit area of cross – section (A).
Magnetic flux density, B 
Number of magnetic lines of force
Unit area of cross - section
Unit: Weber / m2 or Tesla.
9. Define magnetic field intensity.
Magnetic field intensity (H) is defined as the ratio of the magnetic flux density (B)
to the absolute permeability () of the medium.
B
μ
Unit : Ampere – turns / metre.
H
10. Define magnetic moment.
Magnetic moment is defined as the product of magnetic pole strength (m) and
magnetic length (l).
Magnetic moment = ml Wb/m
11. Define intensity of magnetization.
Intensity of magnetization (I) is defined as the magnetic moment per unit volume.
I
Magnetic moment
Wb/m
Volume
12. Define magnetic permeability.
Matnetic permeability () is defined as the ratio of the magnetic flux density (B) to
the applied magnetic field intensity (H).
μ
B
Henry/m
H
13. Define relative permeability.
Relative permeability is defined as the ratio between the absolute permeability of
the medium () and the absolute permeability of free space (0).
14. Define magnetic susceptibility.
Magnetic susceptibility (m) is defined as the ratio of the intensity of magnetisation
(I) to the magnetic field strength (H).
I
H
Where m is a dimensionless quantity.
χm 
15. Explain the term remanence and coercivity with its units.
Remanence / Retentivity: It is the residual intensity of magnetisation retained by
the specimen even when the external magnetic field is cut off.
Unit: Wb m–2
Coercity: It is the strength of reverse magnetic field required to completely remove the
residual magnetisation (or) demagnetise the material.
Unit :
Ampere turn
metre
16. Mention the different classifications of magnetic materials.
Magnetic materials are broadly classified into the following types,
1. Diamagnetic materials
2. Paramagnetic materials
3. Ferromagnetic materials
4. Antiferromagnetic materials
5. Ferrimagnetic materials.
17. What are diamagnetic materials? Give their properties.
The materials which when placed in a magnetic field acquire weak magnetism in a
direction opposite to that of the field are known as diamagnetic materials.
Properties:
1. They have no permanent dipole moment.
2. They repel the magnetic lines of force.
3. They exhibit negative magnetic susceptibility.
4. Their susceptibility is independent of temperature.
5. They have relative permeability slightly less than unity.
Examples: Hydrogen, air, water, gold, silver etc.
18. What are paramagnetic materials. Give their properties.
The materials which when placed in a magnetic field acquire weak magnetism in
the same direction as that of the applied field are known as paramagnetic materials.
Properties:
1. They have permanent dipole moment.
2. They attract the magnetic lines of force.
3. They exhibit positive magnetic susceptibility.
4. Their susceptibility is dependent on temperature.
5. They have relative permeability slightly greater than unit.
Examples: Chromium, Platinum, Oxygen, Crown glass etc.
19. State Curie’s law.
Curie’s law states that the susceptibility of a paramagnetic substance varies
inversely with the temperature.
1
i.e., χ α
T
C
or
χ
T
Where
C = Curie constant, and
T = Absolute temperature in Kelvin.
20. What is Curie – Weiss law?
The relation given by the equation
C
χ 
is known as Curie – Weiss law.
Tθ
Where  is the Curie temperature.
For paramagnetic materials at low temperatures, Curie – Weiss law can be sued.
21. What are ferromagnetic materials? Give their properties.
The materials which when placed in a magnetic field acquire strong magnetism in
the same direction as that of the applied field are known as ferromagnetic materials.
Properties:
1. They have permanent dipole moment.
2. They attract the magnetic lines of force strongly.
3. They exhibit positive and high value of magnetic susceptibility.
4. Their susceptibility depends on temperature in a complex manner.
5. They have a high value of relative permeability.
6. They exhibit magnetisation even in the absence of magnetic field.
7. They exhibit hysteresis curve.
Examples: Iron, Nickel, Cobalt etc.
22. Discuss the orientation of spin for dia, para and ferro-magnetic substances.
Diamagnetic materials: Here the electron spins are randomly oriented and mostly they
have equal and opposite spins. Thus the net magnetic moment is zero.
Paramagnetic material: Here the spins of electrons will not be equal, which leads to
have some unpaired electrons. Hence there exists some resultant magnetic moment.
Therefore in paramagnetic material the net magnetic moment is not zero.
Ferromagnetic material: In ferromagnetic materials the number of unpaired electrons
will be more. Hence there exists a large resultant magnetic moment in it.
23. Why diamagnetic materials are called weak magnets and ferromagnetic
materials are called strong magnets?
Weak magnets: If a diamagnetic material is kept in an external magnetic field, the
electrons spins in the material reorient in such a way that they align perpendicular to the
field direction and hence the materials will not be easily magnetised. Thus diamagnetic
materials are called weak magnets.
Strong magnets: When a ferromagnetic material is kept in external magnetic field, the
electrons which are already aligned parallel to the direction of magnetic field acquires a
very strong magnetic moment in it. Hence ferromagnetic materials are called strong
magnets.
24. What is Curie temperature?
Curie temperature is the critical temperature below which a material can behave as
ferromagnetic magnetic material and above which it can behave paramagnetic material.
25. What are magnetic domains?
The group of atoms organised into tiny bounded regions in a ferromagnetic material
are called magnetic domains. A magnetic domain will behave like a magnet having its
own magnetic moment.
26. Discuss the effect of external magnetic field over magnetic domains.
When an external magnetic field is applied to the ferromagnetic materials, based on
the strength of the fields, two process occurs viz.
(i) Movement of domain walls take place at weak magnetic field and
(ii) Rotation of domain walls take place at strong magnetic field.
27. What are the four types of energies involved in the growth of magnetic domains.
The four types of energies involved in the growth of magnetic domains are
(i) Exchange energy (or) magneto – static energy
(ii) Anisotropy energy
(iii) Domain wall energy and
(iv) Magneto-strictive energy.
28. What is meant by reversible and irreversible domain?
When the external magnetic field applied to a domain is increased, it starts
expanding. Now when the external magnetic field is removed, if the domain returns to its
original position, it is called reversible domain and if the domain doesn’t returns to its
original position it is known as irreversible domain.
29. What is hysteresis?
When a magnetic material is taken through a cycle of magnetisation, the intensity
of magnetisation (I) and the magnetic flux density (B) lag behind the applied magnetic
field (H). This lagging of I and B behind H is known as hysteresis.
30. What is meant by Hysteresis loop and what do you infer from it?
The closed curve obtained during the cycle of magnetisation of a material is known
as hysteresis loop.
Inference: The area of the loop gives the energy loss (or) hysteresis loss during the cycle
of magnetisation.
31. Define retentivity of a ferromagnetic material.
Retentivity is defined as the magnetisation retained by the specimen when the
magnetising field is reduced from saturation value to zero.
32. Define coercivity of a ferromagnetic material.
The negative value of the magnetic field required to demagnetise a ferromagnetic
specimen is defined as coercivity.
33. What are soft and hard magnetic materials? Give examples.
Materials which are easy to magnetise and demagnetise are called soft magnetic
materials.
Examples: Iron – silicon alloys, Iron – Cobalt alloys etc.
Materials which are difficult to magnetise and demagnetise are called hard
magnetic materials.
Examples : Carbon steel, Tungsten steel, Al-Ni-Co alloys etc.
34. Distinguish the properties of soft and hard magnetic materials.
S.No.
Soft
1.
They can be easily
magnetised and
demagnetised.
2.
Loop area is less and hence
the hysteresis loss is
minimum.
3.
Susceptibility and
permeability is high.
4.
Retentivity and Coercivity
are small.
5.
They have low eddy current
loss.
6.
These materials are free
from irregularities like
strain or impurities.
Hard
They cannot be easily
magnetised and demagnetised.
Loop area is large and hence the
hysteresis loss is maximum.
Susceptibility and permeability
is low.
Retentivity and Coercivity are
large.
They have high eddy current
loss.
These materials have large
amount of impurities and lattice
defects.
35. Give any four application of ferrites.
(i) Ferrites are used in audio transformers, video transformers, radio receivers etc.
(ii) They are used in two port devices such as gyrator, circulator and isolator.
(iii) They are used in computers and data processing circuits.
(iv) They are used in switching circuits and parametric amplifiers.
36. What is a magnetic storage device? Give examples.
Ferro and ferri magnetic materials which are used to store the data in the form of
zeros and ones are called magnetic storage devices.
Examples: Floppy disk, Audio cassettes, Magnetic tapes etc.
37. Define superconductivity.
Superconductivity is defined as the phenomenon of exhibition of zero electrical
resistance by a solid material when it is cooled below a certain temperature called the
critical temperature TC.
38. What are superconductors? Give examples.
The materials which exhibit the property of superconductivity at very low
temperatures are called superconductors.
Examples: Mercury, Lead, Tin, Aluminium, certain alloys etc.
39. Define critical temperature or transition temperature TC.
The temperature at which a solid material exhibit zero electrical resistivity is
defined as the critical temperature or transition temperature TC.
40. Define critical magnetic field HC.
The critical value of the magnetic field which destroys superconductivity is defined
as the critical magnetic field HC.
41. Define critical current density IC.
The critical current density IC is defined as the minimum current that can be passed
through a material without destroying its superconducting state.
42. What is Meissner effect ?
The expulsion of magnetic lines of force by a superconductor below a certain value
of magnetic field is known as Meissner effect.
43. What is Josephson’s effect ?
The tunneling of superconducting electron pairs through a thin insulating barrier
between two superconductors is known as Josephson’s effect.
44. What is meant by persistent current ?
When d.c. current of large magnitude is once induced in a super conducting ring,
then due to the diamagnetic property of the super conductor, the magnetic flux is trapped
inside the ring an shown in fig and hence the current persists in the ring for a longer time.
This current is called as persistent current.
45. Mention the types of superconductors.
Superconductors are classified into two types as
1.
2.
Type I Superconductors or Soft superconductors
Type II Superconductors or Hard superconductors.
and
46. What are Type – I superconductors? Why they are called as soft
superconductors?
The superconductors which strictly follow the Meissner effect are called Type – I
superconductors. They are called soft superconductors because they give away their
superconducting properties at lower magnetic field strengths.
47. What are Type – II superconductors? Why they are called as hard
superconductors?
The superconductors which do not strictly follow the Meissner effect are called
Type – II superconductors. They are called hard superconductors because relatively large
magnetic fields are required to bring them back to the normal state.
48. State BCS theory of superconductivity.
BCS theory states that superconductivity is the result of formation of Cooper pairs
which have the capacity to move through the lattice without any scattering.
49. What is a Cooper pair?
The bound pair of electrons with opposite momentum and spin is called a Cooper
pair.
50. What is coherence length in superconductors?
The distance upto which a Cooper pair travels without breaking its bound state in a
superconductor is known as coherence length. The coherence length is of the order of 10–
6
m.
51. What are high temperature superconductors?
Superconductors with unusually high values of critical temperature (TC > 77K) are
called high temperature superconductors.
52. What is a cryotron?
Cryotron is a type of switching element made by two different super conductors A
and B as shown, with critical fields H CB  H CA . Here the super conducting property
vanishes for the material ‘A’ due to the magnetic field produced by material Band hence
it can be used as relay (or) switching elements.
53. Write a short note on SQUIDS.
SQUID is an abbreviation of Super conducting Quantum Interference Devices. It is
an improved model of Jesephson device and it works under the principle that, “A small
change in magnetic field produces large variation in the quantum flux”.
UNIT IV
DIELECTRIC MATERIALS
1. Define dielectric constant or relative permittivity.
Dielectric constant is the ratio between the permittivity of the medium () and
the permittivity of free space (0). It is a measure of polarisation in the dielectrics. It
has no unit.
εr
i.e.,

ε
ε0
If a medium has high dielectric constant it can be easily polarised and behaves as
a good electrical insulator. The value of r = 1 for air or vaccum.
2. Define dielectric susceptibility and permittivity.
Permittivity of a medium is defined as the ratio between the electric induction
D and electric field intensity E. It determines the easily polarisable nature of the
medium. Its unit is farad meter –1.
εD
i.e.,
E


The polarization vector P is proportional to the applied electric field E .


P α E


P  ε0 e E
where e is a constant known as electric susceptibility.
Electric susceptibility is a characteristics of dielectrics. It determines the easily
polarisable nature of the material.
3. Define electric dipole.
If in a system there are two equal and opposite charges separated by a distance
then it is called as electric dipole.
4. Define dipole moment of a dielectric
The product of magnitude of the charges ‘q’ and distance of their separation
‘d’ is called the dipole moment ‘’ of the electric dipole. Its unit is coulomb – metre.
 = qd
5. Define polarisability in a dielectric material.
If the strength of the applied electric field E is increased, the strength of
the induced dipole also increases. So the induced dipole moment is directly proportional
to the applied field E.
i.e.,
E
=E
Where the proportionality constant  is called polarisability.
6. Define power factor of a dielectric.
Power loss PL = 2 f C V2 tan
Here tan is called the power factor of the dielectric. If f, C, V are constants then
PL  tan
7. Derive the relationship between dielectric constant and susceptibility.

 e 
P

ε0 E

Substituting the value of
P

from equation (5), we get
E
e 
ε 0 (ε r  1)
ε0
i.e.,
e = r – 1
or
r = 1 +
e
8. Mention four types of polarization mechanisms involved in dielectric.
(i)
Electronic Polarisation
(ii)
Ionic Polarisation
(iii)
Orientational Polarisation
(iv)
Space-charge Polarisation
9. Define electronic polarization.
Electronic polarisation occurs due to the displacement of positively
charged directions, when an external electric field is applied. As a result a dipole
moment is created in the dielectric. The induced dipole moment i is
proportional to the applied electric field E.
 E
 = e E
where e is electronic polarisability.
10. What is ionic polarization?
Ionic polarization occurs due to the displacement of cations (– ve ions)
and anions (+ ve ions) in opposite directions by the applied electric field. It
occurs in ionic dielectrics.
11. What is orientational polarization?
Orientational polarisation arises in polar dielectrics. Polar dielectrics have
polar molecules. Polar molecules are the molecules which have permanent dipole
moments even in the absence of an electric field.
When an electric field is applied on the polar dielectric, the molecules
align themselves along the field direction. i.e., Positive portion of the dipole align along
the direction of the applied field and negative portion align in the opposite direction of
the field. Due to this a resultant dipolemoment is produced in that material.
12. What is meant by local field or internal field is a dielectric?
The internal field Ei is defined as the electric field acting on the atom is
equal to the sum of the electric fields created by the neighbouring polarized atoms
and the applied field. This field is responsible for polarising the atoms.
13. Explain the terms dielectric loss and dielectric breakdown.
When a dielectric is subjected to an electric field, the electric energy is
absorbed by the dielectric and certain quantity of electrical energy is dissipated in the
form of heat energy. This dissipation of energy is called dielectric loss.
When the strength of the electric field applied to a dielectric is increased
beyond a critical value, very large current flows through the dielectric.
Hence it losses its insulating property and begins to act as a conductor.
This phenomena is called dielectric breakdown.
14. What are different types of dielectric breakdown mechanisms?
1.
Intrinsic and avalanche breakdown
2.
Thermal breakdown
3.
Chemical and electrochemical breakdown
4.
Discharge breakdown
5.
Defect breakdown
15. Define dielectric strength.
The electric field strength at which the dielectric breakdown occurs is
known as dielectric strength. It is the ratio of dielectric breakdown voltage to
thickness of dielectric.
Dielectric strength 
Dielectric breakdown voltage
Thickness of dielectric
16. What are the requirements of good insulating material?
A dielectric material should have the following characteristics to avoid
breakdown.

It should have high resistivity to reduce leakage current.

It must have high dielectric strength.

Dielectric loss should be low.

It must have sufficient mechanical strength.

It should be fire – proof.

These should not be any defects.

It should not react with oils, liquids and gases.

It must have less density.

Thermal expansion should be low.

It must be in pure form.
17. Define ferroelectricity.
If a material exhibits electric polarization even in the absence of external
electric field, then that material is called as ferroelectric materials. The
phenomenon of exhibiting dielectric polarization, even in the absence of external
electric field is known as ferroelectricity.
18. Explain the phenomenon of spontaneous polarization in ferroelectric
material.
When an electric field is applied to a ferroelectric material, the
polarization increases with applied field rapidly from 0 to A and attains
saturation. This type of polarization is called spontaneous polarization (Ps).
19. Give the function of transformer oil in transformers.
Mineral oil or synthetic oil like askarels area used as transformer oil.
They are used for insulation and for cooling. They have very high dielectric
strength and is highly viscous. They transfer heat from the transformer windings
and core to the outer side and thus help in dissipation of the heat generated.
Sovol, sovotol are some of the synthetic oils widely used in high voltage
transformers.
20. List out any four applications of ferroelectric materials.
1. Ferro electric materials are used in capacitors to store large quantity of
electric energy.
2. The high dielectric constant of ferroelectric crystals is also useful for
storing energy in small sized capacitors in electrical circuits.
3. These are used in electro acoustic transducers such as microphone.
4. Ferroelectric crystals exhibit the piezoelectric property. Using this, we can
find enormous applications of ferroelectric materials.
UNIT-V
MODERN ENGINEERING MATERIALS
1. What is meant by glass transition temperature?
The temperature at which the metals in the molten form transforms into
glasses is known as glass transition temperature.
2. What are metallic glasses?
Metallic glasses are the amorphous metallic solids which have high
strength, good magnetic properties and better corrosion resistance and will
possess both the properties of metals and glasses.
3. State the advantages of using metallic glass as transformer core
material.
Metallic glasses are ferromagnetic materials having low magnetic loss,
high permeability,saturation magnetization and low coercivity.
4. Mention any four properties of metallic glasses.
a) Metallic glasses have tetrahedral closely packed structure.
b) The metallic glasses are very strong in nature.
c) They posses malleability, ductility etc.
d) They exhibit very low hysterisis loss
5. What are shape memory alloys?
When material is heated above the transformation temperature,then
there will be some change in the crystal structure, which cause the material to
return to its original structure. Such materials are called shape memory alloys.
6. What are nano materials?
Nano materials are the materials in which the atoms /grain size is in
the order of 1 to 100 nano-meters and these atoms will not move away from
each other.
7. What do you understand by ‘Martensite’ and ‘Austenite’ phases?
Martensite is an interstitial super solution of carbon in α ion and it
crystallizes into twinned structure. The SMA will have this structure generally
at lower temperatures and it is soft in this phase.
Austinite is the solid solution of carbon and other alloying elements
in γ –iron and it crystallizes into cubic structure. The SMA will attain this
structure at high temoeratures and it is hard in phase.
8. Define pseudo elasticity.
Pseudo elasticity occur in some types of SMA in which the change in
its shape will occur even without change in its temperature.
9. What are the properties of SMA?
i)
The transformation occurs not only at a single temperature rather they
occur over a range of temperatures.
ii)
They have pseudo-elastic and super elastic property.
iii)
They exhibit hysterisis curve, during cooling and heating process.
iv)
Crystallographically the thermo elastic martensites are reversible.
10. What are the types of SMA?
i)
One way shape memory alloy (SMA) : Though there is some
change in its temperature, the SMA remains in the same phase, and
this type of material is called one way shape memory alloy.
ii)
Two way SMA : The type f materials which produces spontaneous
and reversible deformation just upon heating and cooling even
without load are called two way shape memory alloys.
11. Give any four application of SMA.
i)
SMA is used in eye glass frames, toys, helicopter blades,
blood clot filter etc.
ii)
They are also used in fire safety valves, coffee maker,
circuit edge connector etc.
iii)
They are used in relays and activators.
iv)
Ni-Ti SMA are also used in artificial hip-joints, boneplates, pins for healing bones fractures and also in
connecting broken bones
12. What are the properties of nano particles?
i)
Since the size of the particle is very less, the particles are
very close to each other and hence the inter particle spacing
is very less
ii)
The energy band in these materials will be very narrow.
iii)
The ionization potential is found to be higher for nano
materials.
iv)
Due to large magnetic moment these nano materials
exhibits spontaneous magnetization at smaller sizes.
13. What is meant by plasma arching
Plasma arching is a method used to produce nano particles. Here a
starting material is heated to a very high temperature, above its evaporation
point to form plasma. Now,when He gas is passed, the metal vapour nucleates
on the Helium gas to form nano particles on a colder collector rod.
14. What is meant by carbon nanotubes?
Carbon nano tubes are molecular scale tubes of graphitic carbon with
outstanding properties. They are among the stiffest and strongest fibres
researched till date, with remarkable electronic properties and applications.
15. List out the various forms of CNT.
i)
Arm chair structure
ii)
Zig zag structure
iii)
Chiral structure
16. Give the techniques adopted to synhesis CNT.
i)
Chemical vapour deposition
ii)
Caron arc technique
iii)
Pulsed laser deposition method
17. How CNTs can be produced by chemical vapour deposition?
In chemical vapour deposition method hydro-carbon gas such as
methane is heated upto 1100oC and is decomposed. As the gas decomposes, it
produces carbon atoms. These carbon atoms are made to deposit over a cooler
substrate, which contains iron as a catalyst to form carbon nano tubes.
18. What are the properties of CNTs?
i)
The strength of the sp2 carbon-carbon bonds
give high hardness for carbon nanotubes.
ii)
The Young’s modulus of the nanotubes is
approximately 5 times greater than the steel.
iii)
The tensile strength is around 50 times higher
than steel.
iv)
Nanotubes have higher conductivities than tha
of copper.
19. What are the applications of carbon nanotubes?
i) They are used in battery electrodes, fuel cells, einforcing fibers etc.
ii)They are used in the development of flat panel displays for computer
monitors and televisions
iii)
They are used as switching devices
iv)
They are very light in weight, but they are very strong. Hence they
are used in aerospace.
20. What is meant by metallic nan clusters?
The cluster formed by alkali metals, alkaline earth metals and transition
metals are called metallic nano cluster.