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Kathir College of Engineering, Coimbatore

Department of physics

Engineering physics II

Question bank

1. What are conducting materials?

Materials which have the ability to conduct electricity when an electric potential

difference is applied across them are called conducting materials. Also, these materials allow

heat energy to flow easily through them. Normally, metals are good conductors of both heat

and electricity.

2. What are the merits of classical free electron theory?

i. It is used to verify Ohms law

ii. It is used to explain electrical and thermal conductivities of metals.

iii. It is used to derive Wiedemann - Franz law.

iv. It is used to explain the optical properties of metal.

3. How to classify the conducting materials?

The conducting materials can be classified into following three types baed on their

electrical resistivity.

i. Zero resistivity material

ii. Low resistivity material

iii. High resistivity material

4. What is zero resistivity material?

They are a special class of materials which conduct electricity almost with zero

resistance below the transition temperature. These materials are called as superconducting

materials. They are used for saving in power systems, superconducting magnet etc.

5. Define drift velocity.

The mean velocity acquired by the electrons in the presence of an electric field is

defined as drift velocity.

6. Define drift current.

The current due to drift motion of electrons in the presence of an electric field is

defined as the drift current.

7. Define electrical conductivity.

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Electrical conductivity is defined as the quantity of electricity that flows in unit time per

unit area of cross-section of the conductor per unit potential gradient.

8. Define mobility of electron.

Mobility of the electron is defined as the drift velocity acquired by the electron per unit

electric field applied to it.

i.e ., Mobility =

9. What is meant by mean free path of an electron?

The average distance traveled by an electron between any two successive collisions is

called the mean free path of the electron.

10. Define collision time?

It is defined as the average time between two successive collisions in the presence of

electric field.

11. Define relaxation time.

The average time taken by the electron between any two successive collisions in the

presence of electric field is defined as the relaxation time.

12. Mention some of the sources of electrical resistance in metals.

Thermal vibrations, impurities, plastic deformations, lattice defects, etc., are some the

sources of electrical resistance in metal.

13. What do you mean by electron theory of metals?

The study of valence electrons which controls the various properties of metals such as

electrical conductivity, thermal conductivity, magnetic properties, etc., is known as electron

theory of metals.

14. What are free electron or conduction electrons?

The valence electrons are so weakly attached to their nuclei that they can easily remove

or detached. Since these valence electrons are free of their corresponding nuclei, they are

called free electrons. These free electrons are capable of electrical conduction and hence are

known as conduction electrons.

15. Mention any two important features of quantum free electron theory of metals?(April 2003)11

It shows that the energy levels of an electron are discrete

The maximum energy level upto which the electrons can be filled is denoted by Fermi

energy level.

16. What is the source of resistance in metals? (Nov. 2003)

i. Impurities present in the materials

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ii. Temperature of the metal

iii. Number of free electrons

17. State the Wiedemann-Franz law.

The Wiedemann-Franz law states that the ratio of thermal conductivity (K) to the

electrical conductivity () is constant for all metals and is proportional to the absolute

temperature T.

18. Define thermal conductivity?

Thermal conductivity of the material is defined as the amount of heat conducted per

unit area per unit time maintained at unit temperature gradient.

19. Give postulates of classical free electron theory?

Postulates of classical free electron theory:

i. Revolving electrons around the nucleus constitute an atom and a metal is

composed of such atoms.

ii. The movement of free electron obeys the laws of classical Kinetic theory of gases.

iii. The free electrons move in a completely uniform potential field due to ions fixed

in the lattice.

20. Drawbacks of classical free electron theory:

i. From the classical free electron theory the value of specific heat of metals is given by

4.5 R, where R is called the universal gas constant. But experimental value of specific

heat is nearly equal to3R.

ii. Further according to classical free electron theory, the value of the electronic specific

heat is equal to (3/2) R. But actually it is about 0.01R only.

iii. Classical free electron theory states that all free electrons will absorb the supplied

energy but quantum free electron theory states that only few electrons will absorb the

supplied energy.

iv. Electrical conductivity of semiconductors or insulators could not be explained using

this theory.

21. What is Fermi? (OR) What is Fermi energy?

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i. Fermi level is that state at which the probability of electron occupation is at

any temperature above 0 K and also it is the level of maximum energy of the

filled states at 0 K.

22. What is Lorentz Number?

The ratio between thermal conductivity (K) of a metal to the product of electrical

conductivity () of a metal and absolute temperature (T) of the metal is a constant. L = K /T

23. Calculate the Lorentz number?

Lorentz number is a constant and it is given by

L = 3/2(K/e)Where K =1.3810 JK (Boltzmanns Constant)

e=1.602110 coulomb

L = 3/2 ( 1.3810 )

( 1.602110 )

=1.1210 WK

i.e., L=1.1210 WK

24. List the merits of the classical free electron theory.

a. It successfully verifies Ohms law

b. It explains the high electrical and thermal conductivity of metals.

c. This model is in food agreement with the Wiedemann-Franz law

d. The various optical properties of metals can be explained with the classical free electron

theory.

25. List the demerits of the classical free electron theory.

a. This theory cannot account for the linear variations in resistance with temperature

b. The predicted values of specific heat and Para magnetism as obtained by the classical

theory are much higher than the observed experimental values.

c. This theory fails to explain blackbody radiation, Compton Effect and Photoelectric

effect.

26. What are degenerate and non degenerate energy states?

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a. Energy states having different wave functions but the same energy values are said to be

degenerate energy state.

b. If there is only one wave function for a particular energy state, then such a state is

known as a non degenerate state.

27. Define density of energy states.

The density of energy states is defined as the number of energy states per unit time per

unit volume in an energy interval. The density of energy states represents the number of states

that could be occupied by charge carriers.

28. Mention any two important features of quantum free electron theory of metals.

i. It shows that the energy levels of an electron are discrete

ii. Maximum energy level upto which the electrons can be filled in denoted by Fermi

energy level.

29. Define Fermi distribution function.

The probability F(E) of an electron occupying a given energy level at temperature T is

known as Fermi distribution function.

EF Fermi level

K Boltzmanns constant

T Absolute temperature

E Energy of the level whose occupancy is being considered.

30. What is carrier concentration?

The carrier concentration i.e., the number of electrons per unit volume in a given energy

range which is determined by summing the product of the density state Z (E) and the

occupancy probability F (E).

31. Define Fermi level. (OR) What is meant by Fermi energy of a metal ?(April 2002,Nov 2002)

Fermi level is defined as the reference energy level which separates filled and vacant

energy levels at absolute zero. The Fermi level is the occupied energy level at absolute zero.

32. write the Fermi Dirac distribution function and give its importance?( April 2003,Nov

2003,dec,2003)

Fermi-Dirac distribution function represents the probability of an electron occupying a

given energy level. It is given by

33. Where - Boltzmann constant

1

F (E) =

1 + e (E EF

) / KT

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T - Absolute temperature

34. How does the Fermi function varies with temperature?(Nov 2003)

The Fermi function varies with respect to the temperature here at 0 k all the energy

states below are filled and all those above it are empty. Now when the temperature isincreased, the electron takes an energy T and hence the Fermi function falls to zero

35. Define mobility of electrons.

In addition to thermal motion, electrons drift due to the applied field, Mobility of the

electrons is the magnitude of the drift velocity per unit field.

= Vd / E

36. What is a semiconductor? And give examples?

Semiconductors are materials which have electrical conductivity values lying between

that of a good conductor and a perfect insulator.

Example: Germanium and silicon

37. What are elemental semiconductors?

The elemental semiconductors are formed by single element from the fourth coulumn

of the periodic table. Example: Si and Ge.

38. What are compound semi conductors?

The compound semiconductors are formed by the combination of elements of third &

fifth column elements or second & sixth column elements of the periodic table.

Example: GaAs , Inp.

39. Define conductivity of a semiconductor.

In a semiconductor, the charge carriers are electrons and holes. The conductivity of a

semiconductor due to electrons in the conduction band is given by n = nee

Similarly, the conductivity of the semiconductor due to holes is given by p = neh

The conductivity of a semiconductor is = e (ne + nh)

40. What are intrinsic semiconductors? Give examples.

Intrinsic semi conducting materials having energy gap of the order of 1eV. Charge

carriers are generated due to the breaking of covalent bonds. Example: Ge and Si

T = 0 K T = 0 K

1

F (E) F (E) T 0K

0.5

0 E EF 0 E EF

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41. What are extrinsic semiconductors? Give examples.

Semiconductors in which the charge carriers originate from impurity atoms added to the

material are called extrinsic semiconductors. The addition of impurity increases the carrier

concentration and hence the conductivity of the conductor.

42. Define carrier concentration.

Carrier concentration or density is defined as number of electrons in the conduction

band per unit volume or the number of holes in the valence band per unit volume of the

semiconducting material.

(OR)

Carrier concentration pr density is defined as number of electrons in the conductance

band per unit volume ( or the number of holes in the valence band per unit volume of

the semiconducting material.

43. What is an extrinsic semiconductor and give examples.

A semiconducting material in which the charge carriers originate from impurity atoms

added to the material is known as extrinsic semiconductor or impure semiconductor.

They are obtained by adding pentavalent impurity atoms or trivalent impurity atoms.

44. What is an n-type semiconductor?

n-type semiconductors are obtained by adding pentavalent impurity atoms As, P, Sb,etc., in pure semiconductors like Si and Ge. Electrons are majority carriers which are generated

from added impurity.

45. What is a p-type semiconductor?

p - type semiconductors are obtained by doing trivalent impurity atoms to pure

semiconductor like Si and Ge. In p-type semiconductor, majority carriers are holes which are

created due to lack of an electron in the impurity atom to complete the four tetrahedron bonds.

46. What is Hall Effect?If a semiconductor or a conductor carrying a current is placed in a transverse magnetic

field, a potential difference is produced in the direction normal to both the current and magnetic

field directions. This phenomenon is called Hall Effect and the generated voltage is called the

Hall Voltage.

47. List out two important applications of Hall Effect?

a. The sign of the charge carrier is determined.

b. The carrier concentration, n = -1/eRH and p = 1/eRH can be determined.

c. We can measure the mobility of charge carriers directly.

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d. Electrical conductivity of the material can be determined experimentally.

48. Compare elemental and compound semiconductor.(or)what are the differences between indirect

band gap semiconductor and direct band gap semi conductor.(april 2002)

S.No Elemental semiconductor (or) indirectband gap semiconductor

Compound semiconductor (or) Direct bandgap semiconductor

1 They are made of single element

E.g: Ge, Si, etc.

They are made of compounds.

E.g: Gae, GaP, CdS, MgO etc

2 They are called as indirect band gap

semiconductors.

They are called as direct band gap

semiconductors.

3 Electron-hole recombination takes

place through traps, which are present

in band gap.

Electron-hole recombination takes place

directly with each other.

4 Here Heat is produced due to

recombination

Here photons are emitted during

recombination (this effect is in LED)

5 Life time of charge carriers is more

due to indirect recombination.

Life time of charge carrier is less due to

direct recombination.

6 Current amplification is more Current amplification is less

7 They are used for the manufacture of

diodes and transistors., etc.

They are used for making LEDS, Laser

diodes, ICS etc.

49. What is meant by Intrinsic semiconductor and extrinsic semiconductor?(nov 2003,Dec 2003,april

2002)

S.No Intrinsic semiconductor Extrinsic semiconductor

1 Semiconductor in a pure form is called

intrinsic semiconductor.

Semiconductor which is doped with impurity

is called extrinsic semiconductor.

2 Here charges carriers are produced

only due to thermal agitation.

Here the charge carriers are produced due to

impurities and may also be produced due to

thermal agitation.

3 They have low electrical conductivity. They have high electrical conductivity.

4 They have low operating temperature. They have high operating temperature

5 At OK, the Fermi level exactly lies

between conduction band and valance

band.

At OK, Fermi level lies closer to conduction

band in n type semiconductor and lies near

valence bond in p type semiconductor.

6 Examples: Si, Ge Examples: Si and Ge doped with Al, In,P, As

50. Give the expression for Fermi energy of an intrinsic semiconductor and extrinsic semiconductors

at 0K?(april 2003)

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Fermi energy level is the energy level which distinguishes the filled and empty states

(or) It is the minimum energy level upto which the electrons are filled at 0K.

The Fermi energy of an intrinsic semiconductor is i.e.,

The Fermi energy level exactly lies between the lowest energy level of the

conduction band and highest energy level of valence band.

The Fermi energy of n-type semiconductor is a ,i.e.,

The Fermi energy level lies exactly between minimum energy level of

conduction band and donor energy level.

The Fermi energy of a p-type semiconductor is ,i.e.,

Fermi energy level lies between the acceptor energy level and the maximum

energy level of valence band.

51. Mention the applications of Hall effect?(Nov 2003,may 2005)

(i) It is used to determine whether the material is p-type or n-type semiconductor.(i.e.,) if

is negative then the material is n-type. if the is positive then the material is p-type .

(ii) It is used to find the carrier concentration = and

(iii) It is used to find the mobility of charge carriers and .

(iv) It is used to design magnetic flux meters and multipliers on the basis of the Hall voltage.

(v) It is used to determine the sign of current carrying charges.

(vi) It is used to find the power flow in an electromagnetic flow.

52. Mention any four advantages of semiconducting materials?(April 2002, April 2003)

(i) It can behave as insulators at 0K and as conductors at high temperatures.

(ii) It possesses some properties of both conductors and insulators.

(iii) On doping we can produce both n and p-type semiconductors with charge

carriers of electrons and holes respectively.

(iv) If possess many applications in electronic field such as manufacturing of diodes,

transistors, LEDS, ICs etc..,

53. What are magnetic materials?

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The materials which get magnetized in a magnetic field are called as magnetic materials.

When a magnetic material is placed in an external magnetic field, permanent magnetic moment

is produced in it.

54. Define magnetic dipole moment.

It is defined as the product of magnetic pole strength (m) and distance two magnetic

poles.

Magnetic dipole moment (M) = m.l (wb-m)

55. What is magnetic field?

Magnetic field is the space around the magnet (or) the current carrying conductor where

the magnetic effect is felt.

56. 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.

i.e., B = /A

57. Define magnetic field strength.

The magnetic field strength (H) is defined as the force experienced by a unit North Pole

placed at a given point in a given magnetic field.

Hence H = F /m (A/m)

58. What is Bohr magnetron? And give its value?

Bohr magnetron is the atomic unit of magnetic moment. Both the orbital magnetic moment

and the spin magnetic moment of an electron present in a atom are expressed in this unit.

1 Bohr Magnetron = e / 2m B = 9.2710 -24 (Am2)

where, is permeability, B is magnetic flux density.

59. Define magnetic susceptibility.

The magnetic susceptibility () is defined as the ratio of intensity of magnetization

produced in a sample to the magnetic field strength.

i.e., = 1/H

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where,

= magnetic susceptibility

l = intensity of magnetization produced

H = magnetic field strength

60. Define magnetic permeability.

The magnetic permeability () is defined as the ratio of the magnetic flux density (B) in the

sample to the applied magnetic field intensity (H). It is a measure of the amount ofmagnetic

lines of forces penetrating through a material.

1. = B/H

where,

B is magnetic flux density

H is magnetic field density

61. Define relative permeability.

Relative permeability () is defined as the ratio between the permeability of the medium

() and permeability of free space ().

Hence, permeability of the medium

r= / =

permeability of the free space

62. What are diamagnetic materials?

Diamagnetic materials are generally referred to as non-magnetic materials. They exhibit

extremely weak response to an external magnetic field. In a diamagnetic material, the dipoles

are oriented in such a way that the resultant dipole moment is zero.

63. What are paramagnetic materials?

In paramagnetic materials, the dipoles are randomly oriented. The spins in two opposite

directions will not be equal. Some unpaired electrons present in the atom gives rise to spin

magnetic moment. Therefore the resultant magnetic moment will not be equal to zero.

64. What are ferromagnetic materials?

Ferromagnetic materials exhibit spontaneous magnetization, i.e., atomic magnetic

moments are aligned even in the absence of an external magnetic field. This implies that there

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is a strong internal field within the material that makes the atomic magnetic moments align

with each other.

65. What anti-ferromagnetic materials?

In this magnetic material, the magnetic interactions between any to two dipoles alignthemselves anti-parallel to each other. All the dipoles are equal in magnitude and they are

aligned in anti-parallel manner due to unfavourable exchange interaction among them.

Therefore, the resultant magnetization is zero.

66. What are ferromagnetic materials?

In these magnetic materials, the magnetic interactions between any two dipoles align

themselves anti-parallel to each other. But the magnitude of any adjacent dipoles is not equal.

67. What are domains in a magnetic material?

A single crystal of ferromagnetic material is divided into large number of small regions

called domains.

68. What is magneto-static energy?

The potential energy stored in the magnetic material is known as magneto-static energy.

69. What is anisotropy energy?

The excess of energy required per unit volume of a substance to magnetise it along a

particular direction with respect to an easy direction is known as anisotropy energy.

70. What is domain wall energy?

The equilibrium thickness of the domain wall should have a minimum potential energy

which is the sum of the exchange energy and the anisotropy energy. This minimum potential

energy of a domain wall is known as the domain wall energy.

71. What is magnetostriction energy?

When the material is magnetized there exists a deformation (i.e.,) change in its dimension.

This phenomenon is known as magneto-striction and the energy produced in this effect is

known as magneto-striction energy.

72. What is hysteresis curve?

When a ferromagnetic material is taken through a cycle of magnetization, the variation

of B (magnetic induction) with respect to H (applied magnetic field) can be represented by a

closed hysteresis loop (or) curve. i.e., it refers to the lagging of magnetization behind the

magnetizing field.

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73. What is remanetivity?

It is a residual magnetization of the material in the absence of the external field. i.e., it

measures the strength of the ferromagnetic material as a permanent magnet.

74. What is coercive field?

If a permanently magnetized magnetic material has to be taken back to its original zero

magnetization configuration, an external magnetic field has to be applied in the reverse

direction. As the field is increased in the reverse direction from zero, the magnetization

decreases from M, as shown by the portion cd in the hysteresis curve. The magnetization

becomes zero at H = - H. This field is called the coercive field.

75. What are soft magnetic materials?

Soft magnetic materials are materials which can be easily magnetized and demagnetized.

76. What are hard magnetic materials?

Hard magnetic materials are materials which are difficult to magnetize and

demagnetize. In hard magnetic materials, the rotation of the domain wall is very difficult.

77. What is A-site in the structure of ferrite?

In this structure, each divalent metal ion (mg) is surrounded by four O ions in a

tetragonal fashion and it is called A-site.

78. What is B-site in the structure of ferrite?

In this structure, each trivalent metal ion (Fe) is surrounded by six O ions in a

octahedral fashion and it is called B-site.

79. Give some applications of ferrites.

Soft ferrites such as MeO.FeO have many applications in high frequency

transformer cores and computer memories.

Soft ferrites are used in the computer hard disks and floppy disks, credit cards, audio

cassettes, video cassettes, recorder-head, transformer cores, etc,.

80. What is superconductivity?

Certain metals and alloys have almost zero resistivity (i.e., infinite conductivity) when

they are cooled to sufficient low temperatures. This phenomenon is called superconductivity.

81. What is transition temperature?

The temperature at which a normal conductor is converted into a superconductor is

known as transition temperature (or) critical temperature (T).

82. What is critical magnetic field?

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The critical value of the magnetic field which destroys the superconductivity is known

as critical magnetic field (H) and varies as a function of temperature.

83. What is critical current density?

It is the measure of the conductors current carrying capacity. Actually J is notmeasured, but it is calculated from the critical current I and dividing it by the specimens

cross-sectional area A.

i. i.e., J = I/A

84. What are ferrites and ferrox cubes?(Nov 2003)

Ferrites are the modified structure of iron with no carbon in which the magnetic moments

are of unequal magnitudes. They are made by two (or) more different kinds of atoms. Its general

formula is given by . Where is a divalent metal ion such as ,

etc.,

Ferrox cubes are the soft magnetic material for which the hysteresis loop will be in the form of

a narrow rectangle.

85. Give some applications of ferrites.

a. Soft ferrites such as MeO.Fe2O3 have many applications in high frequency transformer

cores and computer memories.

b. Soft ferrites are used in the computer hard disks and floppy disks, credit cards, audio

cassettes, video cassettes, recorder-hard, transformer cores etc.,

86. What are the required magnetic parameters for recording?(Nov 2003)

The basic parameters required for recording are

i. Electromagnetic induction should occur in materials.

ii. The materials should easily acquire magnetism.

iii. It should posses magneto-resistance i.e.., the electrical resistance should vary with

respect to the magnetisation.

iv. Soft magnets should be used for temporary storage and hard magnets should be used for

permanent storage.

87. What is meant by magnetic bubble? How they are formed.(May 2005)

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Magnetic bubbles are soft magnetic materials with magnetic domains of few

micrometers in diameter. When the magnetic field is applied from the magnetic garnets like

Gadolinium Gallium garnet, Small cylindrical domain area know as magnetic bubble is formed.

These bubbles have a magnetic region of one polarity (either north (or) south) surrounded by

the other polarity.

88. What is superconductivity?

Certain metals and alloys have almost zero resistvity when they are cooled to sufficient

low temperatures. This phenomenon is called superconductivity.

89. What is transition temperature?

The temperature at which a normal conductor is converted into a superconductor is

known as transition temperature (or) critical temperature (Tc).

90. What is critical magnetic field?

The critical value of the magnetic field which destroys the superconductivity is known

as critical magnetic field and varies as a function of temperature.

91. What is BCS theory of superconductivity?

This theory states that the electrons experience a special kind of mutual attraction,

overcoming the coulomb forces of repulsion between them, as a result cooper pairs (i.e.,)

electron pairs are formed. At low temperature, these pairs move without scattering (I.e.,)

without any resistance through the lattice points and the material becomes super conductor;

here the electron lattice electron interaction should be stronger than electron electron

interaction.

92. What is Type II superconductors?

The material which loses its super conducting property gradually due to the increase in

magnetic field are called Type II super conductors.

93. Explain meissner effect?

When a super conducting material is kept in an external magnetic field under the

conduction when and , the magnetic flux lines are completly excluded from

the material and this phenomenon is known as Meissner Effect.

94. Mention any four property charges that occur in super conductor (or) what are the properties of a

super conductor.

a. They have Zero resistivity.

b. They exhibit perfect diamagnetism.

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c. The super conducting property can be destroyed due to the application of magnetic and

electric fields.

d. The transition temperature varies due to the presence of isotopes.

e. The entropy and specific heat decreases at transition temperature.

f. The elastic properties, crystal structure and thermal expansion remains constant.

95. Give any two applications of superconductors in engineering and medical field?

Engineering applications:

It is used in magnetic levitation, cryotrons, Josephson devices, SQUIDS etc .

It is used in computers transmission lines etc.

Medical application:

Super conductors are used in NMR (Nuclear Magnetic Resonance) Imaging

equipments

Superconducting solenoids are used in magneto hydrodynamic power

generation to maintain plasma in the body.

96. What is the principle of magnetic levitation?

Electro-magnetic induction is used as the principle (i.e.,) when there is a relative motion

of a conductor across the magnetic field, current is induced in the conductor and vice versa.

97. What are high temperature superconductors?

The super conductors having high transition temperature (Tc) values are called as High

Temperature Superconductors.

98. Give some properties of High Tc superconductors.

a. The transition temperature Tc is very high.

b. They have PEROVSKITE crystal structure.

c. They are direction dependent.

99. On the basis of spin how the materials are classified as para, ferro, anti ferro and ferri magnetic?

(Nov 2002,Nov 2003)

a. Paramagnetic materials have few unpaired electron spins of equal magnitudes.

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b. Ferro magnetic materials have many unpaired electrons spins with equal magnitudes.

c. Anti ferro magnetic materials have equal magnitude of spins but in anti parallel manner.

d. Ferri magnetic materials have spins in anti parallel manner but with unequal

magnitudes.

100. Give curie-Weiss law and its importance?(May 2003)

Curie-weiss law is given by

Where C - Curie constant

T - Absolute temperature- Curie temperature

Importance: It determines the susceptibility of the magnetic materials in terms of

temperature. i.e., if the temperature is less than Curie temperature, paramagnetic materials

become diamagnetic and if the temperature is greater than the Curie temperature, a

ferromagnetic material becomes a paramagnetic material.

101. 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.

102. What is Bohr magnetron? And Give its value.

Bohr Magnetron is the atomic unit of magnetic moment. Both the orbital magnetic

moment and the spin magnetic moment of an electron present in an atom are expressed in this

unit.

103. Define magnetic susceptibility.

The magnetic susceptibility is defined as the ratio of intensity of magnetization

produced in a sample to the magnetic field strength.

104. Define magnetic permeability.

The magnetic permeability is defined as the ratio of the magnetic flux density in the

sample to the applied magnetic field intensity. It is a measure of the amount of magnetic lines

of forces penetrating through a material.

105. Define relative permeability.

Relative permeability is defined as the ratio between the permeability of the medium

and permeability of free space.

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106. Give any two properties of diamagnetic materials.

a. Diamagnetic materials repel the magnetic lines of force. The behavior of diamagnetic

materials in the presence of the magnetic field.

b. The magnetic susceptibility is negative and is independent of temperature and applied

magnetic field strength.

107. What do you understand by the terms magnetic domains and domain walls? (Nov

2002,2003)

Magnetic domains are the small regions in ferromagnetic materials which has group of

atoms. These atoms can be completely magnetized by favorable exchange spin-spin interaction.

The walls of these small regions (or) domains are called domain walls.

108. What are soft and hard magnetic materials? (Or)Compare the soft and hard magnetic materials

on basis of hysteresis loop. give examples.(April 2002,Nov 2002,2003,Nov 2005)

S.No Soft magnetic materials Hard magnetic materials

1

2.

3.

4.

5.

They can be easily magnetized and

demagnetized

Movement of domain wall is easy and

hence even for a small applied field

large magnetization occurs.

The hysteresis loop is very steep.

Loop area is less and hence the

hysteresis loss is minimum

Examples: Iron, Silicon alloys,

Ferrites, Garnets etc.

They cannot be easily magnetized (or)

demagnetized.

Movement of domain wall is not easy due

to the presence of impurities and hence

large field is required for magnetization.

The hysteresis loop is very broad.

The loop area is large and hence the

hysteresis loss is maximum

Carbon steel, tungsten steel and

chromium steel. Cu-Ni-Fe (Cunife)

109. Define energy product and give the importance in the case of permanent magnets?(May 2003)

The product of retentively ( ) and the coercively ( ) is known as energy product. It

represents the maximum amount energy stored in the specimen.

Importance: It gives the maximum energy stored in the magnets which helps in

distinguishing a weak and strong magnet. Therefore for permanent magnets the value of energy

product should be very high.

110. What are dielectric materials?

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The dielectric materials are non-metallic materials of high specific resistance and have

negative temperature coefficient of resistance. The function of dielectric materials is to store

electrical energy,

111. What is electric polarizability?

When an electric field is applied for a dielectric material, electric charges are displaced

causing polarization in it. The polarization P of an elementary particle is directly proportional to

the strength of electric field (E). The polarizability is defined as the polarization per unit electric

field.

112. What is dielectric constant?

Dielectric constant is measure of the degree to which a medium can resist the flow of

charge. The dielectric constant of a material is defined as the ratio of permittivity of the medium to

the permittivity of free space.

113. What is electric susceptibility?

The polarization vector P is directly proportional to the total electric flux density e and is in

the same direction of e. Therefore polarization vector can be written as P E

114. What is electronic polarization (OR) Define electronic polarisation?

This is the polarization that results from the displacement of the positively charged nucleus

and the negatively charged electron of an atom in the opposite directions on the application of an

electric field.

The induced dipole moment

Where, - electronic polarization

115. What is ionic polarization?

This polarization is the result of displacement of the positively charged ions (cations) and

negatively ions (anions) in the ionic crystals in the opposite directions on the application of an

electric field. This type of polarization is produced in ionic molecules such as Nacl, KBr, KCl, and

LiBr.

116. Mention any two active and passive dielectrics with there applications?

S.No Active dielectrics Passive dielectrics

1.

2.

3.

Dielectrics which can easily adapt itself

to store the electrical energy in it is called

active dielectrics.

Examples: Piezo electrics Ferro electrics,

Pyro electrics

It is used in the production of ultrasonics

Dielectrics which restricts the flow of

electrical energy in it are called passive

dielectrics

Examples: Glass, mica, plastic.

It is used in the production of sheets,pipes

etc.,117. What is meant by loss tangent? How does it occur?

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The power loss , where is called as loss tangent and is called loss angle.

This dielectric loss mainly occurs due to the imaginary part of complex dielectric constant.

118. What is meant by dielectric strength?

Dielectric strength: It is the minimum strength (voltage) required per unit thickness of the

dielectric material to produce dielectric breakdown.

119. Explain electrochemical breakdown in dielectric?

This type of breakdown occurs due to the presence of coirs and their mobility. This impurity

causes leakage current and energy loss in the materials. Thus, when the temperature of the dielectric

material is increased, due to the presence of coirs the chemical reaction is accelerated and hence the

material becomes conducting, causing electrochemical breakdown.

120. Mention the various dielectric breakdown mechanisms?

a. Intrinsic breakdown

b. Thermal break down

c. Discharge break down

d. Electrochemical break down

e. Defect break down

121. What are the ways in which the dielectric breakdown can be minimized?

The dielectric break-down can be minimized by selecting the materials with following

properties.

i. It should posses high dielectric strength and dielectric loss.

ii. It should have less density and thermal expansion.

iii. It should have resistivity and resistivity and sufficient mechanical strength.

iv. It should be pure.

v. It should not possess any defects.

122. What is the effect of temperature on polarization?

The electronic and ionic polarizations are independent of temperature. The orientation

polarization decreases with increases in temperature where as the space charge polarization increases

with increases in temperature.

123. What is the effect of frequency of an a.c field on polarization?

In an a.c field, the polarization decreases with increases in frequency. At optical frequencies

electronic polarization is present where as orientation and space-charge polarization will be absent. The

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ionic polarization is present at infrared frequencies. The orientation and space charge polarization will

occur at only radio and audio frequencies.

124. What is electric field strength?

Electric field strength at any point is define as the force experienced by a unit positive charge atthe point

E= =

125. What is displacement vector?

Electric displacement vector (D) at any point is defined as electric flux passing normally

through the unit cross section at that point.

D=

126. What is electric dipole moment?

Two electric charges of equal magnitude and opposite polarizability are separated by some

distance is called an electric dipole. Dipole moment is equal to the product of one of the charges and

distance of separation between them.

Dipole moment () =q1(coulomb/meter)

127. What is electric polarizability?

When electric field is applied for a dielectric material, electric charges are displayed causing

polarization in it. The polarization P of an elementary particle is directly proportional to the strength of

electric field (E). The polarizability is defined as the polarization per unit electric field.

=

Where, is proportionality constant known as polarizability.

128. What is dielectric constant?

Dielectric constant is a measure of the degree to which a medium can resist the flow of charge.

The dielectric constant of a material is defined as the ratio of permittivity of the medium () to the

permittivity of free space (0)

Dielectric constant r=

i.e., r=

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129. What is electronic polarization?

This polarization that results from the displacement of the positively charged nucleus

and the negatively charged electrons of an atom in the opposite directions on the application of

an electric field.

130. What is ionic polarization?

The polarization is the result o displacement of the positively charged ions (cations) and

negatively charged ions (anions) in the ionic crystals in the opposite directions on the

application of an electric field. This type of polarization is produced in ionic molecules such as

NaCl, KBr, KCl and LiBr

131. What is orientation polarization?

The orientation polarization arises due to the existence of permanent dipole moment in

the dielectric medium in the absence of electric field. Certain dielectric materials have

permanent electric dipole even in the absence of electric field. Such substances are called polar

dielectric material. Examples are CO, H O, and HCl etc..,

132. What is internal field?

The internal field Ei, which is defined as the electric field acting at the location of a

given by the sum of the electric field created by the neighboring atoms and the applied field.

This field is responsible for polarizations of the individual atoms are molecules.

133. What is dielectric loss?

When a dielectric material is subjected to the a.c.voltage, the electric energy is absorbed

by the material and dissipated in the form of heat. The part of energy is lost or wasted, as no

useful work is done by it, this dissipation of energy is called dielectric loss.

134. What is dielectric breakdown?

When an electric field is applied to the dielectric and if the electric field is increased

solely, when the applied field increased the critical field, the dielectric material losses its

resistivity and permits very large current to flow through it. Then the phenomenon is called

dielectric breakdown.

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135. Give some applications of dielectric materials.

Quartz crystal is used for the preparation of ultrasonic transducers, crystals oscillators,

delay linesetc.

Barium titanate is used for the preparation of accelerometer.

The insulating dielectric liquids are used in transformers, switch gears and generators.

136. What are ferroelectric materials?

In certain dielectric materials, polarization is not a linear function of applied electric field.

Such materials exhibit hysteresis curve similar to that of ferromagnetic materials and are known

as ferroelectric materials.

137.Give some application of ferromagnetic materials.

i. They are used in production of ultrasonic waves

ii. They are used as transducers where mechanical energy is converted in to electrical

energy.