Download Short Answers

BBD NIIT PHYSICS
SHORT QUESTIONS WITH ANSWERS (UNIT – I)
CRYSTAL STRUCTURE AND X RAY DIFFRACTION
1.
2.
3.
4.
5.
6.
7.
8.
Define Space Lattice.
Ans. A space lattice is defined as an infinite array of points in three dimensional space in which each lattice point has identical
surroundings in all respect.
What is Lattice?
Ans. Lattice is a regular periodic array of points in space that looks a net like structure. It has the environment about any particular
point which is in every way the same as that about any other point. It is a mathematical concept.
r’ = r + T = r + n1a + n2b + n3c
Define basis.
Ans. Basis (or motif) consists of an atom (or molecule or ion) or a group of atoms. It is identical for every lattice points in terms of
composition, arrangement, relative orientation and separation.
What is crystal structure?
Ans. The crystal structure is formed only when a basis of atoms is attached identically to each lattice point.
Crystal Structure = Lattice + Basis
Define unit cell.
Ans. Unit cell is the smallest component of the crystal which when stacked together with pure translation repetition reproduces the
whole crystal. It is mainly of two types:
(i)
Primitive Unit Cell – which contain only a single lattice point. It is the minimum volume unit cell.
(ii)
Non-primitive Unit Cell – which contains more than one lattice point.
(iii)
Wigner-Seitz Unit cell
What are lattice parameters?
Ans. Lattice parameters determines the form and actual size of the unit cell. A lattice is characterized by six parameters namely
(i)
Primitives denoted by a, b and c - the intercepts on the three crystallographic axes defines the dimensions of the unit cell.
(ii)
Interfacial angles denoted by α, β and γ - The angles between the three crystallographic axes.
Write the names of seven crystal systems.
Ans.
S. No.
Names of Crystal Systems
Axes
Angles
1
Triclinic
a≠b≠c
α ≠ β ≠ γ ≠ 90°
2
Monoclinic
a≠b≠c
α = γ = 90, β ≠ 90°
3
Orthorhombic
a≠b≠c
α = β = γ = 90°
4
Tetragonal
a=b≠c
α = β = γ = 90°
5
Hexagonal
a=b≠c
β = 90°, γ = 120°
6
Rhombohedral
a=b=c
α = β = γ = 90°
7
Cubic
a=b=c
α = β = γ = 90°
Define simple, body centered, face centered and base centered cells.
Ans. Simple or Primitive Type (P) – It has lattice point only at corners and each corner is common in eight cells. Each unit cell
contains one lattice point per unit cell.
Body centered Type (I) – It has lattice points at corners as well as at centre of each cell. Each unit cell contains two lattice points
per unit cell
Face centered Type (F) – It has lattice points at corners as well as at centre of each face of unit cell. Each unit cell contains four
lattice points per unit cell.
Base centered Type (C) – It has lattice points at corners as well as at centre of top and bottom face of unit cell. Each unit cell
contains two lattice points per unit cell.
9. What is Bravais space lattice?
Ans. According to A. J. Bravais, there are only 14 distinct lattice arrays in which point can be arranged such that each point has
identical surroundings. These are called Bravais space lattice.
10. Define coordination number.
Ans. Coordination number is defined as the number of nearest neighbours around any lattice point in the crystal lattice.
11. What do you mean by atomic radii in a crystal?
Ans. Atomic radius is defined as half the distance between nearest neighbours in a crystal of pure element.
12. Define atomic packing factor.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
Ans. Atomic Packing Factor (APF) or Atomic Packing Density is defined as ratio of the volume of the atoms per unit cell to the
volume of the unit cell.
What are Miller Indices?
Ans. Miller Indices are the three smallest possible integers which have the same ratios as the reciprocal of the intercepts of the
plane on the three crystal axes. Miller Indices are denoted by three numbers (h k l).
Define interplanar spacing in crystal structure.
Ans. The minimum or perpendicular distance between any adjacent planes of a set is called interplanar spacing.
Define lattice planes.
Ans. A crystal may be regarded as a stack of parallel, equally spaced planes of atoms passing through lattice points. These planes
are called lattice planes.
What is reciprocal lattice?
Ans. Reciprocal lattice is an infinite periodic three dimensional array of reciprocal lattice points whose spacing varies inversely as
the distances between the planes in the direct lattice of the crystal.
Mention some properties of NaCl Crystal Structure.
Ans. In NaCl
(i)
Each sodium atom is surrounded by six nearest neighbor Cl atoms and vice-versa.
(ii)
All electrons are tightly bound to the atoms.
(iii)
NaCl possess strong ionic bonding.
(iv)
NaCl possess orthorhombic crystal structure.
(v)
Coordination number of NaCl crystal is 6.
(vi)
Refractive index of NaCl is 1.5442.
Mention some properties of Diamond Crystal Structure.
Ans. Diamond exhibits
(i)
both cubic and hexagonal type structure.
(ii)
Unit cell of Diamond crystal contains 18 atoms. Of these 8 are placed at corners, one on each of the six faces and 4 inside
it.
(iii)
Tetrahedral coordinations of C-atoms.
(iv)
Space lattice of Diamond crystal is fcc lattice.
(v)
Coordination number of NaCl crystal is 4
(vi)
No. of atoms in the Diamond crystal unit cell is 8
Mention two properties of Reciprocal lattice.
Ans. In Reciprocal lattice
(i)
The primitive vectors have the dimensions of the inverse of length.
(ii)
Reciprocal lattice is in fourier space or k-space.
What are the facts of Laue’s experiment?
Ans.
Laue’s expt. Gives the following two important facts:
(i)
X-rays are electromagnetic waves of short wavelength.
(ii)
The atoms of a crystal are arranged in a regular three dimensional lattice.
What are the uses of Laue’s experiment?
Ans.
It is widely used for:
(i)
To determine the microscopic structure of matter in solid state.
(ii)
To determine the angular relationship between different planes in a crystal lattice which have large no. of atoms.
Write the properties of X-rays.
Ans.
(i)
X-rays are electromagnetic waves of short wavelength.
(iii)
They have ionizing power. When they are passed through gas, the gas is ionized.
(iv)
They have high penetrating power.
(v)
They are electrically neutral, hence they are not deflected by electric or magnetic field(vi)They show photoelectric effect.
What are the uses of X-rays?
Ans. They are used to study the internal structure of the crystals. X-rays have high penetrating power, due to this property they are
used to detect the fractures of bones, tuberculosis of lungs and presence of stones in the body. They are also used to detect goods
like gold, hidden parcels etc.
What is Bragg’s Law?
When X-rays are incident on the planes at an angle θ, the reflected beams will interfere. If the path difference is an integral multiple
of wavelength λ, the reflected beams will interfere constructively and gives the maximum intensity. Thus strong reflection will be
observed in the direction which corresponds to a path difference λ, 2λ, 3λ ………..etc. between the rays reflected from consecutive
planes. i.e.
2dSinθ = n λ. This equation is known as Bragg’s law. Where n is an integer and d is lattice spacing.
Unit II Dielectrics and its properties
Q.1 What is a Dielectric?
Ans. A dielectric material (dielectric for short) is an electrical insulator that can be polarized by an applied electric field. When a dielectric
is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from
their average equilibrium positions causing dielectric polarization.
Q.2 What are the polar and non polar molecules?
Ans.When the centre of positive and negative charges of a molecules coincide the molecule will not have a permanent dipole moment. Such
molecules are called non polar molecules. If the centre of positive and negative charges in a molecule do not coincide even in the absence of
external electric field, the molecule will have a permanent dipole moment. Such molecules are called polar molecules.
Q.3 Explain the term Dielectric Constant.
Ans. The dielectric constant is the ratio of the permittivity of a substance to the permittivity of free space.
K = ℰr = ℰ / ℰ0
Or
It is defined as the ratio of the capacitance of a condenser with dielectric to the capacitance of the same condenser without dielectric. K =
C/C0
Q.4 What is Polarization?
Ans. Polarization P in its quantitative meaning is the amount of dipole moment p per unit volume V of a polarized material, P = p/V.
Q.5 Write the relation between polarization (P) and dielectric constant (K).
Ans It is given by: P = ℰ0 E (ℰr -1)
where E is the electric field within the dielectric.
Q.6 What is electronic Polarization?
Ans. Electric polarization is slight relative shift of positive and negative electric charge in opposite directions within an insulator, or
dielectric, induced by an external electric field. Polarization occurs when an electric field distorts the negative cloud of electrons
around positive atomic nuclei in a direction opposite the field. This slight separation of charge makes one side of the atom
somewhat positive and the opposite side somewhat negative.
Q.7 What is Ionic Polarization?
Ans. Ionic Polarization is polarization which is caused by relative displacements between positive and negative ions in ionic crystals (for
example, NaCl). If crystals or molecules do not consist of only atoms of the same kind, the distribution of charges around an atom
in the crystals or molecules leans to positive or negative. As a result, when lattice vibrations or molecular vibrations induce relative
displacements of the atoms, the centers of positive and negative charges might be in different locations. These center positions are
affected by the symmetry of the displacements. When the centers don't correspond, polarizations arise in molecules or crystals.
This polarization is called ionic polarization.
Q.8 What is Orientational Polarization?
Ans. When the polar molecules are placed in external electric field, the randomly oriented dipoles tend to orient themselves along the applied
field. This tendency of orientation of dipoles along the field direction is known as Orientational or dipole Polarization.
Q. 9 What is an internal field?
Ans. The internal field or the local field may be defined as the electric field which acts at the location of a given atom subjected to an external
electric field and is the resultant of the applied field and the field due to al surroundings dipoles.
Q.10 What is Dielectric loss?
Ans. Dielectric loss is loss of energy that goes into heating a dielectric material in a varying electric field. For example, a capacitor
incorporated in an alternating-current circuit is alternately charged and discharged each half cycle. During the alternation of polarity of the
plates, the charges must be displaced through the dielectric first in one direction and then in the other, and overcoming the opposition that
they encounter leads to a production of heat through dielectric loss.
UNIT III
ELECTROMAGNETICS
Q.1 Write the Physical significance of Maxwell’s equations.
A.1 (i) Maxwell’s first equation represents the Gauss’s law in electrostatics, which states that the electric flux over a hypothetical closed
surface is equal to 1/ ε0 times the total charge enclosed within the surface.
(ii) Maxwell’s second equation represents the Gauss’s law in magneto statics, which states that net magnetic flux through any closed surface
is zero which means existence of magnetic monopole is not possible.
(iii) Maxwell’s third equation represents Faraday’s law of electromagnetic induction that electric field is produced by changing magnetic
flux.
(iv) Maxwell’s fourth equation represents modified form of Ampere’s circuital law. It is valid for both steady and time varying fields and
states that the magneto-motive force around a closed path is equal to the sum of conduction current and displacement current.
Q.2 What is the nature of electromagnetic waves?
A.2 The electromagnetic waves are transverse in nature which means that the electric and magnetic vectors are perpendicular to each other as
well as direction of propagation.
Q.3 What is the concept of Displacement current?
A.3 Maxwell showed that a changing electric field in vacuum or in dielectric also produces a magnetic field. So, a changing electric field is
equivalent to a current which flows as long as the electric field is changing and produces the same magnetic effect as an ordinary current.
This is known as Displacement current.
Q.4 Why Maxwell proposed that Ampere’s law require modification?
A.4 To resolve the paradox of charging capacitor and inadequacy of Ampere’s law for time varying field. Maxwell postulated that it is not
only the current in a conductor that produces a magnetic field, but a changing electric field in vacuum, or in dielectric, also produces a
magnetic field. Thus a changing electric field is equivalent to a current called Displacement current.
Q.5 What is equation of continuity?
A.5 Equation of continuity represents conservation of charge which means that electric charges can neither be created nor destroyed.
Continuity equation is represented as
which means that the net amount of charge in an isolated system remains constant.
Q.6 What is Poynting vector. State Poynting theorem.
A. 6 The amount of energy carried by plane electromagnetic wave is given by Poynting vector S=EXH Statement - The rate of energy
transfer (per unit volume) from a region of space equals the rate of work done on a charge distribution plus the energy flux leaving that
region.
Q.7 Write down the dimensions of the magnitude of Poynting vector.
A.7 The unit of Poynting vector is Watt/meter2. Dimensions of Poynting vector is MT-3.
Q.8 What happens to an electromagnetic wave when it enters a conducting medium?
A.8 The electromagnetic wave attenuated greatly while propagating through good conductor.
Q. 9 Define skin depth or depth of penetration.
A.9 The skin depth or depth of penetration is defined as the depth at which the strength of electric field associated with the electromagnetic
wave reduces to (1/e) times its initial value.
Q. 10 Define propagation constant.
A .10 The propagation constant for an electromagnetic wave propagating through a conducting media is complex quantity and given by k = α
+ jß. The real part α represents attenuation constant and imaginary part ß represents phase constant.
UNIT-IV
SEMICONDUCTORS
Q.1 Define Fermi level and Fermi energy.
A.1 Fermi level is the energy that corresponds to the centre of gravity of the conduction electros and holes weighted according to their
energies. Fermi energy may be defined as the maximum energy that any electron may possess at absolute zero.
Q.2 Define density of states.
A.2The density of states represents the number of states within a given arbitrarily narrow interval of energy that the electron may possess.
Q.3 Define valence band, conduction band and forbidden energy gap.
A.3 The range of energies possessed by valence electron is called as valence band. The range of energies possessed by valence electron is
called as valence band. The separation between conduction band and valence band on energy level diagram is known as forbidden energy
gap.
Q.4 What are intrinsic semiconductors?
A.4 A semiconductor in an extremely pure form is known as intrinsic semiconductor. In intrinsic semiconductor, the number of conduction
electron is equal to the number of holes.
Q.5 What are extrinsic semiconductors?
A.5 When a small amount of trivalent or pentavalent impurity is added to a pure semiconductor crystal, the resulting crystal is called extrinsic
semiconductor. Extrinsic semiconductors are of two types: n-type and p-type.
Q.6 How the Fermi level in intrinsic semiconductor depends on temperature?
A.6 Fermi level in an intrinsic semiconductor lies exactly in the middle of forbidden energy gap between the valence band and the conduction
band. The Fermi level in an intrinsic semiconductor is independent of temperature.
Q.7 How the Fermi level in extrinsic semiconductor varies with temperature?
A.7 In extrinsic semiconductors as the temperature increases, the Fermi level progressively moves towards the middle of forbidden energy
gap.
Q.8 What do you mean by conductivity of semiconductors?
A.8 The conductivity of a material is defined as the property of a material that enhances the flow of current through it. The conductivity of a
semiconductor depends on the mobilities of charge carriers as well as on their densities or concentrations.
Q.9 Write the expressions for conductivity of intrinsic semiconductor, N-type semiconductor and P-type semiconductor.
A.9 The conductivity of intrinsic semiconductor is
𝜎i = ne [μe + μh ]
The conductivity of N-type semiconductor is
𝜎n = ene μe
The conductivity of N-type semiconductor is
𝜎p = enh μh
Q.9 Define drift velocity and mobility.
A.9 The electrons can move from one place to other with random velocity. When an external field is applied then in addition to random
velocity,
an
additional
velocity
is
associated
with
each
electron
known
as
drift
velocity
(v d).
The mobility is defined as the drift velocities attain by charge carriers in a conductor when unit electric field is applied.
Q.10 How conductivity of semiconductors varies with temperature?
A.10 As the temperature increases the conductivity of semiconductor increases exponentially.
SUPERCONDUCTIVITY
Q.1 what is superconductivity?
A.1 Superconductivity is a phenomenon, which occurs in certain materials where the resistivity suddenly drops to zero.
Q.2 what is Meissner effect?
A.2 When a material makes transition from normal to superconducting state, it excludes magnetic field from its interior. This is called
Meissner effect.
Q.3 Define transition temperature.
A.3 The temperature at which a substance becomes superconducting is called transition temperature or critical temperature.
Q.4 What is critical magnetic field?
A.4 The minimum value of applied magnetic field when the superconducting material losses its superconductivity is called critical magnetic
field.
Q.5 What are type I superconductors?
A.5 Type I superconductors are generally pure metals. The field is abruptly destroyed when the strength of applied magnetic field rises above
the critical magnetic field.
Q.6 what are type two superconductors?
A.6 Type II superconductors comprises of alloys or intermetallic compounds. They exist in mixed state of normal and superconducting
regions. e.g. alloys of lead-bismuth .
Q.7 What is the effect of temperature of entropy in superconductors?
A.7 Superconducting state is more ordered state since the entropy decreases with temperature below the critical temperature.
Q.8 What is Josephson effect?
A.8 Josephson Effect is the phenomenon in which current flows across two weakly coupled superconductors separated by a thin insulating
material.
Q.9 Describe the features of BCS theory in brief.
A.9 BCS theory explains the sudden vanishing of resistivity of certain metals at low temperatures. It proposes that electrons with opposite
spins pair up to form cooper pairs. This pairing arises from the attraction between electrons related to lattice vibrations .These coupled
electrons can take the character of boson and condense into ground state which leads to superconductivity.
NANOMATERIALS
Q.1Define nanoscience and nanotechnology?
A.1 Nanoscience is the science in which we study the materials in size ranging from 1nm to 100nm. Nanotechnology is the engineering
which deals with the design, characterization, production and application of materials on nanoscale.
Q.2 What is fullerene?
A.2 The fullerenes are a class of allotropes of carbon which conceptually are grapheme sheets rolled into tubes or spheres. Graphene is a oneatom thick layer of graphite.
Q.3 Explain the structure of bucky ball.
A.3 Bucky ball is a hollow cluster of carbon atoms shaped like a football in which each carbon atom is bounded to three adjacent carbon
atoms and arranged in a sphere about a nanometer in diameter. It has carbon atoms at 60 chemically equivalent vertices that are connected by
32 faces, 12 of which are pentagonal and 20 hexagonal.
Q.4 What are nanomaterials?
A.4 Nanomaterials are materials possessing grain size of the order of a billionth of a meter. Materials canbe produced at nanoscale in one
dimension, in two dimension or in all three dimensions.
Q,5 How buckyballs are synthesized?
A.5 Buckyballs canbe created by vaporizing carbon placed between two graphite rods which are placed in low preesure. He atmosphere in a
reaction chamber. When electric discharge is generated between the electrodes, tremendous amount of energy is created which transferred to
carbon. The carbon is evaporated to form bucklyballs.
Q.6 What are the different types of carbon nanotubes?
A.6 Nanotubes are of two types
(i) Single walled carbon nanotube (SWNT)- A nanotube in which a one atom thick layer of graphite is wrapped into a seamless cylinder.
(ii) Multi walled carbon nanotube (MWNT)- A nanotube consists of multi layers of graphite rolled in on themselves to form a tube.
Q.7 How buckyballs are used as antioxidant?
A.7 Bucklyballs can be used as antioxidant to counteract free radical in human body. A free radical is a molecule has unpaired electron and
become very active. When another electron provided by antioxidant to free radical, it become inactive.
Q.8 Write two properties of nanotubes .
A.8 (i) Carbon nanotubes are the strongest and stiffest materials on the earth.
(ii) all nanotubes are good thermal conductor along the axis but good insulator along radial.