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CHAPTER 1
BASIC ELECTRIC CIRCUITS
Q. Define electric current and state its S.I. unit.
Ans: Electric current: Electric current is defined as the flow of electric charge
per unit time. ORIt is the rate of flow of charge.
Let Q be the electric charge, flowing in time tsecond, then I is given by
Charge
Electric current = _________Time
QI = _____
TCoulomb
The unit of current = ____________ = ampereSecond
The S.I. unit of current is ampere and symbol is A
Q. State Ohms law and state its equation with usual meaning of each
symbol.
Ans: Ohms law:When the physical state of metallic conductor remains constant,
the electric current flowing through the conductor is directly proportional to
the potential difference across it.
(Physical state: material, length, cross sectional area, temperature)
I V
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V = constant x I
V___ = constant x I or V= IR
I
Where,
R =Resistance of conductor, i.e. in ohm.
V =Potential difference across it, i.e. in volt.
I =Current flowing through it, i.e. in ampere.
Q. Define specific resistance (or resistivity) of a material and state its S.I.
unit.
Ans: Specific resistance:Specific resistance (or resistivity ) of a given material
of wire is defined as a resistance of a wire of unit length and unit cross
sectional area. It is denoted by
S.I. unit of specific resistance = ohmmeter = m.
Q. Define specific conductance (or conductivity) and state its unit.
Ans: Specific conductance: The reciprocal of specific resistance is called
specific conductance.
1It is denoted by = ____
The S.I. unit of specific conductance is S/m.
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Q. What is the principle of Wheatstone network? Describe with neat
diagram, Wheatstone network and obtain the balancing condition. OR
Obtain the balancing condition for Wheatstone network.
Ans: Principle of Wheatstone network:It network is an arrangement of four
resistances used for measuring one of them in terms of other three under
balancing conditions.
Under balancing condition
R1 R3
___ = ______
R2 R4
Wheatstones network is an arrangement in which resistances R1and R2are
concected in series. Similarly R3, R4are connected in series and then these
tow series combination are connected in parallel. Across AC, battery cell is
connected, and across BD galvanometer is connected.
In this network R1, R2, R3are kept constant and R4is adjusted in such a way
that the galvanometer shows zero deflection.
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When galvanometer shows zero deflection then network is said to be
balanced.
In balanced condition point B and D are at equal potential and this is
possible if
P.D. across AB = P.D. across AD and
P.D across BC = P.D. across D.C.
By using ohms law.
I1R1= I2R3
And I1R2= I2R4
Dividing equation (1) by equation (2), we get
I1R1 I2R3
________ = _______
I1R2 I2R4
R1 R3
_______ = _______R2 R4
Q. State and explain the principle of potentiometer.
Ans: The principleof potentiometer:The potential difference across any length
of the potentiometer wire is directly proportional to its length.
OR
The fall of potential is directly proportional to the length of the
potentiometer wire.
V l
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Consider uniform wire AB of length I having cross sectional area A.
Let R be the resistance of the wire and V be the potential difference
across its ends.
RAV = IR (1) Where = _______
l
l
R = _____A
l
Equation (1) becomes V = I ______A
For given circuit current I, specific resistance and cross sectional Area A
remains constant.
I
V = k. lwhere K = _____A
V 1
This is principle of potentiometer.
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Q. Define potential gradient & state its SI unit.
Ans: Potential gradient: The potential gradient is defined as fall of potential per
unit length of wire:
Potential
PG = ___________
Length
Unit of P.G = V/m.
Q. Mention the uses of potentiometer.
Ans: Uses of Potentiometer
1.
To determine emf of cell.
2.
To determine accurately the potential difference across the component.
3.
To compare the emfs of two cells.4. To find internal resistance of a cell.
Q. What is capacitance? State its S.I. unit.
Ans: The ratio of the charge on either conductor of the condenser to the potential
difference between them is called capacity or capacitance of condenser.
Q
i.e. Capacitance, C = ______
V
The S.I. Unit of capacitance is farad(F).
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Q. Define: a) Condenser or Capacitor b) 1Farad.
Ans: a) Condenser or Capacitor:The arrangement of two parellal metal plates
separated by small distance in which large amount of electric charges can
be stored at low potential is called as condenser or capacitor.
b) 1Farad: When a charge of 1-coulomb is given to plate of condenser to
raise its potential by 1 volt, then the capacity of condenser is said to be 1-
farad.
Q. State &explain the principle of condenser.
Ans: Principle of condenser: The capacity of charged conductor increases, if
another conductor is kept near it, which is earthed.
Explaination:As shown in fig. (a), metal Plate-P and Pare separated by
small distance (d). When an electric charge + Q is given to Plate-P, the equal
amount of Q charges are induced on inner side of plate P while equal
amount of +Q charges are induced on outer side of Plate-P
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The chargesQ decrease the potential of PlateP (V-V) while charges +Q
of Plate P- increases the potential of Plate P (V-V+V), so that there is
no effect on potential of plateP.
But when Plate -p is earthed, then the +Q charges of plate p are absorbed
by the earth so thatQ charges decrease the potential of PlateP (V-V)
(V-V) < V
The new capacity of condenser is given by.
Q
C = ________
(V-V)
As potential V is decreased by V so more charge can be stored on
condenser.
In this way more electric charge can be stored, at low potential by
arrangement of plates. This is the principle of condenser.
Q. State the factors affecting capacitance of a condenser
Ans: The capacity of condenser depends upon following factors.
1. Area of each plate
2. Dielectric of medium
3.
Distance between plates
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Q. State the principle of parallel plate condenser.
Drive an expression (equation) for capacity of parallel plate condenser.
OR
A0k
Show that, C = _______, in case of parallel plate condenser.
D
Where symbols have usual meaning?
Ans: Principle: The capacity of charged conductor increases, if another
conductor is kept near it, which is earthed.
Consider two metal plates A and B as shown above,
Let A = Area of each plate
d = Distance between two plates
+ Q = Charge given to A
Q = Charge induced to inner side of B
V = Potential difference between two electrodes
K = Dielectric constant of medium
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The electric flux density (D) between two plates is given by.
D = 0k.E ----------------- equ. (1)
Where,
0permittivity of air (free space)
kdielectric constant of medium.
EElectrical intensity .
Q V
But flux density, D = ______ and E = ______A d
The equn(i) becomes
Q V
______ = 0 k _______
A d
V
Q = 0 k ________ Ad
Where, V potentialP.D. between plate P and P
Q 0kA
_______ = _________V d
Q
But _____ = CV
A0k
C = ________ -------------------------------------- equn. (2)d
equation (2) represents capacity of parallel plate condenser
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1 1 1 1
Q. Prove that, ____ = _____ + _____ + ______, for condensers in series:
CS C1 C2 C3
OR
Derive the relation for the effective capacitance, k if three condensers of
capacities C1, C2and C3are connected in series. OR
Obtain an expression for effective capacitance when condensers are
connected in series
Ans: When the condensers are connected one after another in the circuit, then the
condensers are said to in series.
Consider three condensers C1, C2& C3are connected in series between twopoints A and B with potential difference of V volt.
When condensers are connected in series the total charge on each condenser
remains the same and the potential difference across each condenser gets
divided into three parts V1, V2 & V3which depends on values of capacitor
V=V1+V2+V3
Q
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But C =
V
Q
But V= C
Q
Potential Difference across C1is V1=C1
QPotential Difference across C2is V2=
C2
QPotential Difference across C3is V3=
C3
Q
Potential Difference across CSis VS=CS
Q Q Q Q= + +CS C1 C2 C3
1 1 1 1
= + +
CS C1 C2 C3
The reciprocal of equivalent capacity of condensers in series
combination i s equal to the sum of the reciprocal of individual capacity of
condensers.This is known as law of condenser in ser ies.
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Q. Obtain an expression for the resultant capacitance when condensers are
connected in parallel OR
Prove that, CP= C1+ C2+ C3, for condensers in parallel.
Derive the relation for effective capacitance, if three condensers of
capacities C1, C2, C3are connected in parallel.
Ans: When numbers of condensers are connected between two common points so
that potential difference across each condenser is same, then the condenser
are said to be in parallel.
Let C1, C2 and C3 be the capacitance of three condensers connected in
parallel. Let V be the potential difference across each condenser.
Let Q be the electric charge flowing through the circuit. At point A, the
electric charge Q is splitted into electric charge Q1, Q2 and Q3 flowing
through the condensers C1, C2and C3respectively.
At pointA
Q = Q1+ Q2+ Q3 ------------------------------- Eqn. (i)
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But Q1= C1V1 But Q2= C2V and Q3 = C3V
Equn(1) becomes
Q = C1V + C2V + C3V
= V (C1+ C2+ C3)
Q
______ = C1+ C2+ C3 ------------------- Eqn. (ii)
V
If the parallel combination of condensers is replaced by signal condenser of
capacity CPthen
Q
Q = CpV ______ = CpV
Equation (2) becomes
CP= C1+ C2+ C3 ------------- Eqn(iii)
The equivalent capacity of condensers in parall el combination is equal to
the sum of the capacity of each condenser.This is known as law of
condensers in paral lel.
Eqn. (iii) represents the equivalent capacity of condensers in parallel
combination.
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CHAPTER 2
SEMICONDUCTOR PHYSICS
Q. What are semiconductors?
Ans: Semiconductors are the material whose conductivity lies in between
conductor & insulator. They are neither good conductors nor insulator at
room temperature. It is a substance which has almost filled valence band &
nearly empty conduction band with a very small energy gap of 1 ev.
Ex.Germanium, silicon, lead, carbon.
Q. What are Intrinsic & Extrinsic Semiconductors?
Ans: Intrinsic semiconductor:A semiconductor which is in extremely pure form
is called as intrinsic semiconductor.
Ex. Silicon, Germanium
Extrinsic semiconductor:A semiconductor with impurity added, is called
as extrinsic semiconductor.
Extrinsic semiconductor is of P-type & N-type semiconductor.
P-type: In P-type, impurities added are gallium, indium, boron,
aluminium
N-type:In N-type, impurities added are arsenic, antimony, phosphorous
Q. What is doping of semiconductor?
Ans: The process of addition of impurity to the intrinsic semiconductor is called
as doping of semiconductor. The need of doping is to increase the
conductivity of intr insic semiconductor .
Ex.Aluminium is added as impurity in pure germanium.
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Q. What are Acceptor (trivalent) impurities & Donor (pentavalent)
impurities?
Ans: Acceptor Impurities:The element from 3rdgroup like boron (B), Aluminium
(Al), Gallium (Ga), Indium (In) etc. accept electron from puresemiconductor are called as acceptor impurities.
Donor Impurities:The elements from 5stgroup like Arsenic (As), Antimony
(sb), Phosphorous (P) etc donate electron to pure semiconductor are called
as donor impurities.
Q. Name two donors (pentavalent) & two acceptor (trivalent) impurities.
Ans: Donor (pentavalent) impurities are
1. Phosphorous 2. Arsenic 3. Antinomy
Acceptor (trivalent) impurities are
1. Boron 2. Aluminium 3. Indium
Q. What are majority & minority carriers in P-type & N-type
semiconductor?
Ans: In P-type semiconductor
1.
Majority carriersHoles
2. Minority carriersElectrons
In N-type semiconductor
1. Majority carriersElectrons
2. Minority carriersHoles
Q. Define P-type & N-type semiconductor
Ans: P-type semiconductor: When a small amount of impurity from 3rd group
element added to pure semiconductor, it is called P-type semiconductor.
N-type semiconductor: When a small amount of impurity from 5 thgroup
element is added to pure semiconductor, it is called N-type semicond
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Q. What is the effect of temperature on conductivity of semiconductor?
Ans: When temperature of semiconductor increases, the number of free electrons
increases and hence current increases through the semiconductor so the
conductivity of semiconductor increases, as temperature increases.
Q. Why semiconductors have negative temperature coefficient of
resistance? OR Explain the effect of temperature on intrinsic
semiconductors
Ans: At absolute zero temperature semiconductor behaves like insulators. At
room temperature because of thermel energy, few electron hole pairs are
generated which constitute a small current i.e. it has small conductivity.Further,if the temperature of semiconductor increases its conductivity
increases. It means as temperature of semiconductor increases its resistance
decreases. Therefore semiconductors have negative temperature coefficient
of resistance.
Q. Define conduction band, valence band & forbidden energy gap.
Ans: Valence band:The band which is occupied by valence electrons or a band
having highest band energy is called valence band. OR The range of energy
possessed by valence electrons is called as valence band.
Conduction band:The band which is occupied by conductivity of electrons
or the lowest unfilled energy and is called conduction band. OR The range
of energy possessed by conduction electrons is called as conduction band.
Forbidden energy gap:There is a energy gap between valence band and
conduction band is called Forbidden energy gap.
For EGWood > 5eV, EGcopper = 0eV
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Q. Classify the solids into conductors, semiconductors and insulators on
the basis of band theory of solids.
OR Distinguish between conductors semiconductors and insulators
Ans: On the basis of band theory of solids, the solids are classified into three
groups:
1 Conductors 2 Semiconductors 3 Insulators
Parameter Conductor Insulator Semiconductor
Conductivity Very high Very low Moderate
Resistivity Very low Very high Moderate
Forbidden
gap
No forbidden gap Large forbiddengap ( Eg= 6 eV )
Mediumforbidden gap
( Eg = 1 to 2 eV )
Number of
electrons
available for
conduction
Very large Very small Moderate
Applications
Examples
Conductor,wires,
bus bars
Aluminium,
copper
Capacitor,
insulation for
wiresPaper,mica,glass
Semiconductor
devices
Silicon.germenium
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Q. Define energy band. Draw energy level (band) diagram for conductors,
insulators and semiconductors.
Ans: The range of energies possessed by an electron in solids, is called energy
band.
The energy level diagrams are as follows:
Q. Write the properties of semiconductor.
Ans: Properties of semiconductor:
1. The conductivity of a semiconductor is less than that of a conductor but
more than that of an insulator.
2. Semiconductors have negative temperature coefficient of resistance
3.
When some suitable impurity element (e.g. Al, Ga) is added to a
semiconductor, its current conducting properties increases.
4.
The forbidden energy gap in semiconductors is about 1eV
5. The conduction band is partially filled at room temperature.
6. The valence band is partially filled at room temperature.
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Q. Why intrinsic semiconductor behaves like insulator at absolute zero (k
or2730C) temperature?
Ans: A pure semiconductor is intrinsic semiconductor. Germanium and silicon is
intrinsic semiconductor. The crystal structure of Germanium is shown in fig.
Germanium atom has four valence electrons. For stable structure, each atom
shares one electron with each of its four neighboring atom in covalent bonds.
So that all covalent bonds are filled with electrons.
At absolute zero temperature, covalent bonds are so strong that electrons are
not available for conduction of electric current through it. Hence at absolute
zero temperature, semiconductor acts as insulator.
Q. Distinguish between P-type semiconductor and N-type semiconductor.
Ans:
P- type N-type
When small amount of impurity from
3
rd
group element is added to puresemiconductor, it is called a P-typesemiconductor.
When small amount of impurity from
5
th
group element is added to puresemiconductor, it is called as N-typesemiconductor.
In this type majority carriers are
holes
In this type majority carriers are
electrons
In this type electrical conduction takeplace due to hole
In this type electrical conduction takeplace due to electrons
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The impurity used in this type iscalled acceptor impurity
The impurity used in this type iscalled donor impurity
Trivalent impurities are boron,
gallium, indium etc.
Pentavalent impurities are arsenic,
phosphorous antimony
Impurity atoms are trivalent Impurity atoms are pentavalent
Q. Differentiate between intrinsic semiconductor & extrinsic
semiconductor.
Intrinsic semiconductor Extrinsic semiconductor
A semiconductor which is in
extremely pure form is called asintrinsic semiconductor
A semiconductor with impurity
added, is called as extrinsicsemiconductor
There is no doping There is dopingNumber of holes are equal to number
of electrons
In N-type no. of electrons more & in
P-type no. of holes are more
Here electrons & holes are thermallygenerated
Here electrons and holes aregenerated due to doping
Conductivity is less Conductivity is more
It is neither p-type no N-type
semiconductor
It is either P-type or N-type
semiconductor
Q. How P-type semiconductor is formed? OR Explain the covalent bonding
in P-type semiconductor.OR Describe P- type semiconductor in detail
Ans: P-type semiconductor: When a trivalent impurity like boron, aluminum,
gallium, indium etc is added to a pure semiconductor, the resulting crystal is
called as P-type extrinsic semiconductor.
1.
As shown in fig. when trivalent boron is added to pure germanium, thethree electrons of boron form covalent bond with the valence
electronsof three germanium atom.
2.
The fourth covalent bond is incompleteas boron atom has only three
valence electrons and there is a deficiency of one electron which is
called hole.
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3. As hole is positively charged as it represent the absence of negative
charge.
4.
Now the remaining fourth electron of the germanium also tries to form a
covalent bond.
5.
The holes are moving randomly due to thermal effect.6. Thus corrresponding to each trivalent impurity added, a hole is created
7. The increased number of holesmake the semiconductor P- type
8.
In this way p-type semiconductor is formed.
Q. How N-type semiconductor is formed? Explain the covalent bonding in
N-type semiconductor. OR Describe N- type semiconductor in detail
Ans: N-type semiconductor:When pentavalent impurities like arsenic, antimony,
phosphorous are added to a pure silicon or germanium atom, then resulting
crystal is called N-type semiconductor.
1. As shown in fig. let us consider, in pure silicon if a pentavalent
impurity like arsenic is added. Then the four electronsof the arsenic
atom form covalent bond with the four valence electrons of silicon
atom.
2.
The fifth electron of arsenic atom is not covalently bonded, but it is
loosely boundedto the arsenic atom.
3. By increasing the thermal energy, this electron can easily be excited
from the valence bandto the conduction band.
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4. Thus corresponding to each impurity atom (Arsenic) an extra electron
becomes available for conduction
5.
This increasesd number ofelectronsmake the semiconductor as N- type
Q. Explain the working of p-n junction diode in forward biased.
Ans: When p-type region of junction diode is connected to positive terminal of a
battery and n-type region to the negative terminal, then p-n junction diode is
said to be in forward biased.
The p-n junction diode is connected in forward biased shown in fig . with
block diagram
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In forward biased:
1. The positive terminal of battery repels the majority current carriers
(holes) in p-type region and forcesthem towards the junction.
2.
Similarly negative terminal of battery repelsmajority current carriers(free electrons) in n-type region and forcesthem towards the junction.
3.
So holes in n-type and electrons in p-type neutralize in depletion layer.
4.
Hence the depletion layer disappearsremoving the barrier potential and
p-n junction diode supports a continuous flow of current.
In this way, p-n junction diode in forward biased.
Q. Explain the working of p-n junction diode in reverse biased..
Ans: When p-type region of junction diode is connected to negative terminal of
batter and n-type region to positive terminal, then the p-n junction diode is
said to be in reverse biased.
The p-n junction diode is connected in reverse biased, shown in fig . with
block diagram
In reverse biased:
1. The positive terminalof the battery attractsthe majority current carriers
(free electrons) in n-type region and forces them away from the
junction.
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2. Similarly the negative terminalof battery attract the majority current
carriers (holes) in p-regionand forces them away from the junction.
3.
So the size of depletion layer increasesand lastly no any type of charge
carriers.
In this way, p-n junction diode works in reverse biased.
Q. Explain forward bias characteristic of p-n junction diode.
Ans: Figure shows I-V characteristics of p-n junction diode in forward biased.
The graph represents the following information.
1.
Up to certain voltage, the current remain zero, the voltage is called
knee voltage.2.
After knee voltage, barrier potentialalmost reduces to zeroand current
increases rapidly.
3. As current is large, forward resistanceof bias is small
Q. Explain reverse bias Characteristic of p-n junction diode.
Ans: When p-n junction diode is in reverse bias, then the current- voltage
characteristic is shown in fig. the graph give the following information.
1.
Current is very small (few uA)
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2. Current increases with voltage and after that it becomes almost
constant. It is called reverse saturation current.
3. Reverse resistanceof the diode is very high (mega-ohm).
4.
For very large reverse voltage, current suddenly increases. This is
called as reverse breakdownor avalanches breakdown.
Q. What is the symbol of photodiode?
Ans: The symbol of photodiode is
Q. What is photodiode? Explain the principle of photodiode.
Ans: Photodiode:A photodiode is a special purpose reverse-biased PN junction
diode that generates charge carriers in response of photons and high energy
particles.
Its working based on photoelectric effect. It is always operated in reverse
bias.
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Principle: When light photon have energy hv>Egenter its depletion region,
then electron-hole pairs are generated. These electron hole flow under the
influence of reverse bias and current in A flows in circuit.
This photocurrent depends upon intensity of incident radiation and isindependent of reverse bias.
Q. State application of Photodiode.
Ans: The applications of photodiode are as follows:
1.
Photodiodes are commonly used for accurate measurements of lightintensities in spectroscopy, medical imagine and photography.
2.
Photodiodes are used to detect optical signal and used in object counters,
optocouplers.
3. Photodiode used as sensor in remote-controlled set, optical positioned
Photodiode used in barcode reader
Q. Why silicon requires 0.7 drop voltage across it before it starts conducting?
Ans: Due to the presence of immobile positive and negative ions on opposite of
the junction,electric field is created across the junction.This electric field is called
as barrier potential. It acts as a barrier to oppose the flow of electrons and holes
across the junction.When it starts in conducting state it requires 0.7 drop voltage to
break this barrier potential.
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CHAPTER 3
MODERN PHYSICS
[A] PHOTO-ELECTRICITY
Q. Define Photoelectric effect.
Ans: When light of suitable frequency is incident on metal surface, electrons are
emitted from it. This effect is called as photoelectric effect.
Q. State Plancks hypothesis.
Ans: According to Plancks hypothesis, Emission and absorption of energy can
occur only in discrete amounts of bundles. Planck called this discrete
amount or bundle of energy as quanta or photon. The energy of each photon
isE=hv. So for emitted or absorbed energy,
E = nhv
Where, h is Plancks constant, has a value of 6.626 x 10-34J s
vis the frequency,
n is an integer = 1, 2, 3, ----
Q. What is photon? State the properties of photon
Ans: Photon:It is small pocket or bundle of energy.
The properties of Photon are as follows
1. It is a mass less particle
2. It is electrically neutral
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3. It travels with speed of light
4.
They are not affected by electric or magnetic field
5.
They do not ionise
Q. Define
a) Stopping Potential
b) Photoelectric Work Function
c) Threshold frequency
d) Threshold Wavelength
Ans: a) Stopping Potential: It is reverse or negative potential which reduces
the photoelectric current to zero.
It is denoted by Vs.
b) Photoelectric Work Function:It is the energy required to detach the
electron from the metal.
It is given by w0= hv0
c) Threshold frequency:It is the minimum frequency of incident light at
which emission just begins.
It is given by v0= c/v0
d) Threshold wavelength:It is the maximum wavelength of incident light
at which emission just begins
It is given by0=
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Q. State the properties (Characteristics) of Photoelectric effect.
Ans: Following are the characteristics of photoelectric effect
1.
A metal emits electrons only when the incident (light) radiation has
frequency greater than critical frequency (v0)
2. A photoelectric current is directly proportional to the intensity of light
and independent of frequency
3.
The velocity of photoelectron is directly proportional to the frequency of
light and independent of intensify.
4. For a given metal surface, stopping potential is directly proportional to
the frequency
5.
This process is instantaneous. i.e. the emission of photoelectrons starts at
the moment light is incident on the metal surface.
Q. Derive (Obtain) Einsteins photoelectric equation.
Ans: 1. According to Plancks Hypothesis, Energy is radiated and also travels in
the form of bundles or quantaknown as photons .Each photon carries an
energy hv
2. When a photon of incident light radiation interacts with an electron
inside an atom, the whole amount of energy is absorbed by the electron.
3. The electron uses part of the incident energy, to make the electron free
from the metal (work function) and the rest part of the energy is converted
into kinetic energy
Thus, hv= 0+1
2mv2max
hv= hv0+1
2mv2max
1
2mv2max= hvhv0
mv
2max = h (vv0)
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Where,1
2mv2maxis the maximum kinetic energy of electrons,
h is Plancks constant,
vis the frequency of incident light,
v0is the threshold frequency
This is the required Einsteins Photoelectric equation
Q. State the Einsteins Photoelectric Equation and explains the significance
of each term involved in it.
Ans: The Einsteins Photoelectric Equation is given by
mv
2max= h (vv0)
Where,
mv
2maxis the maximum kinetic energy of electrons,
h is Plancks constant,
vis the frequency of incident light,
v0is the threshold frequency
Q. Why electrons are not emitted from the surface of metal plate, if
frequency of incident radiation is less than threshold frequency v0
Ans: 1. According to Einsteins Photoelectric Equation
12 m v
2max= h(v-v0)
2. If v< v0, then,12 mv
2
maxbecomes negative
3. But for the emission of photoelectrons the kinetic energy must be positive
4. Thus, due to negative value of kinetic energy the photoelectrons are not
emitted from the surface of metal plate
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Q. Define Photoelectric cell. State itsany two applications.
Ans: A Photocell is an electronic device in which light energy gets converted
into an electrical energy.
It is used to produce a current or voltage when exposed to light or other
electromagnetic radiation.
The two applications of Photoelectric cell are-
1. They are used in exposure meters
2.
They are used in burglar alarms
Q. Explain the construction and working of photoelectric cell.
Ans: Principle:The working principle of photoelectric cell is photoelectric
effect. It is an electronic device which converts light energy into an
electrical energy.
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Construction:
1. A photocell is an evacuated tube consisting of two electrodes i.e. cathode
and anode
2.
Cathode is concave in shape called an emitter, made from a material thatemits electrons easily. Whereas, anode is in the form of thin rod as shown
in the fig
3.
Cathode is connected to negative terminal of battery. An ammeter is
connected in the circuit to measure the current flowing through the
photoelectric cell
Working:
1.
When light is allowed to fall on cathode it emits photoelectrons.2.
The photoelectrons are attaracted by anode.
3.
The photoelectric current starts flowing through the circuit and the
milliammeter shows the deflection.
Q. Draw the diagram of Photoelectric cell.
Ans:
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Q. State the applications of photoelectric cell.(Any four)
Ans: The applications of photoelectric cell are
1. It is used in Burglar alarm
2. It is used in fire alarm
3.
It is used in Lux meter to determine the intensity of light
4.
It is used in Exposure mete
5.
It is used in automatic control of traffic signal
6. It is used in automatic counting objects
7. It is used in automatic shutting and opening doors
8.
It is used in reproducing sound in motion pictures9.
It is used in detecting flaws in metals
10.
It is used in television sets
Q. State the principle of light dependent resistor (LDR) or photo resistor.
Ans: Principle: The electrical resistance of LDR decreases as the intensity of
incident light increases.
Q. Draw the symbol of LDR.
Ans:
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Q. State the applications of LDR.
Ans: Application of LDR:
1.
It is used for detecting ships and air crafts by the radiations given out.
2.
It is used as flame, smoke and burglar detectors.
3. It is used as a automatic lighting controls for street light.
4. It is used in Camera for exposure control.
5. It is used in Xerox machine- to controls the density of toner.
6. It is used in Colorimetric test equipment.
7.
It is used as automated real view mirror etc
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[B] X-RAYS
Q. What are X-rays?
Ans: X ray is electromagnetic radiations of short wavelength ranging from 0.01
A0to 100A
0.
Q. State the properties of X-rays.(Any four)
Ans: Properties of X-rays:
1. X ray are the electromagnetic radiations of very short wavelength
2. X rays travels with speed of light
3.
X rays are electically neutral
4.
X rays can ionises gas
5. X rays can damage or kill living cells
6.
X rays are invisible7. X rays can affect photographic plate
8. X rays are not deflected by electric or magnetic field
Q. Explain the production of X-rays using Coolidge (Modern) X-ray tube.
Ans: X ray is produced whenever fast moving electrons strike a high atomic
weight solid (tungsten) in vacuum.
1. The Coolidge X-ray tube is shown in figure
2. It consists of a highly evacuated hard glass tubecontaining cathode
(K) and the target(T) attached to anode (A).
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Working: 1.When the cathode is heated by electric current it produceselectrons due to thermionic emission.
2. The beam of electronis thenfocussedon the anode (Target).
3. The electrons from cathode are then accelerated by applying high
voltagebetween cathode and anode using step up transformer.
4. When these fast moving electrons are suddenly stopped by tungesten
anode, they lose their kinetic energy and X rays are produced from thetarget.
5. Some amount of kinetic energy is converted to large amount of heat.
Q. State the formula for minimum wavelength of X-rays also state the
meaning of symbols used in it
=/. Where, = minimum wavelength of x rays
h= planks constant, V= applied voltage
C = velocity of light, e =charge of electron
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Q. State the applications of X-rays.
Ans: Applications of X-rays:
A) Industrial [Engineering] applications
1. X rays are used to detect defects within metals, machine parts, and
castings etc.
2.
X rays are used to detect manufacturing defect in rubber tyres or tennis
ball in quality control.
3.
X rays are used to detect cracks in the wall
4. X rays are used to detect the cracks in the body of aeroplane or motor car
5. X rays are use to distinguish real diamond from duplicate one
6.
X rays are used to detect smuggling gold at airport and ship yard
B) Scientific Research applications:
1. X rays are used to investigate the structure of the atom.
2. X rays are used for analyzing the structure of complex organic molecules.
3. X rays are used in determining the atomic number and identification ofvarious chemical elements.
C) Medical Applications:
1. X rays are used in detecting fractures in bones
2. X rays are used to cure skin diseases and destroy tumors.
3. X rays are used to detect bullet position inside the body.
4. X rays are used to cure diseases like cancer
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[C] LASER
Q. State the full form of LASER.
Ans: The full form of Laser is Light Amplification by Stimulated Emission of
Radiation.
Q. State the four properties of LASER.
Ans: The properties of LASER are-
1. Laser source is tremendously Intense source of light.
2. Laser source is a Monochromaticsource of light.
3.
Laser source is a Coherentsource of light.
4.
Laser source is a highly Undirectionalsource of light.
Q. Differentiate between stimulated and spontaneous emission of light.
Ans:
Stimulated emission Spontaneous emission
1 This process can be controlled from
outside
This process cannot be controlled from
outside
2 Multiplication of photons takesplace
Multiplication of photons does not takeplace
3 All the emitted photons move in
same direction
All the emitted photons move randomly
4 It results in monochromatic light It results in non monochromatic light
5 The intensity of emitted light does
not decrease with distance from the
source
The intensity of emitted light decreases
with distance from the source
6 It results in highly intense light It results in low intense light
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Q. What is Population inversion of light?
Ans: The process of increasing the number of atoms in higher energy level, more
than that of ground state is called as population inversion of light
Q. Define optical pumping in LASER. Explain Optical pumping.
Ans: Optical pumping: The process of raising the atoms from lower energy state
to higher excited state using light medium is called as optical pumping.
Let E1, E2 and E3 are energy levels and N1, N2 and N3 are respective
concentrations of atoms.
The atoms in level E1 are excited to E3 by optical pumping. The
concentration N1decreases.
The time for which the atoms can stay in level E3is very short. They lose
some energy and return to energy level E2.
The transition from E3 to E2 is rapid and spontaneous. Since level E2 is
metastable state, hence atoms relax here for longer time.
The no. of atoms in E2 increases and when it is greater than level E1
population inversion takes places. i.e. N2 >> N1
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Q. Describe construction and working of He- Ne laser.
Ans: Construction:
1.
He- Ne laserconsists of a quartz tube of about 80 cmlength and 1.5 cm
diameter
2. The tube is filled with mixture of 90% helium (He) and 10% neon (Ne)
2. At one end perfect reflectoris fixed and at other end partial reflectoris
fixed
Working: (1) When electric discharge is produced in the tube, He and Ne
gas atoms are excited. Some excited levels of helium are close to some
excited levels of neon. Therefore these excited helium atoms collide with
excited atoms of neon and transfer the energy to neon atoms.
(2) The actual lasing action is done by neon atoms. The neon atoms with
extra energy from helium atom are forced to jump in ground state by
emitting a photon. This produces the LASER light. The newly emittedphoton triggers the next neon atom and increases the radiations.
(3) Thus coherent, monochromatic, unidirectional LASER is produced by
He-Ne gas LASER.
The energy level diagram of He-Ne LASER is shown below.
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Q. State any four applications of Lasers.
Ans: The applications of Lasers are as follows:
A)Industrial or Engineering use:
1.
HeNe gas laser is commonalty used to read barcode.2.
High power lasers are used in cutting, drilling, welding
3.
Lasers are used to find defect in material
4. It is used for marking, engraving of number plates .Ex. number plate,
name plate
5. Lasers are used in holography
6. Lasers are used in computer printers
7.
Lasers are used for 3D,Laser scanners
8.
Lasers are used in controlled heat treatment
B)Medical uses:
1.
It is used to repair the detached retina of human eye.
2.
They are used for surgery in medical field.
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3. Dentists use LASER for painless drilling in tooth.
4.
They are used for destroying cancer cells in human body.
C)Communication purpose uses:
1. It is used in fiber optical communication.
2. It can be used for communication between earth and moon due to
unidirectionality and brightness.
3.
It is used to measure the distance between the earth and the satellite.
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CHAPTER 4
PHYSICS OF NANOPARTICLES
Q. What is nanotechnology?
Ans: Nanotechnology:
1. The branch of engineering that deals with things smaller than 100
nanometers is known as nanotechnology.2. It is an advanced technology involving the fabrication and use of devices
at atomic or molecular level.
Q. Define nanoscale.
Ans: Nanoscale:
The size range from approximately 0.2 nm to 100nm is known as nanoscale.It refers to dimensions below 100nm.
Q. What is meant by nanoparticles? OR Define nanomaterials.
Ans: Nanoparticles (Nanomaterials):Any microscopic particle less than about
100 nanometers (nm) in diameter is known as nanoparticles.Nanoparticles
have at least one dimension of 100 nm or less.
Q. Mention nano material of zero and one dimension
Ans: Nano material of zero dimension: Nanoclusters
Nano material of one dimension: Carbon nanotubes, nanofiber
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Q. State the properties of nanomaterials or nanoparticles.
Ans: Properties of nanomaterial
1. Electrical properties
2. Magnetic properties
3. Optical properties
4. Mechanical property
5.
Structural property
6. Thermal property
Q. State the applications of Nanotechnology.
Ans: Electronics Applications:
1.Nanoparticles can be used as switches or sensors due to their smaller size
2. By using nanotube transistor in integrated circuits we can reduce the size
of IC.
3.
Carbon nanotubes are used to produce displays of mobile
laptops,calculator.
A)Automobile Applications:
1.Nano particle paints, provide smooth, thin and attractive coating of
vehicles
2.
Nano coating for glass used in vehicles, is permanent and easy to clean
B)Medical:
1.Nanotechnology has its application in the field of medicine such as
disease diagnosis, drug delivery and molecular imaging.
2.
DNA chips and arrays are useful in diagnostics and genetic research.
3.
The array of nanoparticles used in drug discovery, forensics and
detection of information on disease.
4.Nano-sized sponges can be used to remove any overdose of drugs from
blood stream.
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C)Textile:
1.
It is used in manufacturing of clothes which would give pleasant look and
high comfort.
2.Nanotechnology is used in the manufacturing of special threads and dyes
3.
Nanofibre gives clothes antibacterial properties
D)Cosmetics:
1.
It is used in the manufacturing of sunscreen lotion, hair cream, hair dyes
etc.
E)Environmental:
1. It is used in sensors which are useful in water purification, pesticides etc
2.
Use of nanoparticles as hydrogen storage reduces pollution due to vehicle
F)Space & Defense:
1. It is used to improve performance and multifunctioning of space vehicles
2. It is used in the manufacturing of aircrafts having superior property
Q. Describe four applications of nanomaterial in engineering field.
Ans: Applications of nanomaterial in engineering field
1. Data storage system: Semiconductor material in the form of film can be
deposited on substrate to form the chips
2. In energy sector: The conventional energy sources like coal,fuel are
depleting day by day, thus use of alternative energy source is inevitable
3. In automobiles: High mechanical strength material but light in weight can
be produced by using nanotechnology. Nano painting material can be used toget uniform layer of coating on the body of vehicle
4. In consumer goods:Nanotechnology has wide applications in cosmetics,
domestic products and textiles. Using nanomaterial fiber,one get comfort of
cotton clothes
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Q. Name Physical method of synthesis of nanoparticles
Ans: The physical methods of synthesis of nanoparticles are as follows:
1.
High energy ball milling
2.
Lithography
3.
Physical vapour deposition
4. Chemical vapour deposition
Q. Describe any one physical method of synthesis of nanoparticles.
Ans: High Energy ball milling method:
This process is used for producing the magnetic and catalytic nanoparticles.
In this method larger particles are simply crushed mechanically in the
rotating drums by hard steel and tungsten carbide balls.
Method: In high energy ball milling process
1.
A containeris filled with stainless steel ballsof few millimeter
diameters.
2.
The material to be processed is added in the powderedform of about mgrain size to the container.
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3. Then the container is filled with nitrogen gasafter adding the grain size
material in container.
4.
Then the shaft is rotated, which grind the material for 1 hr to 100hrs.
5. At the end of process we get nanoparticles of desired size.
Advantages: a) This process is simpleb) Have high production rate
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