Applied Physics Notes for Computer,Electrical,Electronics branch by Prof. Atul Waghmare 9657824223

5/21/2018 Applied Physics Notes for Computer,Electrical,Electronics branch by Prof. At...

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1

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



2

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.



3

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.



4

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



5

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.



6

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).



7

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



12

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.



15

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



19

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.



20

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.



22

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.



25

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)



26

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.



27

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.



28

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



29

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=



30

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)



31

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



32

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.



33

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

Documents

Applied Physics Notes for Computer,Electrical,Electronics branch by Prof. Atul Waghmare 9657824223