(Electrical Equipments)

SSC CDS

BANKrAILWAY

ª Concepts with Visual Understanding

ª Core Physics (Detailed Theory)

Physics

ª Practical Applications of Physics

ª Previous year Questions from

1999 to till date

C L A S S E S

Chapter - 08

(Electrical Equipments)

1

Raja Sir Income Tax Inspector

14 Years’ teaching experience

•CAT •GMAT •UPSC •SSC •BANK

ELECTRICAL DEVICES Symbols

Cell

Battery

DC Power Supply

AC Power Supply

Voltmeter

Ammeter

Motor

Generator

Filament Lamp

Diode

LED

LDR

Fixed Resistor

Variable Resistor

Fuse

Thermistor

Heater

Loud Speaker

Switch Open

Switch Closed

2 Pin Plug

3 Pin Plug

Resistor

Inductor

Capacitor

Solar Cell

Transformer

Earthing

Neutral (white)

Live (Black)

2

Manisha Bansal Ma'am (Director & Author)


•CAT •GMAT •GRE •SSC •BANK

Laser:

It is an acronym for Light Amplification By stimulated Emission of radiation. It is a device which emits coherent light i.e a strong

beam oflight of same frequency. It uses the concept ofPopulation Inversion.

Mechanism Involved in Laser:

As we supply energy to the electron it jumps from lower state to higher excited state but this state is unstable so it jumps back to

lower state so it releases out the energy absorbed in the form of radiation. But in laser, Electron directly does not shift to the

ground state rather it moves to a meta stable state present in between the ground state and excited state.Similarly a number of

electrons jump to this meta-stable state soa population of electrons gather at intermediate meta-stable state where actually the

electrons should not be present as it is an excited state. This is known as population Inversion. Now all these electrons will jump to

the ground state altogether releasing out the energy of same wavelength and frequency hence a strong beam of same wavelength is

released out. This beam is known as Coherent light or Laser light.

Depending on the number of meta-stable states, we have

2 level laser system



3




Three and Four Levels Lasers

Super conductivity:

As if Temperature , resistance of conductors also increases and vice-versa i.e.

Temp , resistance and current or conductivity .

The materials which shows zero resistance at certain very low temperatures shows Super conductivity (conductivity = very

high at Resistance 0) are Super conductors.

Critical/Transition Temperature:

The temperature at which resistance becomes zero for a super conductor.

Ex: An alloy of

Plutonium Shows

Cobalt Superconductivity

Gallium

Applications of Super conductors

• To produce very high speed computer

• To make very strong electro magnets

• Used in medical research

• For transmission of high power.

Superconductors have been used to make digital circuits based on rapid single flux quantum technology, radio frequency and

microwave filters for mobile phone base stations.

⦁ Superconductor magnets are used in Magnetic Resonance Imaging(MRI) machines, mass spectrometers and the beam, steering

magnets used in particle accelerators.

⦁ Superconductor magnets can be used in magneto hydrodynamic power generation(i.e a direct energy conversion system which

converts the heat energy directly into electrical energy, without any intermediate mechanical energy conversion)

⦁ The film of superconductor can be used in geophysics, meteorology, etc.

Superconductors In India

Programme management board was constituted in 1987 for development of superconductors.

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⦁ National Superconductivity Science and Technology Board was constituted in 1991.

⦁ Department of atomic energy, Indian Institute of Science, Council for Scientific and Industrial Research, etc., conduct research in

superconductors.

⦁ In July 2012, Dhruba Dasgupta reported that his team observed what possibly appears to be superconductivity at 313 K.

⦁ The researchers used lead zirconate titanate (LZT), a common material used in capacitors and other electronics for the work.

Solar Cell

Charles Frittis in 1883, built the first solar cell. He coated selenium with a very thin layer of gold to form junctions. It is basically a

p-n junction device that converts solar energy into electric energy. The current generated is directly proportional to the

illumination (lumen/metre2) and on the size of the surface area that is illuminated.

Solar cells can be connected together in series to create solar panels. Solar cells are also widely used in producing electrical

power in remote areas, in calculators, in empowering houses and offices with electricity.

Note: Solar thermal is a technology that utilizes the sun ray to generate heat which is further used in electricity generation process.

5




Velocity Filter: It is an arrangement of cross electric and magnetic fields which helps to select out or filter a beam of charged

particles of given (desired) velocity from a beam of particles.

Choke Coil (Ballast)

Copper wire wound over an soft iron core makes a choke coil.

Device used to control current in AC Circuits

It is also used in Fluorescent Lamps

Characteristics

• High Inductance [because Soft iron is used]

• Negligible Resistance [because thick copper wire is used]

• Power in circuit having choke coil is least

• Net impedance or Inductance

Reactance = XL = L= 2L

It is an electrical device used in controlling current in an ac circuit without wasting electrical energy in the form of heat. Ohmic

resistance of an ideal choke coil is zero. Hence, no energy is lost.

Types of Choke Coil

Iron Core Choke Coil

• For low frequency

AC Circuit

• L(Inductance) should be higher

Air Core Choke Coil

• For high frequency

AC Circuit

• L (Inductance) is low

PhotoCopier/ Photo State Machine

• It is a device which makes the duplicate copies of a document.

• It works on the principle of electricity and photo conductivity.

• It contains a light sensitive photo receptor inside the machine which attracts the toner particles and than transfers them to a plain

paper to make a copy of document.

Tape Recorder/ Tape Disk/ Tape Machine

• It is a sound recording and reproduction device which records and plays back the sound.

• It uses magnetic tape for Storage.

• It records a fluctuating (audio) signal by moving a tape across a tape head that polarizes the magnetic domains in the tape in

proportion to audio signal

[Take care, It should not be kept near a magnet]

Tank Circuit

An LC circuit, also called a resonant circuit, tank circuit, or tuned circuit, is an electric circuit

consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C,

connected together in parallel.The main function of an tank circuit is to oscillate with minimum

damping. It is used for producing signals or accepting a signal from a more composite signal at a

particular frequency.

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Electro-Chemical Cells

It is a device by which electric current energy is generated at the cost of

chemical energy due to chemical action looking place in the cell.

Cell

Single unit is called cell

+ –

Group of cells is called Battery

+ –

Types

Primary Cell Secondary Cell/Storage Cell/Accumulator's

• Irreversible

• Cannot be charged once used.

• Chemical energy Electrical Energy

• Ex: Voltaic Cell

Dry Cell

Daniel Cell

Leclanche Cell

• Initial cost is low but operating cost is high

• Reversible

• Can be recharged again and again

• Chemical energy Electrical energy

• Ex:Lead Acid Accumulator (Pb + dil. H2SO4)

Edison Cell or Alkali Accumulator (Ni-Fe)

• Initial cost is high but operating cost is low.

Lightening Conductor or Earthing

It is an instrument used to save big buildings from electrostatic discharge of clouds. Here a metal wire

with sharp edge kept on the top of the building is wired whole along the length of building to its bottom

under the earth. Due to induction, charges are transferred from clouds to metal wire and then through

metal wire to ground. So,it saves big buildings from damage by the lightning flashed.

7




Thermo Couple

A thermocouple is an electrical device made up of two dissimilar wires

joined to form a junction. It is used as a temperature sensor to detect

temperature. Whenever a junction is heated, a certain temperature is

observed and corresponding to it voltage is recorded

Sensors: These are the devices used to detect (sense) any physical

quantity and gives us its value in terms of say current or voltage or any

other measurable quantity.

A.C. Generator or A.C. Dynamo

It is a machine which produces alternating current (A.C.) from mechanical

energy.

Uses Fleming Right Hand Rule

Principal: Based on Electro Magnetic induction

It was designed by Nikola Tesla

The word Generator is a misnomer

As nothing is generated by it Rather just converts

Mechanical Energy Electrical Energy

• M.F.(Direction of magnetic field) N (north pole) to S(south pole)

• As coil is rotatedanticlockwise

AD – out, BC in

• As per Fleming Right Hand Rule,direction of current is fromB to C

• So, Current flows in external circuit.

• As coil rotates further, direction of current is opposite.

So,AC current is generated.

D.C. Generator

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• Here split rings are used instead of slip rings.

• Output is DC in nature (i.e. either +ve or –ve)

DC Motor

• It converts electrical energy to mechanical energy.

• Principle: Current carrying conductor when placed in external magnetic field, experiences a force.

If coil is used, it experience Rotational force called Torque.

• A split-ring device called a commutator is used to reverse the current.

Fleming left hand rule is applied here.

9




Transformers

• A transformer is a device that changes voltagefrom one value to another.

• Power at the input endis equal to the power at the output end.

• Only the voltage will increase or decrease.

Principle:

• Transformers work on the principle of Mutual induction.

• Mutual Induction: Suppose there are 2 inductors if some current flows through coil1,there will be change in the current As a result

there will be change in the magnetic flux in the coil 2 and because of which emf is induced in the coil 2.

Construction:

A Transformer consists of:

1. Primary coil:

Primary coil has ‘np’ number of turns of wire over a piece of soft iron core.

It is the input end.

2. Secondary coil:

Secondary coil has ‘ns’ number of turns of any wire (like copper etc.).

It is the output end as we receive output from this end.

3. Soft iron core:

The hysteresis curve for iron is extremely thin because of which it covers minimum possible area.

As theareaofthehysteresisloopof soft iron is very lesstherefore the energy lost by the transformer will be very less.

Permanent magnet is not suitable for use in transformers because the energy lost will be huge.

Working:

•. An input voltage (AC source)is applied across the primary coil. Asa resultalternating current is produced in the primary coil.

• The alternating current will give rise to alternating flux in the coils.

• Because of change in the magnetic flux emf will be induced.

• There will be 2 Emf' s produced in the circuit.

1. Self –induction 2. Mutual induction.

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• There will be self - induced emf in the primary coil, because of change in the magnetic flux in the primary coil.Therewill be

corresponding change in the magnetic fluxassociated with the secondary coil which will give rise to induced emf in the secondary

coil.

• Mutual induction takes place in the secondary coil.

• Induced emf in the primary coil = ep = -Np (df/dt)

• Where (df/dt) = rate of change of magnetic flux and Np = number of turns in the primary coil.

• Mutual induction in the secondary coil es = - Ns (df/dt)

• Where Ns = number of turns in the secondary coil.

• Assumingresistance =0 in both primary and secondary coils.

• Therefore ep = Vp (Voltage across primary coil)

• Vp = -Np (df/dt) ..............................................equation (1)

• es = Vs (Voltage across secondary coil) = - Ns (df/dt)................................equation (2)

• Dividing equation (1) with (2):

• (Vp/Vs) = (Np/ Ns)

• Vs = (Ns/ Np) Vp

• Power at the input end is same as the power at the output end.

• Therefore Pinput = Poutput

• IpVp =IsVs

Types of Transformers

There are 2 types of transformers:

Step up transformer:

1. This transformer amplifies the voltage. The output is higher than the input which is being supplied.

2. This condition will be true Vs >Vp only whenNs >Np and Ip < Is. i.eNumber of secondary turns must be more than number of

primary turns for step up transformer.

3. Vs = (Ns/ Np) Vp

4. The output of the transformer, voltage will be high and current will be less.

5. They are used in the power stations which supply power to the houses.

Step down transformer:

1. For Vs <Vpto be true thenNs <Np.i.eNumber of secondary turns must be less than number of primary turns for step up

transformer.

2. Is >Ipsothat(Pinput= Poutput).

3. Output is low voltage and high current.

4. This transformer is used in welding

Applications of Transformers

1. Transmission of power over long distances:

Suppose there is a main power station, from there power is send to different sub area power stations and from there it is supplied

to different houses. At the main power station there is step-up transformer, it will amplify the voltage and current is reduced.

When the current is reduced therefore heating will be reduced to a great extent. The power loss is minimized to a great extent till it

reaches the area sub stations. At the area substation step-down transformer is used. This transformer will reduce the voltage and

then supply to the houses.The line power loss will be not very much as the distance between the houses and area power substation

is not very large.

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2. Transformers are used to regulate the voltage. Many appliances use voltage stabilizers which regulate the voltage so that the

electronic devices are not harmed when there is fluctuation in the voltage.

Energy losses in actual transformers

Flux leakage:

• There are air gaps between the primary and the secondary coilsbecause of which the change of flux which is associated with the

primary coil is not completely transferred to secondary coil.

• In order to reduce the loss, secondary coil can be wound over the primary coil.

• For example: In case of toroidal transformer cores, Over the primary coil secondary coil is wound above it. As a result there is no

air gap in between them.

Resistance of windings:

• The wire used for the windings has some resistance and so, energy is lost due to heat produced in the wire (I2R).

• If the area of the cross section of the wire is increased then the resistance will be reduced considerably.

• So the thick wires are used in the windings of primary and secondary coils , As a result resistance will be less.

• The amount of heat lost because of wires will be less as resistance is minimal.

Eddy currents:

• Soft Iron core also gets heated up because of magnetic flux,Asa result eddy currents are developed in the soft iron core.

• Core gets heated up because of eddy currents. This will harm the transformer core.

• In order to prevent this,laminated core can be used. Because of insulated covering, Eddycurrents are not able to produce the

heating effects.

Hysteresis:

• There is energy loss involved during the magnetization of the material of the core.

• Always those materials are to be chosen for which hysteresis loss is minimum.

• That is why Soft Iron core is used instead of permanent magnets.

Motor Starter

It is a device used to start a D.C. motor safely, by introducing high resistance in the

motor circuit.

Normally, the resistance of motor is kept low to reduce copper loss and as R is low,

current flowing the motor is very high as per ohm's low.

V = IR

V

I = R

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or R 1

I

Current and Resistance are inversely proportional to each other.

So, if at the starting phase, initial high voltage is supplied to motor, current in motor will be very high due to low resistance and As

per Joule's law of Heating

H = I2RT

A lot of heat will be produced which may burn the motor. To avoid this condition a high resistance is added to the motor circuit so

that current developed in the initial phase may be small to avoidheating effect.

Galvanometer

It is a device used to detect and measure electric current in a circuit. It can

measure current up to 10-6 A. A galvanometer can be converted into a voltmeter by

connecting a very high resistance in its series.

Dead Beat Galvanometer/Moving coil galvanometer

It is a type of galvanometer in which,

The coil comes to rest immediately when the current is switched off, So deflection

can be noted in no time.

It is made by winding the coil on a conducting frame so that eddy currents may set

up in the core which produce damping effect on the coil and coil may come to rest

at once.Phosphor bronzestrip is used as it does not oxidize easily, has low

torsional constant and is non-magnetic.

Ammeter

It is a device used to measure current and is always connected in seriesin an electric circuit.It's measured value is always less

than the actual value of current to be measured.

Voltmeter

It is a device used to measure potential difference between two points in an electric circuit. It is connected in parallel in an

electrical circuit. Its value is always lesser than the true value.

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Shunt

Alow resistor connected in parallel with a circuit or device that reduces the amount of electric current flowing through it

AMMETER VOLTMETER

• To measure current

• Low Resistance Galvanometer

• To measure voltage

• High Resistance Galvanometer

Low R is connected in parallel to decrease resistance • High R is connected in series to (increase) Resistance

Fuse

Itconsists of a low-resistance metal or wire that is used to close a circuit.

When too much current flows throughthe low resistance element of the fuse,

the element melts and breaks the circuitfollowing the Joule's Law of

heating

Gold Leaf Electro Scope (GLE)

It is an instrument used for detecting the presence of charge and the Polarity

(sign ) of charge.

To check presence of Charge:

• Take an uncharged GLE

• Place rod to be tested on Disc

• If it is charged = leaves would diverge

As through metal disc, charges are transferred to the foils/leaves & As they get

same charge, they diverge.

Higher is the divergence of leaves, more is presence of cha rge

To check polarity of charge

• Take a charged GLE

• Place rod on Disc

• If leaves diverge = charge is of same nature as on GLE

• If leaves converge = Charge is of opposite nature as on GLE

Types of charges acquired on Rubbing by various objects

Positive Charges • Glass rod • Fur • Woolen Item • Nylon • Dry Hair • Comb

Negative Charges • Silk Cloth • Ebonite • Rubber • Plastic • Cloth

Solenoid/Temporary Magnet:It is a long insulating wire closely wound in the form of a helix.

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Its length >>> diameter

Concept: Current carrying conductor behaves as a magnet.

When we switch it ON, current flows, Magnetism appears, Acts as magnet

When we switch it OFF, current = 0, Magnetism Vanishes.

So,It is an electro magnet

Where converted

toElectrical Energy Magnetic Energy

Toroid:Similar to Solenoid, just it is an Endless Solenoid in the form of a ring.

It is a hollow circular ring on which a large number of insulated turns of a metallic wire are closely

wound.

Cyclotron

• Developed by Ernest O Lawrence

• It is an equipment used for accelerating positively charged particles like

• proton • Deuterons • alpha participles

• Functioning: Here charged particle is allowed to move in perpendicular E.F. and M.F. due to which it

moves in a helical path, each time gaining velocity and finally striking the target at very high velocity

(speed).

Vande Graaff Generator

A device used to build up charges of very high potential difference of order of

few million volts.

Use of vande Graaff

• To accelerate charged particles like e, p, ions.

• Used in experiments of Nuclear Physics.

Limitation

It cannot accelerate uncharged particles like neutrons.

Thermistor

It is a heat sensitive device whose resistivity changes very rapidly with change in

Temperature

Use:

It is used to measure small temperature changes. It is available in various forms such as :

16




Miniature Circuit Breakers (MCBs)

In present time, the fuses are replaced by MCBs (Miniature Circuit Breakers) which is an automatically

operated electrical switch designed to protect an electrical circuit from the damage caused by

overload or short-circuit.

The tin-lead alloy is also used as soldering material for joining metals in electronic circuit. Earth pin of

the plug is always made thicker and longer.

Invertor

It is a device used in offices, homes and designed to

⦁ charge the battery by converting AC toDC.

⦁ convert DC from a battery to AC.

It is fitted in main power line. In case of failure of power, it is automatically switch ON and provide power to the domestic wiring in

the homes.

When the main supply is restored, the inverter automatically switches to a mode in which it starts charging the battery depleted

due to use during the period of power failure.

Incandescent Lamp or Filament Lamp

It has tungsten filament connected between two lead wires. When there is a current through it, it gets heated up and

emits light. The melting point of tungsten is about 34000C. The electric bulb contains gas to prevent the evaporation

of tungsten.

Argon is a commonly used gas used to fill incandescent light bulbs. It increases bulb life by preventing the

tungsten filaments from deteriorating too quickly. Other gases such as helium, neon, nitrogen and krypton are

also used in lighting. The gases used in light bulbs are known as inert gases.

Fluorescent Tubes

It contains mercury vapors at low pressure which emits invisible ultraviolet rays when tube is switched on. These ultraviolet rays

fall on the fluorescent coating inside the tube and emits visible light.

Since, there is a little amount of heatproduced in a tube, i.e. almost all electrical energy is converted into light energy. The tubes are

more cheaper and efficient.

Compact Fluorescent Lamps

An incandescent light bulb wastes a lot of electricity in the form of heat while no electric energy is wasted as

heat in a fluorescent lamp. A CFL (Compact Fluorescent Lamp) is miniature fluorescent tube and works on the

same principle.

It is 4 to 6 times more efficient than an incandescent bulb. That's why one can buy a 15 W fluorescent lamp that

produces the same amount of light as a 60 W incandescent bulb.

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Although, initial cost for CFL is more but its lasting is 15 times longer than that of other bulbs. (Fluorescent lamp contains mercury

which is a hazardous substance).

.

LCD stands for “liquid crystal display”

LED, which stands for “light emitting diodes,”

LED differs from general LCD TVs in that LCDs use fluorescent lights while LEDs use those light emitting diodes.

Electric Iron/Heater/Heating Rod

It is based on Heating effect of electric current

Heating Element used – Nichrome (an alloy of Ni + Cr)

Ni = Nickel, Cr = Chromium

Electric Arc

It consists of two carbon rods with a small suitable separation between them. When a high

voltage (40 to 60 volt) is applied, a spark jumps across the gap and a very bright light is

emitted in the gap.

Efficiency of an electric device ()

Efficiency of an electric device is defined as the ratio of its output power to the input power,

i.e.,

= output power

input power

In case of an electric motor, efficiency = output mechanical power

input electric power

Sensitivity of a device:

It is the smallest value of a physical quantity that it can measure.

Ex: If a device A can measure current of0.1 Ampere and Device B can measure current of 0.01 Ampere

than Device B is more sensitive

Air conditioner (AC)

Body comfort depends on temperature as well as humidity. The comfortable conditions for an average person are

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(i) Temperature between 23°C to 25°C and

(ii) Relative humidity between 60 to 65 per cent.

An air conditioner provides these conditions by regulating temperature and humidity.

The cooling capacity of an AC is expressed in tonnes, e.g. 1 tonne, 2 tonne, etc. A 1 tonne AC transfers 12000 BTU (British Thermal

Unit) of heat from the room in an hour. (1 BTU = 1055 joule).

An air conditioner (AC) in a room or a car works by collecting hot air from a given space, processing it to release cool air into the

same space where the hot air had originally been collected. This processing is done using five components:

♦ Refrigerant: It is a coolant fluid

♦ Evaporator: These are coils

♦ Compressor: It compresses the gas to decrease its volume

♦ Condenser: Converts gas into liquid

♦ Expansion valve: To eject out the heat

The warm air is drawn in through a grille at the base of the indoor unit, which then flows over evaporator coils( pipes) through

which the refrigerant (i.e., a coolant fluid) is flowing. The refrigerant liquid absorbs the heat and becomes a hot gas itself. The coils

also wrings out moisture from the incoming air, which helps to dehumidify the room. This hot refrigerant gas is then passed on to

the compressor, the compressor compresses the gas so that it becomes hot, since compressing a gas increases its temperature. This

hot, high-pressure gas then travels to the third component – the condenser and it condenses the hot gas so that it becomes a liquid.

The refrigerant reaches the condenser as a hot gas, but quickly becomes a cooler liquid because the heat of the ‘hot gas’ is

dissipated to the surroundings through metal fins. So, as the refrigerant leaves the condenser, it loses its heat and becomes a cooler

liquid. This flows through an expansion valve – a tiny hole in the system’s copper tubing – which controls the flow of cool liquid

refrigerant into the evaporator, so the refrigerant arrives at the point where its journey started.

A thermostat constantly monitors the temperature of the system so that it is maintained near a users desired point.

SSC CDS

BANKrAILWAY



Physics



1999 to till date

C L A S S E S

Chapter - 09

(EW Waves & Maxwell)

1




ELECTROMAGNETIC WAVE A wave which contains electric field(E.F) and magnetic field(M.F)perpendicular to each other and perpendicular to direction of

propagation of wave is called an electromagnetic wave. The EF & MF are varying continuously with time.

Equation of wave:

Direction of wave = along x- axis

Direction of EF = along y- axis

Direction of MF = along z- axis

E.F.(electric field) Ey = E0 cos (kx -t + )

where,

Ey= value of electric field moving in y

direction at any instant

E0= maximum value of electric field

k = wave number

x= variable representing x direction

angular frequency

t= time period

= phase angle

and

k = 2

, = 2 =

2

T

, =

2

or at x = 0, t = 0, E = E0 cos

Constants terms are k ,w , f, E0

Varying terms are x, y, Ey

Similarly,

M.F. (Magnetic Field) BZ =Bo cos (kx – wt + )

Opposite sign of x and t means wave is moving in +x direction

Same sign of x and t means wave is moving in -x direction

Source of EM wave: Accelerated 'Charge'

Charge

A stationary

Charge

Moving

Charge

Accelerated charge

Constant Velocity Varying velocity

(Accelerated)

Electric Field

(only) Magnetic field + Electric field

(fixed)

Electric field + Magnetic field

(Varying)

Creates EM wave

A current carrying conductor do not generate electric field because net charge = 0 or body is not charged when current flows

and EF field is produced only when a body is charged.

2




3




Current Do not create Electric field but creates only Magnetic Field

So, Source of EM waves

accelerated electron

an excited e– (electron)in atom

Spark

LC Oscillation

Oscillation of e–(electron)

Propagation of E.M. wave

Charge oscillates produces varying E.F.

Varying E.F. produces M.F So , an EM wave is propagated.

Speed of EM wave decreases in a medium due to refractive index but Frequency do not change .

Maxwell equations

There are 4 relations on which complete Electromagnetism is based.

1. Gauss Theorem in Electricity

0

qE. ds

E

When there is Electric field ,there has to be a charge and vice versa

2. Gauss theorem in Magnetism

B. ds 0

Magnetic Field is always a closed line. There is no North or South pole.

3. Faraday's Law of EMI(ElectroMagnetic induction)

(voltage)e= d

dt

= BA

d dB

E.dr BA Adt dt

Whenever there is a change in magnetic field, we get Electric field

4. Ampere & Maxwell Law

0 0 0 0 c d

dB.dl I I I

dt

Changing EF produces displacement current

0 0 0

dB.dl I EA

dt E= EA

Change in Electric Field creates a Magnetic Field

4




Displacement Current

+Q –Q

Capacitor

When charge varies , it creates Electric Field, So, it produces current called displacement current I0.

5




RADAR as an acronym for Radio Detection And Ranging.

Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to

detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain

Electro Magnetic Spectrum

Uses • Treatment of cancer/ tumors

• To preserve food stuffs for

long times • To study

structure of atomic nucleus

• In surgery detection of fractures,

stones, bullets • In radio

Therapy • To study

structure of crystals • To detect

cracks in metals

• To cure undetectable

skin disease.

• Sterilizing • Lasik eye surgery

• To kill germs in

water purifier • to pressure

food stuff • In burglar Alarm

• Vision • Photography • Astronomy

• Optical microscopy

• Physical Therapy i.e. muscular

strain • Photography

during fog, smoke

• Secret writings study on walls

• Weather fore casting

• Purity of chemicals

• Solar water heaters and

cookers

• Radar Systems • Air craft

navigation • Microwave

ovens • To detect

speed of tennis ball, automobile

while in motion

• Wireless Communicat-ion System

• FM, AM • Cellular

phones Communicat

-ion . • Television waves

Detection Photography

Film

• Photography

Film • Geiger tubes

Gamma Rays X Rays UV Rays Visible Rays Infrared Rays Microwaves Radio Waves

• Photography

Film • photocells

• Photography Film • eye

• photocells

• Thermopile • Bolometer •Infrared

photo film

Point contact

Diodes

Receiver's

Aerials

8

Inventors Becquerel Roentgen Reigger Newton W. Herschel Hertz Marconi

6




Practice Questions 1. The conduction current is the same as displacement

current when source is

(a) only AC (b) only DC

(c) Both (a) and (b) (d) Neither (a) nor (b)

2. An oscillating charge is an example of

(a) displacement current

(b) conduction current

(c) accelerating charge

(d) accelerating current

3. There may be a large regions of space, where there is no

conduction current, but there is only

(a) displacement current due to time varying electric

fields

(b) induced current due to time varying electric fields

(c) Both (a) and (b)

(d) Neither (a) nor (b)

4. Which statement represents the symmetrical

counterpart of Faraday’s law and a consequence of the

displacement current being a source of a magnetic field?

(a) An electric field changing with time gives rise to a

magnetic field

(b) A magnetic field changing with time gives rise to an

electric field

(c) A emf changing with time gives rise to an electric field

(d) An displacement current, changing with time gives

rise to an electric field

5. Which is the most important prediction to emerge from

Maxwell’s equations?

(a) Existence of magnetic was

(b) Existence of electrical waves

(c) Existence of radio waves

(d) Existence of electromagnetic waves

6. The total current passing through any surface, of which

the closed loop is the perimeter, is

(a) sum of conduction current and displacement current

(b) difference of conduction current and displacement

current

(c) product of conduction current and displacement

current

(d) fraction of conduction current and displacement

current

7. We have a contradiction, calculated one way, there is a

magnetic field at a point P, calculated another way, the

magnetic field at P is zero. Since, the contradiction arises

from our use of

(a) Ampere’s circuital law

(b) Lorentz’s force law

(c) Fleming’s right hand rule

(d) Fleming left hand rule

8. ‘Time-dependent electric and magnetic fields give rise to

each other’. Which laws gives a quantitative expression

of this statement?

(a) Faraday’s law of electromagnetic induction

(b) Ampere-Maxwell law

(c) Faraday’s left hand rule of electromagnetic induction

(d) Both (a) and (b)

9. Which scientist’s experiment marks the beginning of the

field to communication using electromagnetic waves?

7




(a) Maxwell (b) JC Bose

(c) Hertz (d) Marconi

10. Electromagnetic waves can be deflected by

(a) only electric field (b) only magnetic field

(c) Both (a) and (b) (d) None of these

11. Which waves propagate in a solid, which is rigid and that

resists shear?

(a) Electromagnetic waves

(b) Sound waves

(c) Transverse waves of water

(d) Transverse elastic sound waves

12. In which medium, electric and magnetic fields, oscillating

in space and time, can sustain each other?

(a) Air (b) Vacuum

(c) Free space (d) Water

13. The velocity of light depends on

(a) electric properties of the medium

(b) magnetic properties of the medium



14. During the propagation or electromagnetic waves in a

medium.

(a) Electric energy density is double of the magnetic

energy density

(b) Electric energy density is half of the magnetic energy

density

(c) Electric energy density is equal to the magnetic

energy density

(d) Both electric and magnetic energy densities are zero

15. Out of the following options which one can be used to

produce a propagating electromagnetic wave?

(a) A stationary charge

(b) A charge less particle

(c) An accelerating charge

(d) A charge moving at constant velocity

16. At the time Maxwell predicted the existence of

electromagnetic waves, which was the more familiar

electromagnetic waves at that time?

(a) X-rays (b) -rays

(c) Visible light waves (d) Radio waves

17. Which of the following are electromagnetic waves?

(a) Visible light waves and X-rays

(b) Gamma rays and radio waves

(c) Microwaves and ultraviolet rays

(d) All of the above

18. The classification of electromagnetic waves according to

frequency is called

(a) electromagnetic beam

(b) electromagnetic spectrum


19. Infrared waves are produced by

(a) hot bodies and molecules

(b) cold bodies and molecules

(c) Neither hot nor cold


20. Infrared radiations is trapped by

(a) ozone layer (b) water vapour

(c) CO2 (d) Both (b) and (c)

21. Visible rays in the spectrum runs from about

(a) 4 × 1014 Hz to 4 × 1011 Hz

(b) 4 × 1014 Hz to 7 × 1014 Hz

(c) 4 × 1012 Hz to 7 × 1014 Hz

(d) 4 × 1011 Hz to 7 × 1014 Hz

22. In the electromagnetic spectrum, X-ray region lies

(a) beyond the microwave region

(b) above the ultraviolet region

(c) beyond the UV region

(d) above the infrared ray region

23. Arrange the following electromagnetic radiations in the

order of increasing energy.

I. Blue light II. Yellow light

III. X-ray IV. Radio wave

(a) IV, II, I, III (b) I, II, IV, III

(c) III, I, II, IV (d) II, I, IV, III

24. UV radiation is absorbed by

(a) ordinary glass (b) prism

(c) black glass (d) Both (b) and (c)

25. Wavelength of gamma rays are

(a) 10-10 nm to less than 10-14 m

(b) 10-14 m to less than 10-10 m

(c) 10-11 m to less than 10-14 m

(d) 10-14 nm to less than 10-10 nm

26. Gamma rays are used in medicine to destroy

(a) PB cells (b) cancer cells


8




27. One common way to generate X-rays is that

(a) bombard a metal target by high energy electrons

(b) bombard a metal target by low energy neutrons

(c) bombard a metal target by low energy protons

(d) bombard a metal target by high energy neutrons

28. All components of the electromagnetic spectrum in

vacuum have the same

(a) energy (b) velocity

(c) wavelength (d) frequency

29. The condition under which a microwave own heats up a

food item containing water molecules most efficiently is

(a) the frequency of the microwave must match the

resonant frequency of the water molecules

(b) the frequency of the microwave has no relation with

natural frequency of water molecules

(c) microwaves are heat waves, so always produce

heating infrared waves produce heating in a microwave

oven

(d) infrared waves produce heating in a microwave oven

30. Radio wave diffract around building although light

waves do not. The reason is that radio waves

(a) travel with speed target than c

(b) have much longer wavelength than light

(c) carry news

(d) are not electromagnetic waves

31. X-rays are not used for radar purposes, because they are

not

(a) reflected by target (b) partly absorbed by target

(c) electromagnetic waves

(d) completely absorbed by target

32. I. Electromagnetic waves are self-sustaining oscillations of

electric and magnetic fields in free space or vacuum.

II. No material medium is involved in the vibrations of

the electric and magnetic fields.

III. Sound waves in air are longitudinal waves of

compression and refraction.

IV. Transverse waves on the surface of water consist of

water moving up and down as the were spreads

horizontally and radially onwads.

Which of the following statements are correct?

Choose the correct option.

(a) I, II and III (b) I, III and IV

(c) II, III and IV (d) All of these

9




33. I. The great technological importance of electromagnetic

waves stems from their capability to carry energy from

one place to another.

II. The radio and TV signals from broadcasting stations

carry energy.

III. Light carries energy from the sun to the earth, thus

making life possible on the earth.

Which of the following statement(s) is/are correct?

Choose the correct option.

(a) I and II (b) II and III

(c) I and III (d) All of these

34. Which of the following statement(s) is/are correct?

I. Radio waves are produced by the accelerated motion of

charges in conducting wires.

II. Radio waves are used in radio and television

communication system.

III. Cellular phones use radio waves to transmit voice

communication in the ultra high frequency.

(a) I and III (b) II and III

(c) I and II (d) All of these

35. I. Ultraviolet rays wavelength ranging about 4 × 10-7 m

(400 nm) down to 6 × 10-10 m (0.6 nm).

II. UV radiation is produced by special lamps and very

hot bodies.

III. The sun is an important source of ultraviolet light.

IV. Most of the UV rays absorbed in the ozone ayer in the

atmosphere at an altitude of about 40-50 km.

Which of the following statements are correct?

(a) I, II and III (b) II, III and IV

(c) I, III and IV (d) All of these

36. Which of the following statements are correct?

I. The wavelength of microwave is greater than that of

UV-rays.

II. The wavelength of infrared rays is lesser than that of

UV-rays.

III. The wavelength of microwave is lesser than that of

infrared rays.

IV. Gamma rays have shortest wavelength in the

electromagnetic spectrum.

(a) I and II (b) II and III

(c) III and IV (d) I and IV

III Matching Type

37. Match the items of Column I and with these of Column II

and choose the correct potion from the codes given

below.

Column I Column II

A. EdA = Q/

1. Faraday’s

law

B. BdA = 0 2. Ampere-

Maxwell law

C. Edl = - Bd

dt

3. Gauss law for

electricity

D. Bi dl = 0ic + 00 Ed

dt

4. Gauss law for

magnetism

A B C D A B C D

(a) 4 3 2 1 (b) 3 2 1 4

(c) 3 4 1 2 (d) 1 2 3 4

38. Match the items of Column I with the time of Column II

and choose the correct option from the codes given

below.

Column I Column II

A. Radio 1. 54 Mhz

B. Amplitude

modulated

2. 88 MHz to 108 MHz

C. Short wave bands 3. 530 kMz to 1710 kHz

D. TV wave 4. 500 kHz to 1000 MHz

E. Frequency

modulated

5. 54 MHz to 890 MHz

A B C D E A B C D E

(a) 3 1 5 2 4 (b) 1 5 2 3 4

(c) 4 1 5 2 3 (d) 4 3 1 5 2

39. Match List I (Electromagnetic wave type) with List II (Its

association/application) and select the correct option

from the choices given below the list.

List I List II

A. Infrared waves 1. To treat muscular strain

B. Radio waves 2. For broadcasting

C. X-rays 3.To detect fracture of

bones

D. Ultraviolet 4. Absorbed by the ozone

layer of the atmosphere

A B C D A B C D

(a) 4 3 2 1 (b) 1 2 4 3

(c) 3 2 1 4 (d) 1 2 3 4

40. The wavelength of X-rays; -rays; ultraviolet rays and

microwaves are a, b, c and d, respectively then

(a) a > b (b) d > c (c) d < b (d) c > a

10




41. The source of electromagnetic waves can be a charge

(a) moving with a constant velocity

(b) moving in a circular orbit

(c) at rest

(d) falling in an electric field

ANSWER KEY 1 C 2 C 3 A 4 A 5 D 6 A 7 A 8 D 9 D 10 D 11 D 12 C 13 C 14 C 15 C 16 C 17 D 18 B 19 A 20 D 21 B 22 B 23 A 24 A 25 A 26 B 27 A 28 B 29 A 30 B 31 A 32 D 33 D 34 D 35 D 36 D 37 C 38 D 39 D 40 A,B,D 41 B,D

SOLUTION

1. (c) In conductor, there is no storage of charge, so conduction

current is the same as displacement current when

sources are both Direct Current (DC) and Alternating

Current (AC).

2. (c)

3. (a)

4. (a) The fact that an electric field changing with times gives

rise to a magnetic field, is the symmetrical counterpart

and is a consequence of the displacement current being a

source of a magnetic field.

5. (d) The most important prediction to emerge from

Maxwell’s equations is the existence of electromagnetic

waves, which are (coupled) time-varying electric and

magnetic fields that propagate in space. The speed of the

waves, according to these equations, turned out to be

very closed to the speed of light (3 × 108 ms-1), obtained

from optical measurements. This led to the remarkable

conclusion that light is an electromagnetic waves.

6. (a) The total current passing through any surface of which

the closed loop is the perimeter is the sum of the

conduction current and the displacement current. The

generalized law is E0 c 0 0

dB.dl i

dt

and is known

as Ampere-Maxwell law.

7. (a) Ampere's circuit law is a useful law that relates the net

magnetic field along a closed loop t the electric passing

through the loop.

0B.ds i

8. (d)

9. (d)

10. (d) In electromagnetic waves, the rest mas of a particle is

zero, then net force exerted on a particle is zero. So,

there is no deflection shown by a particle.

11. (d) Transverse elastic (sound) waves can also propagate in a

solid, which is rigid and that resist shear.

12. (c)

13. (c) The velocity of light depends on electric and magnetic

properties of the medium.

14. (c) During propagation of electromagnetic wave electric

energy density = Magnetic energy density.

15. (c)

16. (c)

17. (d) Electromagnetic waves include visible light, X-rays,

gamma rays, radio waves, microwaves and infrared

waves.

18. (b)

19. (a) Infrared waves are produced by hot bodies and

molecules. This band lies adjacent to the low-frequency

or long-wavelength end of the visible spectrum.

20. (d) Infrared waved radiations, are trapped by greenhouse

gases such as carbon dioxide and water vapour.

21. (b) Visible rays is the most familiar form of electromagnetic

waves. It is the part of the spectrum that is detected by

the human eye. It runs from about 4 × 1014 Hz to about 7

× 1014 Hz or a wavelength range of about 700-400 mm.

22. (b)

23. (a) Increasing order of energy

11




Radio wave < Microwave < Infrared < visible light <

ultra-violet < x-rays < Gamma-rays.

24. (a) UV-radiation is absorbed by ordinary glass.

25. (a)

26. (b) Gamma rays are used in treatment of Cancer.

27. (a) One common way to generate X-rays is to bombard a

metal target by high energy electrons. X-rays is to

bombard a metal target by high energy electrons.

28. (b) In electromagnetic spectrum, all components of electric

and magnetic field in vacuum are carrying same velocity

of light i.e., 3 × 108 ms-1.

29. (a)

30. (b) Diffraction takes places when the wavelength of wave is

comparable with the size of the obstacle in path. The

wavelength of radio waves is greater than the

wavelength of light waves. Therefore, radio waves are

diffracted around building.

31. (a) X-rays has wavelength about 1 nm to 10-3 nm which has

minimum wavelength and carries maximum energy ie..,

E ∝1

. So, It penetrates the target and hence are not

reflected back by target.

32. (d) Electromagnetic waves are self-sustaining oscillation of

electric and magnetic fields in free space, or vacuum.

They differ from all the other waves we have studied so

far, in respect that no material medium is involved in the

vibration of the electric and magnetic fields.

Sound waves in air are longitudinal waves of

compression and rarefaction. Transverse wave on the

surface of water consist of water moving up and down as

the wave spreads horizontally and radially onwards.

33. (d) The great technological importance of electromagnetic

waves stems from their capability to carry energy from

one place to another. The radio and TV signals from

broadcasting stations carry energy. Light carries energy

from the sun to the earth, thus making life possible on

the earth.

34. (d) Radio waves are produced by the accelerated motion of

charges in conducting wire. They are used in radio and

television communication system. They are generally in

the frequency range from 500 kHz to about 1000 MHz.

The AM (amplitude modulated) band is from 530 kHz to

1710 kHz. Higher frequencies upto 54 MHz are used for

short wave bands. TV wave range from 54 MHz to 890

MHz.

The FM (frequency modulated) radio band extends from

88 MHz to 108 MHz. cellular phones use radio waves to

transmit voice communication in the Ultra High

Frequency (UHF) band.

35. (d) Ultraviolet rays covers wavelength ranging from about 4

× 10-7 m (400 nm) down to 6 × 10-10 m (0.6 nm). It is

produced by special lamps and very hot bodies. The sun

is an important source of ultraviolet light.

But fortunately, most of it is absorbed in the ozone layer

in the atmosphere at an altitude of about 40-50 km.

36. (d) Increasing order of energy

Radio wave < Microwave < Infrared < visible light <

ultra-violet < x-rays < Gamma-rays.

Decreasing order of energy

Radio wave > Microwave > Infrared > visible light >

ultra-violet > x-rays > Gamma-rays.

37. (c)

38. (d)

39. (d)

40. (a, b, d)

41. (b, d)

Here, in option (b) charge is moving in a circular orbit.

In circular motion, the direction of the motion of charge

is changing continuously, thus it is an accelerated motion

and this option is correct.

Also, we know that a charge starts accelerating when it

falls in an electric field.

SSC CDS

BANKrAILWAY



Physics



1999 to till date

C L A S S E S

Chapter - 10

(Wave & Oscillations)

1




WAVES AND OSCILLATIONS A wave is a disturbance of a field in which a physical attribute

oscillates repeatedly at each point or propagates from each point

to neighboring points, or seems to move through space. Wave Motion

A wave motion is a means of transferring energy and momentum

from one point to another without any actual transportation of

matter between these points.

Types of waves

Mechanical Waves These waves can be produced or propagated only in a material medium. These waves are governed by Newton's laws of motion. For example, waves on water surface, waves on strings, sound waves etc.

Electromagnetic Waves These do not require a material medium for their production and propagation, they can pass through vacuum and any other material medium. Ultra-violet; radio waves; microwaves, X-rays, speed of all EM waves , c = 3 × 108 m/s

Matter Waves (or de Broglie Waves)

These waves are associated with moving particles of matter, like electrons, protons, neutrons, atoms, molecules, big physical bodies etc.

Transverse WaveMotion In this, individual particles of the medium execute simple harmonic motion about their mean position in a direction perpendicular to the direction of propagation of wave motion. For example (i) movement of string of a sitar or violin, (ii) movement of the membrane of a Tabla or dholak, (iii) movement of a kink on a rope, (iv) light waves and all other electromagnetic waves are also transverse waves.

Longitudinal Wave Motion A longitudinal wave motion is that wave motion in which individual particles of the medium execute simple harmonic motion about their mean position along the same direction along which the wave. For example (i) Sound waves travel through air in the form of longitudinal waves, (ii) vibrations of air column in organ pipes are longitudinal, (iii) vibrations of air column above the surface of water in the tube of a resonance apparatus are longitudinal.

Crest A crest is a portion of the medium, which is raised temporarily above the normal position of rest of the particles of the medium, when a transverse waves passes through it.

Trough A trough is a portion of the medium, which is depressed temporarily below the normal position of rest of the particles of the medium, when transverse waves passes through it.

Compression A compression is a region of the medium in which particles are compressed.

Rarefaction A Rarefaction is a region of the medium in which particles are farther apart.

2




3




Waves and Oscillations

Periodic Motion • In this, action repeats regularly after a fixed interval of time.

Sun

Earth revolves

periodically around sun.

Oscillatary Motion Body moves to and for about a fixed point repeatedly. • Action repeats after a fixed interval of time

But period of oscillation is small

Harmonic Harmonic can be expressed in terms of single harmonic function (i.e., sine function or cosine function).

Non-Harmonic Non-Harmonic oscillation is that oscillation which cannot be expressed in term of single harmonic function. A non-harmonic oscillation is a combination of two or more than two harmonic oscillations. Mathematically, non harmonic oscillation may be expresses as

y asin t bsin2 t

2 4 ty asin t bsin

T T

Simple Harmonic Motion

F = –k x or = –C If the restoring force/torque acting on the body in oscillatory motion is directly proportional to the displacement of body/particle and i.e. F x or & this force is always directed towards equilibrium position, then motion is called simple harmonic motion (HM) A harmonic oscillation of constant amplitude and of single frequency is called simple harmonic oscillation. Mathematically, a simple SHM be expressed as y = a sin t = a sin 2 t/T ___ (1) or y = a cos t = a cos 2 t/T ___ (2) Here, y = displacement of body from mean position at any instant t. a = maximum displacement or amplitude of displacement of the body. = angular frequency (= 2 ) = (2 /T) = frequency and T = time period of harmonic oscillation.

Equation of SHM:

(a)F = -ky

(b) d2y/dt2 +ω2y = 0

Here ω = √k/m (k is force constant)

Conditions of Simple Harmonic Motion

4




For SHM to occur, three conditions must be satisfied.

(a) There must be a position of stable equilibrium

At the stable equilibrium potential energy is minimum.

So, dU/dy= 0 and d2U/dy2> 0

(b) There must be no dissipation of energy

(c) The acceleration is proportional to the displacement and opposite in direction.

That is,a = -ω2y

Some important terms

Displacement: The name displacement is given to a change in physical quantity under consideration with time in a periodic

motion. Thus, displacement represent changes in physical quantities with time such as position, angle, pressure, electric and

magnetic fields etc.

Examples:

(i) In a loaded spring:When a body is oscillating under the action of a spring, the displacement variable is its deviation from the

mean position of the oscillation, with time.

(ii) In a simple pendulum:The displacement variable is its angular deviation from the vertical position during oscillations, with

time.

(iii) During the propagation of sound wave in air:The displacement variable is the local change in pressure, with time.

(iv) During the propagation of electromagnetic waves: The displacement variables are electric and magnetic fields which vary

periodically, with time.

Initial Phase or Epoch

It is the phase of a vibrating particle corresponding to time t = 0. When t = 0, = 0.vIts unit is radian.

Measurement of Phases

(i) When the two vibrating particles cross their mean positions at the

same time, moving in the same direction.

y1 = a sin t y2 = b sin t

The phase difference between them is zero.

(ii) When the two vibrating particles cross their mean position at the

same time, moving in the opposite directions and the particle A is ahead

of particle B by half vibration.

y1 = a sin (t + ) y2 = a sin t

The phase difference between them israd or 180°.

(iii) When the particle A is passing from the extreme position with a

further tendency to move towards left hand side and the particle B is

5




passing from the mean position with a further tendency to move towards the right hand side.

y1 = a sin (t + /2) y2 = b sin t

(iv) When the displacement of particle A is ahead of particle B by 1/8 vibration. The motions of two particles can be represented by

y1 = a sin t y2 = b sin (t – /4)

/4 rad or 45°.

Simple Pendulum

A simple pendulum is the most common example of bodies executing SHM.

An ideal simple pendulum consists of a heavy point mass body suspended by a weightless

inextensible and perfectly flexible string from a rigid support about which it is free to oscillate.

Law’s of simple pendulum:

Laws of isochronisms: Its states that (≤4°), the time period of a simple pendulum is independent

of its amplitude.

Laws of length: It states that time period of a simple pendulum varies directly as the square root of

its length.

T∝√l

Law of acceleration due to gravity: It states that, the time period of a simple pendulum varies inversely as the square root of

acceleration due to gravity at that place.

T∝1/√g

So, Time period of simple pendulum,T= 2π√l/g

(a) When placed inside a lift being accelerated upwards, the effective value of gincreases. Thus, the time period of pendulum decreases.

(b) When placed inside a lift being accelerated downwards, the effective value of g decreases. Thus, the time period of pendulum

increases.

(c) Time period of the pendulum increases at higher altitudes due to decrease in g.

(d) Time period of the pendulum at a place below the surface of earth decrease due to increasing g.

(e) At the center of earth (g=0). So, the time period is infinite.

(f) Time period is greater at equator than at poles.

(g) Due to decrease in the value of g due to rotation of earth, the time period of the pendulum increases as the earth rotates about its

axis.

Read TOPIC variation of acceleration due to gravity in Chapter – GRAVITATION

(h) Equation of motion: d2θ/dt2+(g/l)θ = 0

(i) Frequency, f =1/2π √(g/l)

(j) Angular frequency, ω =√(g/l)

• Second Pendulum: A second’spendulumis that pendulum whose time period is two second.

Time period of second pendulum,

dT = 2 sec

So, l = 0.993 m

• Time period of bar pendulum:

T = 2π√I/mgl

Here I isthe rotational inertia of the pendulum.

and

T = 2π√L/g

Here, L = (k2/l)+l

6




• Time period of torsion pendulum:

7




When disc is rotated in horizontal plane, wire gets twisted up and it produces an opposing force due to which disc moves in

Oscillatory motion.

T = 2I

C

I moment of inertia about point C.

C constant opposing twisting motion

TTortional constant of wire.

Conical Pendulum:

Time period, T = 2π√(Lcosθ/g)

Velocity, v = √(gRtanθ)

Compound Pendulum

S point where solid object is pivoted

C Center of mass of object.

l distance between S and C

So,T = 2I

mgl I = mL2

I moment of Inertia about points S.

• Resonance : It is a phenomenon that only occurs when the frequency at which a force is periodically applied is equal or nearly

equal to one of the natural frequencies of the system on which it acts. This causes the system to oscillate with larger amplitude

than when the force is applied at other frequencies

• Frequencies at which the response amplitude is a relative maximum are known as resonant frequencies or resonance

frequencies of the system. Near resonant frequencies, small periodic forces have the ability to produce large amplitude

oscillations, due to the storage of Vibrational energy.

Examples:

Swing

A playground swing is a familiar example of the resonance. When we push the swing, it swings forward and

backwards. If a series of regular pushes are given to the swing, its motion can be built enormously. The man

who is pushing the string has to match the timing of the swing. The pusher has to synchronize with the

timing of the swing. This cause the motion of the swing to have increased amplitude and the swing will go

higher. Once when the swing reaches its natural frequency of oscillation, a gentle push to the swing helps it

maintain that amplitude of swing throughout due to resonance. We call this in-sync motion, as Resonance.

But, if pushes are given irregular, the swing will hardly vibrate, and this out-of-sync motion will never lead

to resonance, and the swing will not go higher.

Bridge

A group of soldiers while marching on the bridge are asked to break their steps very often.

Their rhythmic march can set extreme vibrations at the bridge’s natural frequency. If their

synchronized footsteps resonate with the natural frequency of the bridge, it can shake the

bridge apart. So, When designing such structures, the engineers ensure that the resonant

frequencies of components are different from that of other oscillating components. The

biggest example of this is Tacoma Bridge Collapse, in which the frequency of the air

matches with the frequency of the bridge, leading to its collapse.

8




9




Radio

When we turn the knob of the radio to our favorite channel, we are changing the natural frequency of the receiver. The natural

frequency of the receiver then matches to the transmission frequency of the radio station. When two frequencies match, energy

transfer occurs and we can listen to the selected channel.

Microwave Oven

The food is cooked very efficiently, evenly and quickly in a microwave because of resonance. The microwave oven has a certain

wavelength and frequency. And like all other objects, Molecules of water and fat also have a resonant frequency. At a certain

frequency, the molecules absorb wavelengths and start vibrating, causing cooking and heating up of food.

Oscillations

Damped SHM

If retarding force acts on the system continually, then the

amplitude of wave will decrease continually, it is known as

Damped SHM.

Amplitude decrease with time

Undamped

When a simple harmonic system oscillates with a constant

amplitude which does not change with time, its oscillations

are called undamped simple harmonic oscillations.

Damping is the process of continuous decrease in the amplitude of a signal under effect of an external retarding force.

So, when the amplitude of signal comes to zero, the body is at rest.

Oscillations

Free Oscillations

If it vibrates with its own natural

frequency without the help of any

external periodic force.

Examples: if you strike a tuning fork,

it will begin to vibrate for some time

after you struck it

Forced Oscillations

When a body oscillates with the help

of an external periodic force with a

frequency different from the natural

frequency of the body, its oscillations

are called forced oscillations.

Examples: when you push someone

on a swing, you have to keep

periodically pushing them so that the

swing doesn’t reduce.

Resonant Oscillations

When a body oscillates with its own

natural frequency 0, with the help of

an external periodic force whose

frequency is equal to the natural

frequency of the body, the

oscillations of the body are called

resonant oscillations.

Examples: singer breaking a wine

glass

Condition required for Propagation of Transverse waves through a medium.

For the propagation of transverse waves, the material medium must possess the following four characteristics:

(i) Elasticity: So that particles can return to their mean position, after having been disturbed.

10




(ii) Inertia: So that particles can store energy and overshoot their mean position.

(iii) Minimum Friction: amongst the particles of the medium ensures minimum loss of energy so that waves can travel long

distances.

(iv) Uniform density of the medium.

Standing or Stationary Wave

If two or more waves interfere which have slightly different frequencies, than the intensity of resulting wave will have alternate

maxima and minima.

The number of minima or maxima in one second or number of beats heard per second is called Beat frequency.

Beat frequency = (f1 – f2)= Difference in frequency

Every point on the string vibrates in same phase with same frequency but different amplitudes which depend on position x of point

along string.

This type of wave motion represented by equation is called standing wave because it appears to travel neither to left nor to right.

Nodes: The positions along the string for which the amplitude of oscillation is always zero. (Say at ends)

Antinodes: position along the string for which amplitude of oscillation is maximum.

Standing or Stationary Wave

Longitudinal Stationary (Long wave + long wave)

Longitudinal stationary waves are formed as a result of superimposition of two identical longitudinal waves travelling in opposite directions. For example, Wave produced in organ pipes, Reasonance tube apparatus

Transverse Stationary Waves (Transverse wave + transverse wave)

Transverse stationary waves are formed as a result of superimposition of two identical transverse waves travelling in opposite directions. For example, Stationary waves produced on the vibrating string of a sonometer

Beats

When two sound waves of equal amplitudes and nearly equal frequencies travelling in a medium along the same direction,

superimpose on each other, the intensity of the resultant sound at a particular position rises and falls alternately with time.

If intensity of sound is maximum at time t = 0, one beat is said to be formed when intensity becomes maximum again, after

becoming minimum once in between.

For the formation of distinct beats, difference in frequencies of two sources must be small, say less than 10.

11




Doppler's Effect

According to Doppler's effect, whenever there is a relative motion between a source of sound and listener, the apparent frequency

of sound heard by the listener is different from the actual frequency of sound emitted by the source.

Doppler effect is the motion related change in frequency of sound.

f = foo

s

v v

v v

f observed frequency

fo original frequency

v Speed of wave in medium(always +ve)

vovelocity of observer

vsvelocity of source

12




Sign Convention: All velocities along the direction S to L are taken as positive and all velocities along the direction L to S are taken

as negative.(S- Source,L-Listener)

There is no change in the frequency of sound heard if there is a small displacement of the source and listener at right angle to the

direction of wave propagation.

There will be no Doppler effect, i.e., no change in frequency in the following cases:

1. There is always an increase in frequency or pitch, if source moves towards listener or listener moves towards source or both

move towards each other.

2. While the apparent frequency decreases when either or both move away from each other.

3. This change in frequency depends on speeds of source and listener and not on the distance between them.

Applications of Doppler's Effect

The change in frequency caused by a moving source/observer/listener is called Doppler Shift.

(i) By police to check over speeding of vehicles,

(ii) At airports to guide the aircraft.

(iii) In the military to detect enemy aircrafts.

(iv) By astrophysicists to measure the velocities of planets and stars,

(v) To study heart beats and blood flow in different parts of the body.

13




Practice Questions 1. The correct possible order of interconversion of waves

when our sound is transferred to the receiver via a

mobile phone is

(a) sound waves electric signal electromagnetic

waves (radio waves)

(b) sound waves electromagnetic waves electric

signal

(c) sound waves electric signal sound wave

(d) None of these

2. Which of the following option(s) is/are false for

mechanical waves?

(a) These wave require a medium for propagation, they

cannot propagate through vacuum

(b) They involve oscillation of constituent particles

(c) They depend on the elastic properties of the medium.

(d) They have the same speed in all the mediums.

3. The wave generated from up and down jerk given to the

string or by up and down motion of the piston at end of

the pipe is

(a) transverse or longitudinal

(b) progressive

(c) standing


4. In which of the following case the material medium as a

whole does not move?

(a) A stream (b) A wind

(c) Water wave (d) Both (a) and (c)

5. The picture of a progressive transverse wave at a

particular instant of time gives

(a) shape of the wave

(b) motion of the particle of the medium

(c) velocity of the wave

(d) None of the above

6. In a progressive wave along x – directions, at a particular

location the particles of the medium are executing

(a) oscillatory motion (b) rectilinear motion

(c) rotational motion (d) None of these

Topic – 2

Interference and Superposition of Waves

7. When two wave pulses travelling in opposite direction

overlap, the resultant displacement is

(a) the algebraic sum of the displacement due to each

pulse

(b) always zero

(c) the vector in the direction of the displacement of

right travelling pulse


8. Two pulses having equal and opposite displacements

moving in opposite directions overlap at t = t1s. The

resultant displacement of the wave at t = t1s is.

(a) twice the displacement of each pulse

(b) half the displacement of each pulse

(c) zero

Either (a) or (c)(d)

9. A simple wave motion is represented by y = 5 (sin 4π +

3 cos 4πt). It amplitude is

(a) 5 (b) 5 3

(c) 10 3 (d) 10

10. The correct order of speed of sound in different media is

(consider constant temperature)

(a) min

air solid liquidalu ium water

v v v

(b) minsolid air liquidaly ium water

v v v

(c) minsolid liquid airaly ium water

v v v

(d) None of these

11. The frequency change observed due to motion of the

observer or the source or both is called

(a) Beat frequency (b) Doppler effect

(c) Doppler frequency (d) Both (b) and (c)

12. Doppler’s effect holds goods for

(a) only sound waves

(b) only electromagnetic waves

(c) Both sound waves and electromagnetic waves


13. Match the items in Column I with terms in Column II and

choose the correct options from the codes given below.

Column I Column II

A. Constructive

interference of two

waves of same

1. Beats

14




frequency and

amplitude

B. Destructive

interference of two

waves of same

frequency and

amplitude

2. Nodes

C. Interference of two

waves of nearly same

frequency and equal

amplitude.

3. Antinodes

A B C A B C

(a) 1 2 3 (b) 2 3 1

(c) 3 2 1 (d) 2 1 3

14. Column I has figures showing different modes of

oscillation of the system (a straight tied at both the ends)

and Column II has name of the corresponding mode.

Match the items in Column I with terms in Column II and

choose the correct option from the codes given below.

Column I Column II

A. 1. Fundamental mode

B. 2. Second harmonic

C. 3. Sixth harmonic

D. 4. Fifth harmonic

A B C D A B C D

(a) 4 2 3 1 (b) 4 3 1 2

(c) 3 2 1 4 (d) 2 3 1 4

15. Water wave produced by a motorboat sailing in water

are

(a) neither longitudinal nor transverse

(b) both longitudinal and transverse

(c) only longitudinal

(d) only transverse

16. Speed of sound water in air

(a) is independent of temperature

(b) increases with pressure

(c) increases with increases in humidity

(d) decreases with increases in humidity

17. Change in temperature of the medium changes

(a) frequency of sound waves

(b) amplitude of sound waves

(c) wavelength of sound waves

(d) loudness of sound waves

18. With propagation of longitudinal waves through a

medium, the quantity transmitted is

(a) matter (b) energy

(c) energy and matter (d) energy, matter and momentum

19. A train whistling at constant frequency is moving

towards a station at a constant speed v. The trains goes

past a stationary observer on the station. The frequency

n’ of the sound as heard by the observer is plotted as a

function of time t (figure). Identify the expected curve.

15




20. Speed of sound wave in a fluid depends upon

(a) directly on density of the medium

(b) square of bulk modulus of the medium

(c) inversely on the square root of density

(d) directly on the square root of bulk modulus of the

medium

21. During propagation of a plane progressive mechanical

wave

(a) all the particle are vibrating in the same phase

(b) amplitude of all the particles is equal

(c) particles of the medium executes SHM

(d) wave velocity depends upon the nature of the

medium

22. The motion of satellites and planets is

(a) periodic (b) oscillatory

(c) simple harmonic (d) non periodic

23. The motion of a swing is

(a) periodic but not oscillatory

(b) oscillatory

(c) linear simple harmonic

(d) circular motion.

24. Choose the correct option.

(a) Every periodic motion is oscillatory

(b) Ever oscillatory motion is periodic



25. The example of periodic function is

(a) A tan 2

t (b) 1

cot2

A t

(c) A cos t (d) A cos t sin 2 t

26. SHM could be related to

(a) non – uniform circular motion

(b) uniform circular motion

(c) straight line motion

(d) projectile motion

27. In simple harmonic motion, the force

(a) is constant in magnitude only

(b) is constant in direction only.

(c) varies in magnitude as well as in direction

(d) is constant in both magnitude and direction.

28. In SHM,

(a) PE is stored due to elasticity of system

(b) KE is stored due to inertia of system

(c) Both KE and PE are stored by virtue of elasticity of

system.

(d) Both (a) an (b)

29. The nature of damped oscillation is

(a) absolutely periodic (b) apparently periodic

(c) not periodic (d) None of these

30. Choose x-t graph for an insect climbing up a ramp

uniformly and sliding down and comes back to initial

point and repeats the process identically.

31. A child climbs up a step waited for sometimes on step

and comes down and repeats the process. Choose x-t

graph for this process.

32. The periodic function f(t) = A sin (t) repeats itself

(a) 2π (b) 3π

(c) π (d) π/2

33. Choose the periodic function from the following.

(a) A sin3 (t)2 (b) sin (t) + cos (t)

(c) tan(t)3 (d) et

34. A ball is moving in uniform circular motion in a

horizontal plane, the shadow of ball will execute ……….

On the wall.

(a) projectile motion

(b) uniform circular motion

(c) simple harmonic motion

(d) non-uniform circular motion

35. Match the following columns and choose the correct

options from the codes given below.

16




Column I Column II

A. Uniform circular

motion

1. Projectile

B. Motion of a pendulum 2. Rectilinear

C. Motion of a car on a

straight road

3. Oscillatory

D. Motion of a ball

thrown by a boy at an

angle with horizontal

4. Periodic

A B C D A B C D

(a) 2 3 2 1 (b) 4 3,4 2 1

(c) 3 4 1 2 (d) 4 3 1 2

ANSWER KEY

1 A 2 D 3 D 4 C 5 A

6 A 7 A 8 C 9 D 10 C

11 B 12 C 13 C 14 B 15 B

16 C 17 C 18 B 19 C 20 CD

21 BCD 22 A 23 B 24 B 25 C

26 B 27 C 28 D 29 B 30 C

31 C 32 A 33 B 34 C 35 B

SOLUTION 1. (a) Sound waves are first converted into an electric signal

which in turn generation electromagnetic waves that

may be transmitted by an optical cable or via satellite.

2. (d) Mechanical waves have different speeds in different

mediums.

3. (d) The wave generated from up and down jerk given to the

string or by up and down motion of piston may be

transverse or longitudinal waves. It is also called

progressive wave.

4. (c) In a water wave, it is the disturbance that moves, not

water as a whole.

5. (a) The function of both position x and time t at every

instant gives the shape of the wave at that instant.

6. (a) Along x-direction, particles of the medium are executing

oscillatory motion.

7. (a) When the pulses overlap, the resultant displacement is

the algebraic sum of the displacement due to each pulse.

8. (c) The displacement due to two pulses will exactly cancel

out each other and there is no displacement throughout.

9 (d) Given, equation for wave motion is

y = 5 (sin 4πt + 3 cos 4πt)

= [5 sin 4πt + 5 3 cos 4πt] … (i)

For left travelling wave,

y = A sin (kx + t) + B cos (kx + t) … (ii)

= a sin (kx + t + ϕ)

where,a = 2 2A B ; ϕ = tan- 1 (B/A)

Eq. (i) represents Eq. (ii), where x = 0

On comparing Eqs. (i) and (ii), we get

a = 2

25 5 3 25 75 = 10

10. (c) minsolidalu ium

v = 6420ms-1

liquidwater

v = 1482 ms-1, vair = 343 ms-1

∴ minsolidalu ium

v > liquidwater

v >Vair

11. (b) The motion-related frequency change is called Doppler

effect.

12. (c) Doppler’s effect holds not only for sound waves but also

for electromagnetic waves.

13. (c) Constructive interference produces maximum amplitude

and gives the position of anti-node. Destructive

17




interference gives position of nodes. Beats are formed by

interference of waves of nearly same frequencies.

14. (b) The fixed end points behave as nodes.

(A) Number of nodes = 6

Distance between consecutive nodes =2

From figure,

Length of the string = L = 55

2 2

… (i)

Also, we know

L =2

n; n = 1, 2, 3, … … (ii)

On comparing Eqs. (i) and (ii), we get

n = 5

Thus, the figure represents fifth harmonic ⇒ (A → 4)

(B) Number of nodes = 7

L = 62

… (i)

Again, comparing with L =2

n, we get n = 6

⇒ Sixth harmonic ⇒ (B → 3)

(C) Number of nodes = 2, L =2

So, n = 2 or second harmonic ⇒ (D → 2)

15. (b) Water waves produced by a motorboat sailing in water

are both longitudinal and transverse, because the waves,

produce transverse as well as lateral vibrations in the

particle of the medium.

16. (c) Due to presence of moisture density of air decreases.

We know that speed of sound in air is given by

v =p

For air and p are constants.

17. (c) Speed of sound wave in a medium v ∝ T (where T is

temperature of the medium)

Clearly, when temperature changes speed also changes.

As,v = v

where v is frequency and is wavelength.

Frequency (v) remains fixed

⇒v ∝ or or v

As v does not change, so wavelength () chanes.

18. (b) Propagation of longitudinal waves through a medium

leads to transmission of energy through the medium

without matter being transmitted.

There is no movement of matter (mass) and hence

momentum.

19. (c) Let the original frequency of the source is n0.

Let the speed of sound wave in the medium is v.

As observer is stationary

Apparent frequency na =s

v

v v

n0

[when train is approaching]

=s

v

v v

n0 = na> n0

When the train is going away from the observer

Apparent frequency na =s

v

v v

n0 = na< n0

Hence, the expected curve is (c).

20. (c, d) Speed of sound waves in a fluid is given by

v =B

, where B is bulk modulus and is density of the

medium.

Clearly, v ∝1

(∴ for any fluid, B = constant)

and v ∝ B (∵ for medium, = constant)

21. (b, c, d,)

During propagation of a plane progressive mechanical

wave, like shown in the diagram, amplitude of all the

particle is equal.

(i) Clearly, the particle O, A and B are having different

phase.

(ii) Particles of the wave shown in the figure are having

up and down SHM.

(iii) For a progressive wave propagation in flud.

Speed = v =B

Hence, v ∝1

(∵ B is constant)

As depends upon nature of the medium, hence v also

depends upon the nature of the medium.

18




22. (a) The motion of planets and satellites are repetitive and

repeats after a fixed interval of time. These type of

motions are known as periodic motion.

23. (b) Swinging in a swing, motion of the pendulum of a wall

clock, motion of branches of tree in a wind, piston of a

car engine all are the examples of a to and from

respective motion abut a mean position. This type of

motion is known as oscillatory motion.

24. (b) Every oscillatory motion is periodic but every periodic

motion need not to be oscillatory. The motion of moon

around the earth is periodic but not oscillatory. Circular

motion is a periodic motion, but it is not oscillatory.

Motion of simple pendulum is oscillatory and periodic

both.

25. (c) A cos (t) is an example of periodic function. This

function is periodic in time with time periodic (T) such

that T =2

. tan (t)2 is non-periodic, also cot (t)1/2 and

A cos t sin 2t are non-periodic.

26. (b) SHM could be related to uniform circular motion. The

projection of uniform circular motion on a diameter of

the circular follows simple harmonic motion.

27. (c) In SHM force varies in magnitude as well as in direction.

28. (a) In SHM, potential energy depends on its spring behavior

and kinetic energy on its inertial behavior.

In case of mass m oscillating on spring. KE is due to m

and PE is due to spring.

29. (b) In damped oscillations the energy of the system is

continuously decreasing, still its motion is apparently

periodic. In damped oscillation, energy of system is

dissipated continuously.

30. (c) An insect climbing up a ramp and falling down with

uniform speed is represented by the following graph.

For uniform velocity motion, the x-t graph is a straight

line.

The upward motion is represented by straight line with

positive slope (considering motion in upward direction

as positive) and the downward motion is represented by

a straight line with negative slope.

31. (c) If a child climbs up a step, comes down and repeats the

process, its height above the ground can be represented

by the below the graph.

In the above graph, in the interval (0 – t1) the child is at

ground level. During interval (t2 – t1) child climbs up the

step and that too with uniform velocity. During interval

(t3 – t2) the child is at the step waiting, so height is not

changing during the interval.

During interval (t4 – t3) the child is returning back to

ground, again with uniform velocity. The same shape of

the graph repeats after every T (i.e., time period)

seconds.

32. (a) A periodic function repeats itself after a time period T.

and; f(t) = f(t + T)

As, sin (t) = sin (t + 2π)

∴ period of function is 2π.

33. (b) (sin t + cos t) = 1 1

2 sin t cos2 2

t

= 2 cos sin sin cos t4 4

t

= 2 sin4

t

So, above function has amplitude 2 with phase

constant π/4. This is periodic function with period 2π/.

34. (c)

35. (b) circular motion → Periodic ⇒ A → 4

Motion of a pendulum → Oscillatory as well as periodic

⇒ B → 3,4

Motion of car on a straight road → Rectilinear ⇒ C → 2

Motion of a ball thrown by a boy → Projectile ⇒ D → 1 at

an angle with horizontal.

SSC CDS

BANKrAILWAY



Physics



1999 to till date

C L A S S E S

Chapter - 11

(Semiconductor Electronics)

1




SEMICONDUCTOR ELECTRONICS In terms of the electrical properties, materials can be classified into three groups: conductors, semiconductors, and insulators.

Metals (Conductors): The energy band diagram for a metal is such that the conduction band is partially filled with

electrons, the conduction and valence band partly overlap each other and there is no forbidden band gap in between.

In this situation, large number of electrons are available for electrical conduction, hence the resistance of such a material is low or

the conductivity is high. Even if a small electric field is applied across the metal, these free electrons start moving in a direction

opposite to the direction of electric field(As electric field is from positive to Negative and Electrons move from negative to

positive). Due to it, a current begins to flow through it and hence metal behaves as a conductor.

Insulators: Here, the valence band is completely filled, the conduction band is empty and energy gap is quite large

(Eg> 3 eV). For example, in case of diamond, the energy gap is of 6 eV.Due to large energy gap, no electron is able to go from the

valence band to the conduction band even if electric field is applied. Hence electrical conduction in these materials is impossible

and they behave as insulators.

Semiconductors: Here, the valence band is totally filled and the conduction band is empty but the energy gap between conduction

band and valence band is quite small. It is less than 3 eV. For example, the energy gap for germanium is of

0.72 eV and for silicon it is of 1.1 eV. At zero kelvin temperature, electrons are not able to cross even this small energy gap and

hence the conduction band remains totally empty. Therefore, the semiconductor at zero kelvin behaves as insulator. However,

at room temperature, some electrons in the valence band acquire thermal energy greater than energy band gap less than 3 eV and

jump over to the conduction band where they are free to move under the influence of even a small electric field. As a result of it, the

semiconductor acquires small conductivity at room temperature. The resistance of semiconductor would not be as high as that

of insulator.

A pure semiconductor which is free from every impurity is called intrinsic semiconductor.

The electronic configuration of silicon and germanium are as follows:

Silicon (14), 1s1, 2s2, 2p6 3s2 3p2

Germanium (32), 1s2, 2s2 2p6,3s2 3p6 3d10, 4s2 4p2

Both the atoms have thus four valence electrons. The four valence electrons of a germanium atom form four covalent bonds by

sharing the electrons of surrounding four germanium atoms. Each covalent bond shares two electrons one from each atom.

At temperature 0K, for Ge-structure, the valence band is all full. The energy gap is 0.72 eV and the conduction band is totally

empty. Since no electron is available for conduction therefore, the Ge-crystal at 0K acts as electrical insulator. The conduction is

possible if some of the electrons break away from their covalent bonds and become free. The minimum energy required to break a

covalent bond is 0.72 eV for Ge and 1.1 eV for Si.

2




3




When an electron breaks away from a covalent bond, the empty place or vacancy left in the bond is called a hole. When an external

electric field is applied, these free electrons and holes move in opposite directions and constitute a current flow through the

germanium crystal. The number of free electrons (in conduction band) and holes (in valence band) are exactly equal in an

intrinsic semiconductor.

ne = nh = ni

In a pure semiconductor, at room temperature, the number of intrinsic chargecarries (electrons and holes) is very small ( 1016 m–3).

That is why the pure semiconductor has low conductivity.

It is important to note that

1. In an intrinsic semiconductor

(i) there are two types of current carriers(i.e., free electrons and holes)

(ii) number of electrons is equal to number of holes.

2. An intrinsic semiconductor is electrically neutral as a whole.

3. In a semiconductor the total current is due to the movement of both the free

electrons and holes.

A hole is considered as a seat of positive charge, having charge equal to that of an

electron.

Doping is a process of addition of a desirable impurity atom to a pure semiconductor to modify its properties in a controlled

manner. The impurity atoms added are called dopants.

Conditions for Doping

The impurity added may be 1 part per million (ppm).

Doping of a semiconductor increases its electrical conductivity to a great extent.

Methods of doping: Doping is achieved in many ways; for this, we can

1. Add the impurity atoms in the melt of the semiconductor

2. Heat the crystalline semiconductor in an atmosphere containing dopant atoms or molecules so that the latter diffuse into the

semiconductor

3. Implant dopant atoms or molecules by bombarding the semiconductor with them.

A doped semiconductor or a semi-conductor with suitable impurity atom added to it, is called extrinsic semiconductor.

INTRINSIC SEMICONDUCTOR EXTRINIC SEMICONDCUTOR

1. It is pure semiconducting material and no impurity atoms

are added to it.

2. Examples are crystalline forms of pure silicon and

germanium.

3. No. of e– = no. of holes

4. Its electrical conductivity is low.

5. Its electrical conductivity is a function of temperature

alone.

1. It is prepared by doping a small quantity of impurity

atoms to the pure semiconducting material.

2. Examples are silicon and germanium crystals doped

with phosphorous,indium, boron, aluminium etc.

3. Its electrical conductivity is high.

4.Its electrical conductivity depends upon the

temperature as well as on the quantity of impurity atoms

doped in the structure.

4




Extrinsic semi-conductors are of two types:

(i) n-type semiconductor (ii) p-type semiconductor

n-type semiconductor p-type semicondcutor

1. Group 5 + Group 4

2. The impurity atoms added, provide extra electrons in

the structure, and are called donor atoms.

3. The electrons are majority carriers and holes are

minority carriers.

4. the electrons density (ne) is much greater than the hole

density (nh).

5. The donor energy level is close to the conduction band

and far away from valence band.

6. The fermi energy level lies in between the donor energy

level and conduction band.

1. It is an extrinsic semiconductor which is obtained by

doping the impurity atoms of III group of periodic tableto

the pure germanium or silicon semiconductor.

Group 4 + Group 3

2. The impurity atom added, create vacancies of electrons

(i.e., holes) in the structure and are called acceptor atoms.

3. The holes are majority carriers and electrons are

minority carriers.

4. The hole density (nh) is much greater than the electron

density (ne)

5. The acceptor energy level is close to valence band and

is far away from conduction band.

6. The fermi energy level lies in between the acceptor

energy level and valence band.

5




Fermi Level: It is the energy level between Valence band and conduction where the probability of finding an electron is 50%.

Fermi energy is the maximum possible energy possessed by free electrons of a material at absolute zero temperature (i.e., 0 K).

The value of fermi energy for different materials is different.

Due to doping, the position of Fermi Level changes

as it moves towards valence band in P-type semiconductor

and towards conduction band in N-type semiconductor

Effect of temperature on semiconductors

With the increase in temperature, the mobility of electrons and holes in a semiconductor actually decreases, like the decreases in

mobility of electrons in metals. But, there is a large increase in the charge carrier concentration due to more breakage of covalent

bonds with the increasing temperature. It is so large that conductivity increases with increase in temperature and the decrease in

mobility has no influence.

With the increase in temperature the number of electrons in conduction band increases. Due to it, the conductivity of

semiconductor increases with increase in temperature.

With the rise in temperature

(i) the resistance of a metallic conductor increases

(ii) the conductivity of a metallic conductor decreases

(iii) the resistance of a semiconductor decreases

(iv) the conductivity of a semiconductor increases due to production of more number of electrons and holes.

Vacuum Diode or CRT's

6




Vacuum tubes (also called valves) like the diode valve which has two electrodes(i.e anode (called plate) and cathode. Triode

valve which has three electrodes (i.e., plate, cathode and grid).

Tetrode valve having four electrodes (i.e., plate, cathode and two grids).

Pentode valve having five electrodes (i.e., plate, cathode and three grids).

In these vacuum tubes, the electrons are provided by heating the cathode using low tension battery. The controlled flow of

electrons is achieved by varying the voltage between its different electrodes. The electrons strike the screen and give us a pictorial

representation of data. A vacuum is created between inter-electrodes so that the moving electrons may not lose their energy on

collision with air molecules in their way. In these vacuum tubes, the electrons can flow only in one direction (i.e., from cathode to

anode), hence they are called as valves. The vacuum tube devices are bulky, operating at high voltages (more than 100 V),

consume more power, having limited life and low reliability.

Advantages of Semiconductor Devices over Vacuum Tubes

They are small in size, operate at low voltage, consume small power, having long life and high reliability. The semi-

conductor junctions led to the discovery of integrated circuits which have revolutionized the electronic industry as they have been

used in the working of television and computer which are very commonly used in our daily life.

The PN Junction:

When you take a block of silicon and dope part of it with a trivalent impurity and the other part with a pentavalent impurity, a

boundary called the pn junction is formed between the resulting p-type and n-type portions. The pn junction is the basis for

diodes, certain transistors, solar cells, and other devices. It is called a DIODE or generic diode.

7




A p-type material consists of silicon atoms and trivalent impurity atoms such as boron. The boron atom adds a hole when it bonds

with the silicon atoms. However, since the number of protons and the number of electrons are equal throughout the material,

there is no net charge in the material and so it is neutral.

An n-type silicon material consists of silicon atoms and pentavalent impurity atoms such as antimony. As we know, an impurity

atom releases an electron when it bonds with four silicon atoms. Since there is still an equal number of protons and electrons

(including the free electrons) throughout the material, there is no net charge in the material and so it is neutral.

If a piece of intrinsic silicon is doped so that part is n-type and the other part is p-type, a pn junction forms at the boundary

between the two regions and a diode is created. The p region has many holes (majority carriers) from the impurity atoms and only

a few thermally generated free electrons (minority carriers). The n region has many free electrons (majority carriers) from the

impurity atoms and only a few thermally generated holes (minority carriers).

Formation of the Depletion Region:

The free electrons in the n region are randomly drifting in all directions. At the instant of the pn junction formation, the free

electrons near the junction in the n region begin to diffuse across the junction into the p region where they combine with holes

near the junction, when the pn junction is formed; the n region loses free electrons as they diffuse across the junction. This creates

a layer of positive charges (pentavalent ions) near the junction. As the electrons move across the junction, the p region loses holes

as the electrons and holes combine. This creates a layer of negative charges (trivalent ions) near the junction. These two layers of

positive and negative charges form the depletion region.

After the initial surge of free electrons across the pn junction, the depletion region has expanded to a point where equilibrium is

established and there is no further diffusion of electrons across the junction. This occurs as follows: As electrons continue to diffuse

across the junction, more and more positive and negative charges are created near the junction as the depletion region is formed. A

point is reached where the total negative charge in the depletion region repels any further diffusion of electrons (negatively

charged particles) into the p region (like charges repel) and the diffusion stops. In other words, the depletion region acts as a

barrier to the further movement of electrons across the junction.

Barrier Potential: The potential difference of the electric field across the depletion region is the amount of voltage required to

move electrons through the electric field. This potential difference is called the barrier potential and is expressed in volts.

Biasing:It is the method of connecting a power source to a device to make it functional.

8




Depending on the polarity of connections we can bias a diode in

2 ways:

1. Forward Bias: When positive terminal of the battery is connected to

the p layer of the diode and negative terminal is connected to the n-

layer of the diode. Diode acts as a closed switch.

2. Reverse Bias: When positive terminal of the battery is connected to

the n layer of the diode and negative terminal of the battery is

connected to the p layer of diode. Diode acts as an open switch.

The p side is known as anode and the n side is known as cathode

Diffusion Current

The motion of charge carriers, due to difference in their

concentration in the two regions of a p-n junction, gives rise to diffusion current across the junction.

Drift Current

The motion of these charge carriers due to electric field is called drift. As a result of it, a drift current starts, which is opposite in

direction to the diffusion current.

Injection

The process of crossing of the depletion region by electrons and holes is known as injection.

Characteristics of a P-N Junction Diode

(i) Forward Characteristics

These are the graphical relations between forward bias voltage applied to p-n junction and the forward current through the p-n

junction.

Knee Voltage

It is that forward voltage beyond which the current through the junction starts increasing rapidly with voltage, showing the

linear variation. But below the knee voltage the variation is non-linear.

(ii) Reverse Characteristics

These are the graphical relations between the reverse bias voltage applied to the p-n junction and the reverse current across the p-

n junction.

In reverse biasing of p-n junction diode, the reverse current is very small ( A) and is voltage independent upto certain reverse

bias voltage, known as breakdown voltage. It is called reverse saturation current. If the reverse bias voltage is equal to

9




breakdown voltage, the reverse current through the p-n junction will increase abruptly. If this current exceeds the rated value of p-

n junction (specified by the manufacturer), the p-n junction will get damaged.

Rectifier:It is an electrical device composed of one or more diodes that converts alternating current (AC) to direct current

(DC).A diode is like a one-way valve that allows an electrical current to flow in only one direction. This process is called rectification.

Ripple factor of a rectifier = value of a.c. component

value of d.c. component

That ripple factor for half wave rectifier is 1.21 and for full wave rectifier is 0.48.

Filter Circuits

Filter circuits are used to smoothen the fluctuating or pulsating voltage obtained from the rectifier. In fact filter circuit is a device

which is used to filter out the a.c. components from the output of rectifier.

The commonly used two simple filter circuits are discussed below:

10




11




1. Inductor Filter: It consists of an inductor L of high inductance connected in series with a

load resistance RL

Here an inductor of self inductance L offers a reactance XL to the current flowing through it,

which is given by

XL = L = 2 L

For d.c., = 0, therefore XL = 0

For a.c. = finite, therefore, XL = finite

As a result of it, the a.c. component of output of the rectifier is obstructed by inductor and d.c. component of output of the rectifier

will pass through inductor. Due to it, a smooth d.c. voltage appears across the load resistance RL.

2. Capacitor Filter: It consists of a capacitor C of high capacitance connected in parallel with its

load resistance RL

Here the capacitor of capacitance C offers a reactance to the current flowing through it, which is

given by

XC = 1 1

C 2 C

For d.c., = 0, therefore, XC = 1

0 = (infinity)

For a.c. , = finite, therefore XC is small but finite.

As a result of it, the a.c. component of output of the rectifier is bypassed or filtered out and the d.c. component of input is collected

at output of the rectifier.

Such filter is widely used in power supplies.

Special Purpose P-N Junction Diodes

(a) Zener Diode

It was invented by C-Zener. It is designed specially to operate in the reverse breakdown voltage region continuously without

being damaged.

Zener diode is fabricated by heavy doping of both p-side and n-side of the p-n junction. As a result of it, the depletion region of

Zener diode is very thin (< 10–6 m) and the electric field set up across the junction is extremely high.

Zener Diode as Voltage Regulator

Its working is based on the fact that in reverse breakdown (zener) region, a very small change in voltage across the zener diode

produces a very large change in current through the circuit but the voltage across the zener remains constant. So it provides

constant output voltage.

(b) Photo Diode

Photodiode is a p-n junction which is an optoelectronic device in which current carriers are generated by photons through photo

excitation, i.e., photoconduction by light.

When the photodiode is reverse biased with a voltage less than its breakdown voltage and no light is incident on its junction, the

reverse current is extremely small (almost negligible). This current is called dark current.

The photodiodes are used in reverse bias conditions because the change in reverse current through the photodiode due to

change in light flux or light intensity can be measured easily as the reverse saturation current is directly proportional to the light

flux or light intensity. But it is not so when photodiode is forward biased.

(c) Light Emitting Diode (LED)

12




Light emitting diode is a photoelectronic device which converts electrical energy into light energy, under forward bias condition.

It is a heavily doped p-n junction diode which under forward bias emits spontaneous radiation. The diode is covered with a

transparent cover so that the emitted light may come out.

LED's emit no light when reverse-biased.

(d) Solar Cell

Solar cell is basically a solar energy converter. It is a special p-n junction device which converts solar energy into electric energy.

(e) Schottky diode

The Schottky diode (named after the German physicist Walter H. Schottky), also known as Schottky barrier diode or hot-

carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage

drop and a very fast switching action. In Schottky diode the junction is in between N type semiconductor to Metal plate.

The schottky barrier diode has electrons as majority carriers on both sides of the junction. So it is a unipolar device

(f) Tunnel diode

A Tunnel diode is a heavily doped p-n junction diode in which the electric current decreases as the voltage increases. It is the P-N

junction device that exhibits negative resistance. In tunnel diode, electric current is caused by “Tunneling”. The tunnel diode is

used as a very fast switching device in computers.

Symbols of some common diodes

13




Transistor

A junction transistor is three terminal semiconductor device consisting of two p-n junctions formed by placing a thin layer of doped

semiconductor (p-type or n-type) between two thick similar layers of opposite type. A transistor has three doped regions forming

two p-n junction between them.

It is called as Bipolar Junction Transistor because it uses both holes and electrons as charge carriers.

It is named as Transistor because it transfers the signal from low resistance to high resistance.

Transistors are of two types

1. n-p-n2. p-n-p

A junction transistor is formed by sandwiching a thin layer of a p-type semiconductor between two layers of n-type semiconductor

or by sandwiching a thin layer of n-type semiconductor between two layers of p-type semiconductor. There are three terminals in

it, naming

emitter (E)base (B)collector (C).

It is used as an amplifier in the circuits.

Emitter (E): It is the left hand side thick layer of moderate size of the transistor which is heavily doped.

Base (B): It is a central thin layer of transistor which is lightly doped.

Collector (C): It is the right hand side thick layer of large size than that of emitter of the transistor; which is moderately doped.

The function of emitter is to emit the majority carriers. Function of collector is to collect the majority carriers. Base provides the proper

interaction between the emitter and the collector.

The direction of arrowhead indicates the direction of flow of positive charge or conventional current and arrow is always

present on Emitter.

Concept of an Amplifier

An amplifier is a device which is used for increasing the amplitude of variation of alternating voltage or current or power.

There are two input terminals for the signal to be amplified and two output terminals for connecting the load; and a means of

supplying power to the amplifier.

Feed Back Amplifier and Transistor Oscillator

A process in which a portion of the output of an amplifier is fed back to the input is termed as feedback and such amplifier is known

as feedback amplifier.

A process is said to be a positive feedback if the portion of the output is fed back in phase to the input and a process is said to be

a negative feedback if the portion of the output is fed back out of phase with the input.

The overall gain (Afb) of an amplifier with positive feedback is Afb = A

1 A and with negative feedback is, Afb =

A

1 A

Where is the feedback factor and A is the voltage gain without feedback. Here, A is called loop gain. If A = 1, then for positive

feedbackAfb = , i.e., gain becomes infinite, which states that there is output without any input. It means, the output is self sustained.

In this situation the amplifier becomes oscillator. the condition A = 1 is known as Barkhausen Criterion for Oscillations.

Transistor as An Oscillator

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An oscillator is a device of generating an alternating voltage of high frequency and of constant amplitude. The energy required for

this purpose is taken from the dc source. It means oscillator is a device for converting energy of dc source into alternating voltage

of high frequency.

Tank circuit is a basic oscillatory circuit in which inductance L is connected in parallel to capacitance C. The frequency of

oscillations setup in the tank circuit is given by = 1

2 LC

Due to internal resistance of the inductance coil etc., the L-C circuit shows damped oscillations whose amplitude of oscillations goes

on decreasing with time. The undamped oscillations can be produced by using transistor as an oscillator, in which positive feedback is

applied to the transistor used as an amplifier. The same can be achieved by having inductive coupling of amplifier with L-C network.

Advantages of Semiconductor Devices

1. Semiconductor devices are much smaller in size and weight as compared to vacuum tubes.

2. Semiconductor devices are not to be heated for emission of electrons. They start operating instantly. This saves a lot of electric

power.

3. A transistorized equipment does not get heated, while operating. Therefore, no cooling arrangement is required.

4. The semiconductor devices are more rugged than the vacuum tubes. They can withstand rough handling.

5. Semiconductor device have much longer life as compared to the life of vacuum tubes.

6. Semiconductor devices are cheaper than vacuum tube devices.

7. Semiconductor devices are low power devices.

Disadvantages of Semiconductor Devices

1. Semiconductor devices are very sensitive to changes of temperature whereas the vacuum tubes are less sensitive.

2. It is difficult to produce semiconductor devices with exactly identical characteristics.

3. The noise level in semiconductor devices is higher than that of vacuum tubes.

4. Semiconductor devices cannot handle as much power as vacuum tubes.

In modern times, semiconductor devices are almost replacing vacuum tubes on account of their merits.

Applications

1. In rectification: The semiconductor devices (known as rectifier) can convert a.c. power into d.c. power with very high efficiency,

which can be used for charging storage battery, in electroplating etc.

2. In amplification: The semiconductor devices (known as amplifiers) can raise the strength of weak signal. These amplifiers are

used in telecommunications.

15




3. In automatic control: The electronic devices are used to provide automatic control over the speed of motor, voltage across the

refrigerator etc.

4. In high frequency a.c. generator: The electronic devices (known as oscillators) are used to convert d.c. power to a.c. power of

high frequency. This high frequency a.c. causes strong heating which is used for annealing and hardening.

5. In conversion of light into electricity (i.e. photo-electricity): The photoelectric devices are used in Burglar alarms, sound

recording on motion pictures etc.

6. In conversion of electricity into light: This property is utilized in the television and radar.

Logic Gate

A digital circuit which either allows a signal to pass through or stops it, is called a gate. Such gate allows the signal to pass through

only when some logical conditions are satisfied. Hence they are called logic gate.

The logic gates are the building blocks of a digital system. Each logic gate follows a certain logical relationship between input and

output voltages.

Each basic logic gate is represented by a symbol and its function is defined either by a truth table or by a Boolean expression.

In digital circuits, we mostly use junction diodes and transistors as switches to change from one voltage level to another. In the

analysis of the digital circuits, we find out whether a voltage is low or high. In digital circuits low and high voltages are often

represented by levels 0 and 1 respectively.

Truth Table: It is a table that shows all possible input combinations and the corresponding output combinations for a logic gate. It

is also called a table of combinations.

Boolean expression: George Boole invented a kind of algebra which deals with logical statements that have only two values,

namely either a true or a false value. The logical statements are called Boolean variables. The true value of a Boolean variable is

denoted by 1 and the false value by 0. i.e., on or off states.

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INTEGRATED CIRCUIT

An integrated circuit, or IC, is small chip that can function as an amplifier, oscillator, timer,

microprocessor, or even computer memory. An IC is a small wafer, usually made of silicon, that can hold

anywhere from hundreds to millions of transistors, resistors, and capacitors.

A discrete circuit is constructed of components which are manufactured separately. Later, these

components are connected together by using conducted wires on a circuit board or a

printed circuit board(PCB).

ADVANTAGES OF IC 1. The entire physical size of IC is extremely small than that of discrete circuit. 2. The weight of an IC is very less as compared entire discrete circuits. 3. It’s more reliable. 4. Because of their smaller size it has lower power consumption. 5. It can be easily replaced but it can hardly repair, in case of failure. 6. Because of an absence of parasitic and capacitance effect it has increased operating speed. 7. Temperature differences between components of a circuit are small. 8. It is suitable for small signal operation. 9. The reduction in power consumption is achieved due to extremely small size of IC.

DIS-ADVANTAGES OF IC

1. Coils or indicators cannot be fabricated. 2. It can handle only limited amount of power. 3. High grade P-N-P assembly is not possible. 4. It is difficult to achieve low temperature coefficient. 5. The power dissipation is limited to 10 watts. 6. Low noise and high voltage operation are not easily obtained. 7. Inductors and transformers are connected to the exterior of a semiconductor chip as it is not possible to fabricate inductor and

transformers on the semiconductor chip surface. 8. Low noise and high voltage operation are not easily obtained.

17




TYPES OF INTEGRATIONS OF INTEGRATED CIRCUITS

Name Signification Year Number of transistors Number of logic gates

SSI Small-scale integration 1964 1 to 10 1 to 12

MSI Medium-scale integration 1968 10 to 500 13 to 99

LSI large-scale integration 1971 500 to 20 000 100 to 9999

VLSI Very large-scale integration 1980 20 000 to 1 000 000 10 000 to 99 999

ULSI Ultra-large-scale integration 1984 1 000 000 and more 100 000 and more

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Practice Questions 1. Bonding in a semiconductor is

(a) metallic (b) ionic

(c) van der Walls (d) covalent

2. Following diagram shows energy band positions in a

semiconductor at 0 K.

3. Energy band gap diagram for an intrinsic semiconductor

at temperature T > 0 K is

4. To make a p-type semiconductor, germanium is doped

with

(a) gallium (b) boron

(c) aluninium (d) All of these

5. For extrinsic semiconductor.

(a) the conduction band and valence band overlap

(b) the gap between conduction band and valence band

is more than 16 eV

(c) the gap between conduction band and valence band

is near about 1 eV

(d) the gap between conduction band and valence band

will be 10 e V and more

6. The forbidden energy gap in the energy bands of

germination at room temperature is about

(a) 1.1 eV (b) 0.1 eV

(c) 0.67 eV (d) 6.7 eV

7. Which of the following has negative temperature

coefficient of resistance?

(a) Metal (b) Insulator

(c) Semiconductor (d) All of these

8. Potential difference of p and n-side which prevents

diffusion of electrons is called

(a) potential gradient (b) potential different

(c) barrier potential (d) depletion potential

9. The depletion layer in the p-n junction region is caused

by

(a) drift of holes

(b) diffusion of charge carriers

(c) migration of impurity ions

(d) drift of electrons

10. The barrier potential of a p-n junction depends on

(i) type of semiconductor material

(ii) amount of doping

(iii) temperature

(iv) temperature

Which one of the following is correct?

(a) (i) and (ii) (b) (ii)

(c) (ii) and (iii) (d) (i), (ii) and (iii)

11. The electrical resistance of depletion layer is large

because

(a) of strong electric field

(b) it has a large number of charge carriers

(c) it contains electrons as charge carriers

(d) it has holes as charge carriers

12. In an unbiased p-n junction, holes diffuse from the p-

region to n-region because

(a) free electrons in the n-region attract them

(b) they moves across the junction by the potential

difference

(c) hole concentration in p-region is more as compared

to hole concentration in n-region


13. A diode is a

(a) piece of a covalent crystal

(b) piece of a semiconductor crystal with metallic

contacts provided at two ends

(c) p-n junction with metallic contacts provided at two

ends

(d) piece of a metal which is sprayed over by a

semiconductor

19




14. In forward bias, forward current obtained from the p-n

junction diode is

(a) due to injection of electrons is p-side

(b) due to injection of holes in n-side

(c) both (a) and (b)

(d) due to flow of electrons from negative terminal of

supply to its positive terminal

15. In a reverse biased p-n junction diode,

(a) current under reversed bias is not very much

dependent on applied voltage

(b) current under reversed bias is directly proportional

to applied voltage

(c) current initially depends on applied voltage, then it

becomes independent

(d) no current flows in reversed bias

16. If reverse biasing potential is increased beyond a certain

critical (breakdown) value, then

(a) diode gets destroyed due to overheating

(b) no current flows through the diode

(c) after breakdown a heavy current flows from p to n-

side

(d) potential barrier becomes zero

17. Characteristic curve of a p-n junction is

18. Threshold or knee voltage for a forward biased

germanium and silicon diodes have respective values

(a) 0.2 V, 0.7 V (b) 0.7 V, 1.1 V

(c) 1.2 V, 0.7 V (d) 0.7 V, 0.2 V

19. Diode primarily allows the flow of current only in one

direction (forward bias). The forward bias resistance is

low as compared to the reverse bias resistance. In a

circuit, a diode acts like a

(a) valve (b) switch

(c) amplifier (d) multi-way passage

20. If no external voltage is applied across p-n junction,

there would be

(a) no electric field across the junction

(b) al electric field pointing from n-type to p-type side

across the junction

(c) an electric field pointing from p-type to n-type side

across the junction

(d) a temporary electric field during formation of p-n

junction that would subsequently disappear

21. Which is reverse biased diode?

22. A semiconductor X is made by doping a germanium

crystal with arsenic (Z = 33). A second semiconductor Y

is made by doping germanium with indium (Z = 49). The

two are joined end to end and connected to a battery as

shown. Which of the following statements is correct?

20




(a) X is p-type, Y is n-type and the junction is forward

biased

(b) X is n-type, Y is p-type and the junction is forward

biased

(c) X is p-type, Y is n-type and the junction is reverse

biased

(d) X is n-type, Y is p-type and the junction is reverse

biased

23. If an alternating voltage is applied across a diode in

series with a load, then

(a) a continuous DC voltage appears across load

(b) an AC voltage appears across and

(c) a pulsating DC voltage appears across load

(d) no voltage appears across load

24. Which of this is a half-wave rectifier circuit?

25. Input to an half-wave rectifier is given as follows

Its output will be

26. Output of a full-wave rectifier is

(a) pure DC voltage

(b) pure AC voltage

(c) pulsating DC voltage

(d) pulsating AC voltage

27. Fitters are used alongwith a full-wave rectifier to

(a) remove AC part from the output

(b) remove DC part from the output

(c) mix AC and DC


28. In the given circuit,

Capacitor C is used

(a) for storing potential energy

(b) as a bypass to DC component to get AC in RL

(c) to remove sparking

(d) as a bypass to AC component to get DC in RL

29. A zener diode which is used in reversed biased is used as

a

(a) voltage regulator (b) voltage rectifier

(c) current regulator (d) current rectifier

30. A zener diode differs from a p-n junction that

(a) zener diode is made from very lightly doped p-n

junction

(b) zener diode is made from a heavily doped p-n

junction

(c) zener diode is made from a metal piece

(d) zener diode is made from a heavily doped p-type

semiconductor

31. High current observed at breakdown of a zener diode

due to emission and movement of electrons from p to n-

side is known as

(a) thermionic emission

(b) external field emission

(c) internal field emission

(d) photoemission

32. Correct circuit using a zener diode as a voltage regular to

21




33. Optoelectronic devices are

(a) CFL’s

(b) light based semiconductor diodes

(c) bulbs

(d) discharge tubes

34. A photo detector is a

(a) photodiode used for detecting optical signals

(b) LED’s which are used for detection of infrared signals

(c) an evacuated tube consisting of a photosensitive

cathode


35. In active state of a transistor, the emitter base junction

acts as a ….A…. resistance and base-collector junction

acts like a ….B…. resistance. Here, A and B refer to

(a) low, low (b) low, high

(c) high, low (d) high, high

36. In case of an n-p-n transistor, the collector current is

always less than the emitter current because

(a) collector side is reverse biased and emitter side is

forward biased

(b) after electrons are lost in the base and only

remaining ones reach the emitter back

(c) collector side is forward biased and emitter side is

reverse biased

(d) collector being reserve biased attracts less electrons

37. Before invention of transistor, vacuum tubes were used

and these were named according to number of

electrodes they have.

Now, match these with number of electrodes

Column I Column II

A. Pentode

B. Tetrode

C. Triode

D. Diode

1. 2

2. 3

3. 4

4. 5

A B C D A B C D

(a) 1 2 3 4 (b) 2 3 4 1

(c) 3 4 1 2 (d) 4 3 2 1

38. Match the elements or compounds with their respective

energy gaps values. (Energy gap existing between

conduction and valence bands)

Column I Column II

A. Diamond

B. Aluminium

C. Germanium

D. Silicon

1. 1.1 eV

2. 0.71 eV

3. 0.03 eV

4. 6 eV

A B C D A B C D

(a) 1 2 3 4 (b) 2 1 4 3

(c) 4 3 1 2 (d) 4 3 2 1

39. Match the following columns.

22




A. Moderate size and

heavily doped

1. Base

B. Very thin and lightly

doped

2. Collector

C. Moderately doped and of

large size

3. Emitter

A B C A B C

(a) 1 2 3 (b) 1 3 2

(c) 3 1 2 (d) 3 2 1

40. Match the following symbols with their names.

Column I Column II

A. 1. OR

B.

2. AND

C.

3. NAND

D.

4. NOR

E.

5. NOT

A B C D E

(a) 1 2 3 4 5

(b) 3 1 2 4 5

(c) 5 1 2 5 4

(d) 5 1 2 3 4

41. Match the following Column I with Column II.

Column I Column II

A. n-p-n transistor

1.

B. p-n-p transistor

2.

C. Light emitting diode

3.

D. Zener diode

4.

A B C D

(a) 3 4 2 1

(b) 4 2 1 3

(c) 2 4 3 1

(d) 4 3 2 1

42. A hole is

(a) an anti-particle of electron

(b) a vacancy created when an electron leaves a covalent

bond

(c) absence of free electrons

(d) an artificially created (particle)

43. For the flow of electrons in a vacuum tube, vacuum is

required, because

(a) electrons are not ejected from cathode

(b) Vacuum helps in extracting electrons from remaining

gas molecules or atoms

(c) in vacuum work function of cathode is reduced

(d) electrons may lose their energy on collision with air

molecules in their path

44. Semiconductor devices (diodes, transistors) are smaller

than vacuum tubes because

(a) they are made from silicon/germanium crystals

(b) they have very high density

(c) large crystals of semiconductors have large

resistance

(d) flow of charge carriers are within the solid itself

45. If a solid transmits the visible light and has a low melting

point, it possesses

(a) metallic bonding (b) ionic bonding

(c) covalent bonding (d) van der Walls bounding

46. Bonding in a semiconductor is

(a) metallic (b) ionic

(c) van der walls (d) covalent

47. The SI unit of conductivity is

(a) 1( m) (b) 1m

(c) Sm-1 (d) S

48. Correct one is

(a) semiconductor insulator metal

(b) metal semiconductor metal

(c) semiconductor metal insulator

(d) insulator semiconductor metal

23




49. In a crystal, atomic separation is around 2 to 3A. At this

separation due to interaction, interaction, energies of

(a) outermost electrons are changed

(b) innermost electrons are changed



50. There is no hole current in good conductors, because

they

(a) have large forbidden energy gap

(b) have no energy gap due to overlapping valence and

conduction bands

(c) are full of electron gas

(d) have no valence band .

51. At elevated temperature, few of covalent bonds of Si or

Ge are broken and a vacancy in the bond is created

Effective charge of vacancy or hole is

(a) Positive (b) negative

(c) neutral

(d) Sometimes positive and sometimes negative

52. In pure form, Ge Si, a semiconductor is called

(a) intrinsic semiconductor, ne = nh= ni

(b) extrinsic semiconductor, ne= nh = ni

53. In equilibrium condition, the rate of generation of

electron – hole pairs

(a) is more than rate of recombination of electron and

hole pairs

(b) is less than of recombination of electron and hole

pairs

(c) equals to rate of recombination of electron and hole

pairs

(d) is always zero

54. In intrinsic semiconductor at room temperature, number

of electrons and holes are

(a) equal (b) zero

(c) unequal (d) infinite

55. A pure semiconductor behaves as a good conductor at

(a) room temperature (b) Low temperature

(c) high temperature (d) Both (b) and (c)

56. At absolute zero, Si acts as

(a) non-metal (b) metal

(c) insulator (d) None of these

57. Doping is

(a) a process of adding an impurity to a pure

semiconductor

24




(b) a process of obtaining semiconductor from its ore

(c) melting of a semiconductor

(d) purification of a semiconductor

58. Doping of intrinsic semiconductor is done

(a) to neutralise charge carriers

(b) to increase the concentration of majority charge

carriers

(c) to make it neutral before disposal

(d) to carry out further purification

59. An n-type and p-type silicon can be obtained by doping

pure silicon with

(a) arsenic and phosphorous, respectively

(b) Indium and aluminium, respectively

(c) Phosphorous and indium, respectively

(d) Aluminium and boron, respectively

60. Which of the following statement is correct for an n-type

semiconductor?

(a) The donor level lies below the bottom of the

conduction band

(b) The donor level lies closely above the top of the

valence band

(c) The donor level lies at the halfway mark of the

forbidden energy gap

(d) None of the above.

61. Number of electrons present in conduction band due to

doping

(a) Shows a heavy increase with increase of temperature

(b) shows a heavy decrease with increase of temperature

(c) independent of change in ambient temperature

(d) reduced to zero at temperature above room

temperature

62. In a p-type semiconductor, the majority and minority

charge carriers are respectively,

(a) protons and electrons

(b) electrons and protons

(c) electrons and holes

(d) holes and electrons

63. Which statement is correct?

(a) n-type germanium is negatively charged and p-type

germanium is positively charged

(b) both n-type and p-type germanium are neutrals

(c) n-type germanium is positively charged and p-type

germanium is negatively charged

(d) both n-type and p-type germanium are negatively

charged

64. Three semiconductors are arranged in the increasing

order of their energy gap as follows. The correct

arrangement is

(a) tin, germanium, silicon

(b) tin, silicon, germanium

(c) silicon, germanium, tin

(d) silicon, tin, germanium

65. In an n-type semiconductor, which of the following

statements is true?

(a) Electrons are majority charge carriers and trivalent

atoms are the dopants

(b) Electrons are minority charge carriers and

pentavalent atoms are the dopants

(c) Holes are minority charge carriers and pentavalent


(d) Holes are majority charge carriers and trivalent


66. A p-n junction contains

(a) a p-type semiconductor is joined with an n-type

semiconductor by glue

(b) a p-type semiconductor is bolted with an n-type

semiconductor

(c) a p-type semiconductor is formed with an n-type

semiconductor on same semiconductor crystal wafer

67. Due to diffusion, the space charge region on either side

of p-n junction is developed. this space charge region is

called

(a) dilution region (b) diffusion region

(c) depletion region (d) ionic region

68. Symbol of a p-n junction diode is an arrow, its direction

indicates

(a) nothing its just a symbol

(b) direction of flow of electrons

(c) direction of conventional current when it is forward

biased

(d) direction of electric field

69. In the case of forward biasing of P-n junction, which one

of the following figures correctly depicts the direction of

flow of charge carriers?

25




- +

P - + n

- +

- + →

e- - + e-

Vp

(a)

- +

P - + n

- + → - + →

e- - + e-

Vp

(b)

- +

P - + n

- + → - + e- - + e-

Vp

(c)

- +

P - + n

- +

- + e- - + e-

Vp

(a)

70. In a p-n junction diode,

(a) the current in the reverse biased condition is

generally very small

(b) the current in the reverse biased condition is small

but that in forward biased condition is independent of

the bias voltage

(c) the reverse biased current is strongly dependent on

the applied bias voltage

(d) the forward biased current is very small in

comparison to reverse biased current

71. A photodiode converts

(a) variation in intensity of light into current amplitude

variation

(b) Variation of current amplitude into variation in

intensity of emitted light

(c) variation of voltage into variation of current

(d) variation of intensity of light into variation of volume

72. An LED cannot be used in reverse biased as a voltage

regulator because

(a) reverse breakdown voltage is very low for them

(b) reverse breakdown voltage is very high for them



73. Substance used to make re LEDs is

(a) silicon

(b) germanium

(c) gallium arsenide phosphide

(d) indium phosphide

74. A solar cell is

(a) photodetector

(b) photovoltaic device

(c) light emitting diode

(d) photogenerator

75. In. an LED, when it glows, electron moves from A to B,

when an appropriate bias is applied. A and B are

respectively.

(a) conduction band, valence band

(b) valence band, conduction band

(c) conduction wires, conduction band

76. Photodetectors and LED’s are used in

(a) road construction works

(b) optical telecommunication band

(c) power generation from falling water near dam

(d) radio transmitters

26




77. LED’s are not used for room lighting (although they are

used for automobile bulbs and in industrial lighting)

because

(a) our eyes are not comfortable with very intense light

(b) our eyes are not comfortable with very intense light

(b) our eyes are not comfortable with monochromatic

light

(c) LED’s are much costlier than bulbs tube lights and

CFL’s

(d) LED manufacture in mass production will be a very

polluting process

78. A transistor has

(a) Two doped regions forming a large p-n junction

(b) three doped regions forming two p-n junctions

(c) two p-n junctions connected by a conducting wire


79. For an n-p-n transistor shown below:-

n p n

I II III

Regions marked I, II and III are respectively,

(a) emitter, collector, base

(b) base, collector, emitter

(c) emitter, base, collector

(d) collector, emitter, base

80. When a transistor is biased as follows.

n

p

n

__ + __ +

Then, it is said to be in

(a) Solid state (b) active state

(c) inactive state (d) passive state

81. Analog signals are

(a) Continuous (b) discrete value signals

(b) intermittent signals (d) erratic waveforms

82. Digital signals are

(a) continuous waveforms (b) discrete value signals

(c) intermittent signals (d) erratic waveforms

83. A NOT gate is called an invertor, because

(a) it produces an output which changes with time

(b) it produces I as output when input is 0 and vice-versa

(c) it produces no output for any input

(d) it has only a single input

84. Truth table for an OR gate is

(a)

A B Y

0 0 0

1 0 0

0 1 0

1 1 1

(b)

A B Y

0 0 0

1 0 1

0 1 1

1 1 0

(c)

A B Y

0 0 1

1 0 0

0 1 0

1 1 0

(d)

A B Y

0 0 1

1 0 1

0 1 1

1 1 1

85. Truth table for the logic gate whose symbol shown is

A

B Y

(a)

A B Y

0 0 0

1 0 1

0 1 1

1 1 1

(b)

A B Y

0 0 0

1 0 0

0 1 0

1 1 1

(c)

A B Y

0 0 1

1 0 0

0 1 0

1 1 0

(d)

A B Y

0 0 1

1 0 0

0 1 0

1 1 0

86. Inputs to a NAND gate are A and B are made as below.

1

A

0

1

B

0

t1 t2 t3 t4 t5 t6

Out put of the NAND gate is

27




(a)

(b)

(c)

(d) t1 t2 t3 t4 t5 t6

87. When a forward bias is applied to a p-n junction. It

(a) raises the potential barrier

(b) reduces the majority carrier current to zero

(c) lowers the potential barrier


88. For transistor action, which of the following statements

are correct?

(a) Base, emitter and collector regions should have

similar size and doping concentrations

(b) The base region must be very thin and lightly doped

(c) The emitter junction is forward biased and collector

junction is reverse biased

(d) Both the emitter junction as well as the collector

junction are forward biased

89. In an n-type silicon, which of the following statements is

true?

(a) Electrons are majority charge carriers and trivalent


(b) Electrons are minority charge carriers and

pentavalent atoms are the dopants

(c) Holes are minority charge carriers and pentavalent


(d) Holes are majority charge carriers and trivalent


90. In –an unbiased p-n junction, holes diffuse from the p-

region to n-region because

(a) free electrons in the n-region attract them

(b) they move across the junction by the potential

difference

(c) hole concentration in p-region is more as compared

to n-region


91. Conductivity of a semiconductor increases with increase

in temperature, because

(a) number density of free charge carriers increases

(b) relaxation time increases

(c) both number density of free charge carriers and

relaxation time increases

(d) number density of free charge carriers increases

relaxation time decreases but effect of decrease in

relaxation time is much less than increase in number

density.

answer key 1 D 2 A 3 D 4 D 5 C 6 C 7 C 8 C 9 B 10 D 11 A 12 C 13 C 14 C 15 A 16 A 17 A 18 A 19 A 20 B 21 B 22 D 23 C 24 B 25 D 26 C 27 A 28 D 29 A 30 B 31 C 32 D 33 B 34 A 35 B 36 A 37 D 38 D 39 C 40 D 41 A 42 B 43 D 44 D 45 D 46 D 47 C 48 B 49 A 50 B 51 A 52 A 53 C 54 A 55 C 56 C 57 A 58 B 59 C 60 A 61 C 62 D 63 B 64 A 65 C 66 D 67 C 68 C 69 D 70 A 71 A 72 A 73 C 74 B 75 A 76 B 77 B 78 B 79 C 80 B 81 A 82 B 83 B 84 D 85 B 86 B 87 C 88 B,C 89 C 90 C 91 D

28




SOLUTION

1. (a) Semiconductors have 4 valancy and so they form

covalent bonds.

2. (a)

3. (d)

4. (d) Gallium, boron and aluminium all are trivalent

impurities. These impurities make germanium p-type

semiconductor.

5. (c) Doping does not change energy gap, it is still around l e

V.

6. (c) For Ge, the energy gap, Eg is 0.7 eV.

7. (c) Conductivity of a semiconductor increases with

temperature.

So, they have negative value of temperature coefficient .

8. (c) Potential tends to prevent the movement of electron

from the n-region into the p-region, it is often called

barrier potential.

9. (b) It is cause by diffusion of charge carriers.

10. (d)

11. (a) E opposes movement of charge.

12. (c) Diffusion and drift current of electrons and holes is due

to concentration difference.

13. (c) A semiconductor diode is basically a p-n junction with

metallic contacts provided at the ends for the application

of an external voltage. It is a two terminal device.

14. (c) Due to concentration gradient, the injected electrons on

p-side diffuse from the junction edge of p-side to the

other end of p-side. Likewise, the injected holes on n-side

diffuse from the junction edge of n-side to the other end

of n-side.

This motion of charged carriers on either side gives rise

to current. The total diode forward current is sum of hole

diffusion current and conventional current due to

electron diffusion.

15. (a) The diode reverse current is not very much depends on

the applied voltage. Even a small voltage is sufficient to

sweep the minority charge carriers from one side of the

junction to the other side of the junction.

The current is not limited by the magnitude of the

applied voltage but is limited due to the concentration of

the minority charge carrier on either side of the junction.

16. (a) The current under reserve bias is essentially voltage

independent upto a critical reverse bias voltage, known

as breakdown voltage (Vbr). When V = Vbrr, the diode

reverse current increases sharply from n to p side.

Even a slight increase in the bias voltage causes large

change in the current. If the reverse current is not

limited by an external circuit, the p-n junction will get

destroyed. Once it exceeds the rated value, the diode gets

destroyed due to overheating.

17. (a) Typical V-I characteristics of a silicon diode are as

shown.

18. (a) In forward bias, the current first increase very slowly,

almost negligibly till the voltage across the diode crosses

a certain value.

After the characteristic voltage, the diode current

increases significantly (exponentially), even for a very

small increase in the diode bias voltage. This voltage is

called the threshold voltage or knee voltage (~ 0.2V for

germanium diode and ~0.7 V for silicon diode).

19. (a) By allowing current only in forward bias it acts like a one

way valve.

20. (b) Without an external bias an electric field exists which

points from n to p-side and opposes any diffusion of

electrons.

21. (b) In reverse bias, Vp-side – Vn-side = Negative

22. (d) Arsenic is pentavalent, X is n-type and indium is trivalent

and Y is p – type.

So, the junction is in reverse bias.

23. (c) If an alternating voltage is applied across a diode in

series with a load, a pulsating voltage will appear across

29




the load only during the half cycles of the AC input

during which the diode is forward biased.

24. (b) In a half-wave rectifier, the secondary of a transformer

supplies the desired AC voltage across terminals A and B.

When the voltage at A is positive, the diode is forward

biased and it conducts. When A is negative, the diode is

reverse biased and it does not conduct.

The reverse saturation current of a diode is negligible

and can be considered equal to zero for practical

purposes.

25. (d)

26. (c) The rectified voltage is in the form of pulses of the shape

of half sinusoids. Though, it is unidirectional, it does not

have a steady value.

27. (a) To get steady DC output from the pulsating voltage

normally, a capacitor is connected across the output

terminals (parallel to the load RL).

One can also use an inductor in series with RL for the

same purpose. Since, these additional circuits appear to

filter out the AC ripple and give a pure DC voltage, so

they the called filters.

28. (d) As, XC =1

C, for AC component when is high, then XC is

less and, so a capacitor let AC part bypass through it, so

only DC part reaches RL, the load resistance.

29. (a) Zener diode is a special purpose semiconductor diode. It

is designed to operate under reverse bias in the

breakdown region and used as a voltage regulator. The

symbol for zener diode is shown in figure.

30. (b) Zener diode is fabricated by heavity doping both p-sides

and n-sides of the junction. Due to this, depletion region

formed is very thin (< 10-6 m) and the electric field of the

junction is externally high (~ - 5 × 106 V/m) even for a

small reverse bias voltage.

31. (c) When the reverse bias voltage V = Vz, then the electric

field strength is high enough to pull valence electrons

from the host atoms on the p-side which are accelerated

to n-side.

These electrons account for high current observed at the

breakdown. The emission of electrons from the host

30




atoms due to the high electric field is known as internal

field emission or field ionization.

32. (d) Zener diode must be attached in reverse bias

33. (b) Semiconductor diodes in which charge carriers are

generated by photons (photo-excitation) are called

optoelectronic devices. Optoelectronic devices are

(i) photodiodes used for detecting optical signal (photo

detectors). Used near automatic doors.

(ii) light Emitting Diodes (LED) which convert electrical

energy into light.

(iii) photovoltaic devices which convert optical radiation

into electricity (solar cells).

34. (a) A photo detector detects any change in intensity of light

by changing either potential difference across it or by

changing current through it.

35. (b) Low; High. When emitter-base junction is forward biased

and base-collector junction is reverse biased, then

transistor, is aid to be active state. In active state of

transistor the emitter-base junction acts as low

resistance while the base collector junction acts as high

resistance.

36. (a) iE = iB + iC ⇒ iC = iE – iB.

Emitter side is forward biased, collector side is reverse

biased.

37. (d)

38. (d) The give material in decreasing order of conductivity are

Al > Ge > Si > Diamond (C), so aluminium has least

energy gap and carbon has largest. Diamond has energy

gap 6 eV. Eg (Germanium) = 0.71eV.

39. (c)

40. (d)

41. (a)

42. (b)

43. (d) In a vacuum tube, the electrons are supplied by a heated

cathode and the controlled flow of these electrons in

vacuum is obtained by varying the voltage between its

different electrodes.

Vacuum is required in the inter-electrode space;

otherwise the moving electrons may lose their energy on

collision with the air molecules in their path.

44. (d) The supply and flow of charge carriers in the

semiconductor devices are within the solid itself.

No external heating or large evacuated space is required

by the semiconductor devices, so they have small size.

46. (d) Semiconductor have 4 valency and so they form covalent

bonds.

47. (c) The SI unit of conductivity is Siemen per metre

(Sm–1).

48. (b)

(i)Metal They possess very low resistivity (or high

conductivity).

p ~ 10-2 – 10-8Ω -m

σ ~ 102 – 108 Sm-1

(ii)Semiconductor They have resistivity or conductivity

intermediate to metal insulator.

~ 10-5 – 106Ω-m

σ ~ 105 – 10-6 Sm-1

(iii)Insulator They have high resistivity (or low

conductivity).

~ 1011 – 1019Ω-m

σ ~ 10-11 – 10-19 Sm-1

As, 108> 105> 10-19, σmetal> σsemiconductor> σ insulator

49. (a) Due to atomic interactions, the energies of other- must

electrons are changed in larger amounts.

50. (b) In good conductors, which are metals there is no gap

between valence band and conduction band. Hence, no

holes exist.

51. (a) In semiconductor as the temperature increases, more

thermal energy becomes available to electrons and some

of these electrons may break away (becoming free

electrons contributing to conduction).

The thermal energy affectivity ionizes only a few atoms

in the crystalline lattice and creates a vacancy in the

bond. These holds behave as positive charge carriers.

52. (a) Pure semiconductors are called intrinsic semiconductors

ne = nn = ni,

53. (c) In equilibrium condition, there is no net current through

the semiconductor. This shows that the rate of

generation of electron-hole pairs is equal to rate of

recombination of electron-hole pairs.

54. (a) An intrinsic semiconductor will behave like an insulator

at T = 0 K. It is the thermal energy at higher temperature

(T > 0 K), which excites some electrons from the valence

band to the conduction band. The thermally excited

electrons at T > 0 K, partially occupy the conduction

31




band. These have come from the valence band heaving

equal number of holes there.

55. (c) Number density of electron-hole pairs in increased with

temperature, so at high temperature semiconductors

have higher conductivity.

56. (c) Semiconductors have negative temperature coefficient of

resistance i.e., the resistance of a semiconductor

decreases with the increase in temperature and vice-

versa. Silicon is actually an insulator at absolute zero of

temperature but it becomes a good conductor at high

temperatures. Because on increasing temperature of

semiconductor some of the electrons jumps from valence

band to conduction band.

57. (a) Process of adding an impurity is called doping.

58. (b) Doping increases concentration of majority charge

carriers.

59. (c) To form an n-type semiconductor doping is done by

using a pentavalent impurity like phosphorus and to

form a p-type semiconductor doping is done by using a

trivalent impurity like indium.

60. (a)

Donor energy level is slightly less than energy level

lowest to conduction band.

61. (c) The number of electrons made available for conduction

by dopant atoms depends strongly upon the doing level

and is independent of any increase in ambient

temperature.

62. (d) In a p-type semiconductor, holes are majority charge

carriers and electrons are minority charge carriers.

63. (b) In both n-type and p-type semiconductors, number of

electrons is exactly equal to number of protons. Both are

neutrals.

64. (a) Tin < germanium < silicon (increasing order of energy).

65. (c) The n-type semiconductor can be produced by doping an

impurity atom of valence 5, i.e., pentavalent atoms.

66. (d) Consider a thin p-type silicon (p-Si) semiconductor

wafer. By adding precisely a small quantity of

pentavalent impurity, part of the p-Si wafer can be

converted into n-Si. So, p-n junction is formed.

67. (c) When a hole diffuses from p → n due to the

concentration gradient. It leaves behind and ionized

acceptor (negative charge) which is immobile. As the

holes continue to diffuse, a layer of negative charge (or

negative space-charge region) on the p-side of the

junction is developed. Similarly, a layer of positive space-

charge is developed on n-side because of electron

departure. This space-charge region on either side of the

junction together is known as depletion region.

68. (c) The direction of arrow indicates the conventional

direction of current (when the diode is under forward

bias).

69. (d)

70. (a) In RB potential barrier increases hence movement of

majority carrier decreases. But strong E pushes the

movement of minority carrier towards their respective

side and contributes small current.

71. (a) The variation of intensity results in change in number of

incident photons (per second) and hence a

corresponding change in generation rate of electron and

holes occurs. This causes a change in current amplitude.

32




72. (a) The reverse breakdown voltages of LEDs are very low.

Typically around 5 V. So, care should be taken that high

reserve voltage do not appear across than.

73. (c) For red LEDs,

GaAS0.6 P0.4 – Gallium Arsenic Phosphide (Eg = 1.9 eV) is

used. This correspond to ≈ 700 nm.

74. (b) Solar cells uses light energy (photons) to generate an

emf.

75. (a) Electron moves from conduction to valence band, in LED

when it glows

76. (b) An optical telecommunication link,

77. (b) Light from an LED is highly monochromatic.

78. (b) A transistor has three doped regions forming two p-n

junctions between them. Obviously, there are two types

of transistors.

(i) n-p-n transistor (ii) p-n-p transistor

79. (c) n-p-n transistor Here, two segments of n-type

semiconductor (emitter and collector) are separated by a

segment of p-type semiconductor (base).

80. (b) The transistor works as an amplifier with emitter base

junction forward biased and base collector junction

reversed bias, a transistor is said to be in active state.

81. (a) An analog signal is a continuous waveform as

82. (b)

83. (b) Because inverter gives output zero when input is one

and vice-versa. Same thing happens in NOT gate.

84. (d) A + B = OR operation

A B A+B

0 0 0

0 1 1

1 0 1

1 1 1

85. (b) AND gate An AND gate has two or more inputs and one

output. The output Y of AND gate is l only when input A

and input B are both l. The logic symbol and truth table

for this gate is given by

86. (b) For an NAND gate, truth table is

So, no output occurs when both inputs are at higher

potentials (1). Till time t4, output (1) occurs because

both inputs do not become (1) together.

87. (c)

88. (b,c) For a transistor, = C CB

B

I II

I

base region is thin, so

that majority carrier of emitter will reach the collector.

Rinput = input input

B C

V V.

I I i.e., Rinput ∝

c

1

I

Therefore, Rinput is inversely proportional to the collector

current. For high collector current, the Rinput should be

small for which the base region must be very thin and

lightly doped for a transistor action, the emitter junction

is forward biased and collector junction is reverse

biased.

33




89. (c) In an n-type semiconductor, it is obtained by doping the

GE or Si with pentavalent atoms. In n-type

semiconductor, electrons are majority charge carriers

and holes are minority charge carriers.

90. (c) In an unbiased p-n junction, the diffusion of charge

carriers across the junction takes place from higher

concentration to lower concentration. Therefore, hole

concentration in p-region is more as compared to n-

region.

91. (d) When temperature increases, number density of free

charge carriers increases and mean relaxation time

decreases due to increased lattice vibrations. Effect of

decrease in relaxation time is much less as compared to

effect of increase in number density.

SSC CDS

BANKrAILWAY



Physics



1999 to till date

C L A S S E S

Chapter - 12

(Sounds)

1




SOUND & ACCOUSTICS Sound is a form of energy, which produces the sensation of hearing.

• These are longitudinal mechanical waves.

• Sounds waves have low frequency and high wavelength.

• These waves cannot travel in vacuum.

Velocity of Sound or Speed of Sound

AIR 345 m/s

Water 1483 m/s

Iron 5130 m/s

Acoustics: The properties or qualities of a room or building that determine how sound is transmitted in itor the branch of physics

concerned with the properties of sound.

Characteristics of a Wave

(i) Amplitude

Low Amplitude

High Amplitude

The maximum height of a wave is its amplitude.

(ii) Frequency ()

High Frequency Low Frequency

Hertz

1111

The number of waves passing through a point in one second.

Frequency = 1

time period

It is denoted by (new)

2




3




(iii) Time period (T)

High Frequency

Sec T

The time taken by a wave to complete one cycle.

Time period = 1/Frequency

(iv) Wavelength ()

m, cm, Å

The horizontal distance covered by a wave in one cycle.

It is denoted by (lembda)

c =

Speed = wavelength × frequency = Wavelength/Time period

4




Loudness It depends on the amplitude of wave.

Pitch • It depends on frequency of wave. • Tells shriekness of sound.

Sound

Audible Range

Less than

20 Hz

Infrasonic (sound) 20 Hz – 20 k Hz

greater than

20 k Hz

Ultrasonic (Sound)

Characteristics of Sound Wave

Large Amplitude Small Amplitude

Loud Sound

Girl Sound Boy Sound

High frequency High pitch

Low frequency Low pitch

Feeble Sound

5




According to their frequency range, waves are divided into the following categories

1. Audible or Sound Waves

The longitudinal mechanical waves, which lie in the frequency range 20 Hz to 20000 Hz are called audible or sound waves. These

waves are sensitive to human ear.

2. Infrasonic Waves

The longitudinal mechanical waves having frequencies less than 20 Hz are called infrasonic waves. These waves are produced by

sources of bigger size such as earthquakes, volcanic eruptions, ocean waves, elephants and whales.

3. Ultrasonic Waves

The longitudinal mechanical waves having frequencies greater than 20000 Hz are called ultrasonic waves. Human ear cannot

detect these waves. But certain creatures like dog, cat, bat, mosquito etc., can detect these waves.

Note: Bat not only detect but also produce ultrasonic waves.

Applications of Ultrasonic waves

⦁ For sending signals.

⦁ For measuring the depth of sea.

⦁ For cleaning clothes, aeroplanes and machinery parts of clocks.

⦁ For removing lamp-shoot from the chimney of factories.

⦁ In sterilizing of a liquid.

⦁ In ultrasonography.

⦁ In SONAR

Characteristics of Sound

Intensity or Loudness

6




Intensity of sound at any point in space is defined as the amount of energy passing normally per unit area held around that point

per unit time. Its SI unit is watt/metre. Loudness depends on intensity of sound,

Unit of loudness is bel and 1

10th of bel is decibel (dB).

Pitch or Frequency

The pitch of a sound is the characteristic which distinguishes between a shrillness or graveness of sound.Pitch depends upon

frequency. A still and sharp sound has higher pitch and grave and dull sound has lower pitch.

Quality or Timbre of Sound

Quality is that characteristic of sound, which enables us to distinguish between sound produced by two sources having the same

intensity and pitch. It depends on harmonics and their relative order and intensity.

Beat

When two sound waves of nearly same frequencies are produced simultaneously, then the intensity of resultant sound wave

increases and decreases with time. This change in the intensity of sound is called phenomenon of beats. Beat frequency is equal to

the difference in frequency of two sound sources. Artists use this phenomenon often while tuning their musical instruments with

each other.

Speed of Sound

The speed of sound basically depends upon elasticity and density of medium. Speed of sound in air is 332 m/s, in water is 1483

m/s and in iron is 5130 m/s.

When sound enters from one medium to another medium, its speed and wavelength changes but frequency remains unchanged.

Methods for determination of speed of sound in gas------ Kundt's tube method

Methods for determination of speed of sound in air------ Resonance column method

Note: Liquids and solids generally have higher speed of sound than in gases

Effect of physical parameters on velocity of sound

Effect of

Temperature

If Temperature

then

velocity of sound

Effect of Pressure

If temperature is

constant, Pressure

has NO effect on

velocity of sound

Effect of Wind

If wind is blowing in

the direction of

sound then velocity

of sound .

Humidity

• Humid air High velocity

• Dry air Low velocity

Effect of

Frequency

NO effect

( means Increase)

Reflection of sound

The rebouncing back of sound, when it strikes a hard surface is

called reflection of sound.

The repetition of sound due to reflection of sound wave is

called an echo.

These cannot be heard on the surface of the moon and in outer

space because there is no air on the moon.

The flash of light is seen first and the sound of thunder is

heard a little later.

7




Because the speed of sound in air is very slower as compared to the speed of light in air.

Speed of Light = 3 × 108 m/sec,Speed of sound = 332 m/sec

Echo :It is a reflection of sound that arrives at the listener with a delay after the direct

sound.

Delay is directly proportional to the distance of the reflecting surface from the source and the

listener.

Examples:

• Echo produced by the bottom of a well,

• by a building,

• by the walls of an enclosed room

• by the walls of an empty room.

A true echo is a single reflection of the sound source

Different processes involved with waves

Refraction of Sound Waves

When a sound wave moves from one mechanical medium to another mechanical medium, then the waves are refracted or

transmitted. This phenomenon is called refraction of sound.

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The refracted waves deviates from the original path of the incident waves. The main reason for occurrence of refraction in sound is

difference inspeeds of sound, in different media at different temperatures. Range of sound and temperature depends on each other,

in the following ways

⦁ On a warm day, the air near the ground is warmer than the air above and so the speed of sound waves near the ground is higher.

Thus, causes bending of the sound away from the ground.

⦁ On a cold day or at night, the reverse happens and the sound waves bend towards the earth. Thus, on a cold day, sound can be

heard over larger distance.

Diffraction of Sound Waves

When sound waves are originated by a vibrating source, then they

spread in the medium. If the medium is homogeneous, then these

waves have spherical wave fronts from the point source. Far from the

source, the wave fronts are nearly planes and the shape of the wave

fronts is changed, when the sound waves meet an opening in its

path.This leads to bending of sound waves around the edges. Such

bending of sound waves from an opening is called diffraction. Since,

the wavelength of the sound waves is approximately 1 m and of the

same order as the doors and windows, etc., of our houses. Therefore,

the sound waves diffracted broadly and one person easily hears the

voice of the another person.

Reverberation Time (T):Persistence of sound even after sound has

stopped.

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Sabine Formula:

Formula developed by Wallace Clement Sabine that allows designers to plan

reverberation time in a room in advance of construction and occupancy

Formula: T=0.049(V/A) where T = reverberation time or time required (for sound to

decay 60 dB after source has stopped) in seconds. V = Volume of room in cubic feet. A

= Total square footage of absorption in sabines.

Part of it is absorbed andtransmitted

Remaining is Reflected

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Absorption Coefficient = Sound energy absorbed by Surface

Total incident energy on surface

Standard unit to measure = OWU(Open window unit)

Total Absorption = (Surface Area)

Absorption coefficient

Doppler Effect

It is the phenomenon of change in apparent pitch of the sound due to relative motion between the source of sound and the listener.

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Sonar

The acronym SONAR stands for Sound Navigation and Ranging.

It is a device, that uses the ultrasonic waves to measure the distance, direction and speed of objects under water.

The ultrasonic sound pulse travel from the ship to the bottom of the sea, and back to the ship.

In other words, the SONAR measures the time taken by the echo to return to the ship.

Half of this time gives the time taken by the ultrasonic sound to travel from the ship to the bottom of the sea.

Applications of Sonar

⦁ Target location for torpedoes.

⦁ Resource location for mines.

⦁ Submarine navigation.

⦁ In aircraft.

⦁ Remotely operated vehicles.

⦁ Detecting the vehicle location.

Noise Reduction in Recording Media

Dolby Laboratories Inc is a music recording company, which has developed techniques to reduce noise levels in recorded

music.Dolby noise reduction, employed during recording and during playback, works in tandem to improve the signal-to-noise

ratio.

Dolby A was company's first noise reduction system, intended for use in professional recording studios. It provided about 10 dB of

broadband noise reduction.

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Dolby B was developed to achieve about 9 dB noise reduction primarily for cassettes. It was much simpler than Dolby A and

therefore less expensive to implement in consumer products. From the mid1970s, Dolby B became standard on commercially pre-

recorded music cassettes.

Dolby C provides about 15 dB noise reduction. It first appeared on top-end cassette players in the 1980s.

Dolby SR(Spectral Recording) system is much more aggressive noise expensive to implement than Dolby B or C, but it is capable of

providing up to 25 dB noise reduction in the high frequency range.

Dolby S is found on some Hi-Fi and semi-professional recording equipment. It is capable of 10 dB of noise reduction at low

frequencies and upto 24 dB of noise reduction at high frequencies.

(Electrical Equipments)

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