applied physics notes mehanical by prof. atul waghmare

5/20/2018 Applied Physics Notes Mehanical by Prof. Atul Waghmare

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CHAPTER 1

MOTION

[A] Rectilinear and Angular Motion

Q. State three equations of motions in case of rectilinear motion.

Ans: The three equations of rectilinear motion are

1. V = u + at

2. S = ut +1

2at2

3.

V2 = u2+ 2as

Where, u = Initial velocity

V = Final velocity

t = Time taken by a particle to change velocity from u to v

s = Distance covered in t second

a = Uniform acceleration

Q. State the equation of distance travelled by a particle (body) in nth

second.

Ans: The equation of distance travelled by a body in nthsecond is

Snth= u +

2(2n1)


a = Uniform accelerationSnth= Distance travelled in time n.

If the body freely falling under gravity then the equation of distance

travelled by the body in nth second will be

Snth= u +

2(2 n1)


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Q. State the equations of motion for a body falling freely due to gravity

along with the meaning of each symbol.

Ans: The equations of motion when a body falling freely due to gravity is as

follows.

1.

V = u + gt

2. S = ut +1

2gt2

3.

V2= u2+ 2gs


V = Final velocity

g = Gravitational acceleration

t = Time taken

s=Vertically downward distance covered by body.

Q. State the equations of motion when a body moves vertically upward

(against gravity)

Ans: The equations of motion when a body moves (through) vertically upward are

as follows

1. V = ugt

2.

S = ut1

2gt2

3. V2= u22gs


V= Final velocity

g = Gravitational acceleration

t= Time taken

s= vertically upward distance coved by body


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Q. Define following terms

1. Uniform velocity

2. Uniform acceleration

3. Uniform retardation

Ans: A) Uniform velocity

If a body covers equal displacement in equal interval of time, then it is said

to be in uniform velocity.

For e.g. A car moving with same speed (10 m/s) in the same direction.

B) Uniform acceleration

If the acceleration of a body is uniform in magnitude & direction with

respect to time then it is called as uniform acceleration.

For e.g. A ball falling down.

C) Uniform retardationIf the acceleration of a body is negative & uniform in magnitude &

direction with respect to time then it is called as uniform retardation.

For e.g. A ball moving up.


1. Angular displacement

2. Angular velocity

3. Angular acceleration

Ans:

1.

Angular displacement:

It is defined as the angle through which radius vector turns when the

particle in circular motion moves from one position to other.

It is denoted by .Its S.I. unit is radian (rad).

Unit: m/s or cm/s

Unit: m/s2or cm/s2


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2.

Angular velocity:

The rate of change of angular displacement with respect to time is called

as angular velocity of a particle.

It is denoted by = /t

Its S.I. Unit is red/sec.

3. Angular acceleration

The rate of change of angular velocity with respect to time is called as

angular acceleration of a particle.

It is denoted by Its S.I. Unit is red/s2

= /t

Q. State the relation between linear velocity & angular velocity.

Ans: The relation between linear velocity and angular velocity is given by.

V = r WhereV = Linear velocity

r = Radius

= Angular velocity

Q. Derive the relation between linear velocity and angular velocity.

Ans: Consider a particle particle performing (undergoing) uniform circular

motion. It moves from point A to point B in time T shown in figure.


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We have,

Linear velocity =

V = s/t

Were, S = Linear displacement

= /t (s = )

(= angular displacement)

= x /t (

= )

V =

Where, = Radius of circle= Angular velocity

Thus, linear velocity is radius times the angular velocity.

Q. State the three equations of angular motion along with meaning of each

symbol.

Ans: The three equations of angular motion are as follows

1.I = I+ t

2. = It +1

2t2

3.F2= I

2+ 2


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Where, = Angular displacement, = angular accelerationI= Initial angular velocityF= Final angular velocityt = Time taken by a particle to change velocity from

I to F

Q. State the equation of angular distance travelled by a body in nth

second.

Ans: The equation for angular distance travelled by a particle in nth second is

given by,

nth = 1 +

2(2n1)

Where,

nth= Angular distance travelled by a particle body in nthsecond1 = Initial angular velocity = Constant angular acceleration


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[B] KINETICS


1. Momentum

2. Impulse

3. Impulsive force

Ans:

1. Momentum:

Momentum of a body is the amount of motion that a body or a particle

has. OR

It is defined as the amount (quantity) of motion possessed by movingbody.

Mathematically, momentum is the product of mass & velocity. It is

represented by

Momentum = Mass x Velocity

P = mxv

Its S.I.unit is N sec or Kgm/s

It is vector quantity

2. Impulse:

It is defined as the change in momentum

Impulse = change in momentum

Impulse = mv- mu

Wher, m= Mass of a body

u= Initial velocity

v= Final velocity

Its S.I.unit is kg m/s or N sec,

It is vector quantity.


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3. Impulsive force:

Impulsive force is defined as the force which acts over short time;

producing rapid change in the motion of a body.

Mathematically, impulsive force is the rate of change of impulse withrespect to time.

Impulsive force =

=()

=

()

Its S.I.unit is N

Q. State Newtons First law of motion OR State law of inertiaAns: Statement: It states that every body continues in its state of rest or of

uniform motion in a straight line, unless, it is acted upon by some external

unbalanced force

This means in the absence of unbalanced force, every object has a tendency

to resist any change in its state of rest of motion.

This tendency is inertia so this law is also known as law of inertia.

Examples: 1.A car moving with uniform velocity continues its motion until

we apply breaks

2.A book kept on table, remains at rest until we apply force

Q. State and explain Newtons second law of motion.

Ans: Statement: It states that the rate of change of momentum of a body is

directly proportional to the applied force & takes place in the direction of

force.

Thus,

F

()

F or

()

= Constant x F

ma = F [

= a & assuming proportionality constant = 1]


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Where, F = Force

mv = Final momentum

mu = Initial momentum

t = Timem = Mass

a = Acceleration

u = Initial velocity

v = Final velocity

Q. State Newtons third law of motion(Law of action reaction).

Ans: Statement: For every action there is equal and opposite reaction.

The means that for every force there is a reaction force that is equal in size

but opposite in direction.

Ex:Action:While swimming our body push the water backwards.

Reaction:The water pushes our body forward.

Action:When we hit the ball on wall

Reaction: It bouncesback

Q. Explain recoil of a gun? OR What is recoil velocity of a gun & derive

the equation of recoil of gun.

Ans: If a bullet is fired from a gun then bullet moves (shoot) out with a large

velocity & at the same time gun moves back (jerk) with the little velocity.

This backward movement (jerk) is known as recoil of the gun andthe

velocity with which the gun moves backward is called the recoil velocity of a

gun

1.

Before firing

Let Mgbe the mass of the gun & Mbthat of the bullet


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2.

After firing

Let Vg be the velocity of the gun & Vb that of the bullet.

But, according to law of conservation of momentum.

Initial momentum of Final momentum of

gun & bullet gun & bullet

But, initial momentum of the gun & bullet is equal to zero because they are

initially at rest.

Therefore,

Final momentum after firing = MbVb+ Mg(- Vg) = 0

The negative sign indicates that the gun move in the backward direction

MbVbMgVg= 0

MbVb= MgVg

Vb=

Mg Vg

Mb

This is the equation of recoil velocity.


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[C] WORK, POWER &ENERGY

Q. Define Work, Power & Energy. Write their S.I. Unit.

Ans: Work:Work is defined as the product of force acting on the body & the

displacement produced.

Work = Force x Displacement

The S.I. Unit of work is Newton meter

M.K.S. unit is joule

C.G.S.unit is erg.

Power: Power is defined as the rate (capacity) of doing work

Power =

The S.I. Unit of power is watt

Energy: The energy of a body is defined as its capacity to do work

S.I. unit of work isjoule or ergAll forms of energy are transferable

E.g.Light energy can be converted into electrical energy.

Q. Define potential energy & kinetic energy with equation.

Ans: Potential energy:The energy possessed by a body due to its position is

called as potential energy.

Potential energy = weight x height

P. E. = mgh

S. I. unit Nm or joule


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Kinetic energy: The energy possessed by a body virtue of its motion is

called as kinetic energy.

K.E. =1

2

mv2

Where, m = Mass of body

v = Velocity

S.I. unit Nm or joule

Q. State & explain work energy principle.

Ans: Statement:It states that the work done by a system of force acting on a

body between any two points is equal to the change in kinetic energy of a

body between these same two points.

Consider a body of mass m

Let, F be the force on it

V1be the velocity at position P1

V2be the velocity at position P2

d. be the displacement of a body

K.E. at P1 = 1/2 mv12

K.E. at P2 = 1/2 mv22

Change in kinetic energy = 1/2 mv221/2mv1

2

= 1/2 m (v22

v2

1)Work done = Force X Displacement

= [F] x d

= (m x a) x d

Using equations of motion, it can be proved that

Work done = change in kinetic energy


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Q. Define following terms.

1. Projectile motion

2. Trajectory

3. Angle of projection

4. Time of flight5. Range of projectile

6. Maximum height of projectile

Ans:

1. Projectile motion:Projectile motion of a body thrown (projected) in air at

angle [less than 900] with the horizontal.

Ex: 1. Motion of football kicked in air. 2. Motion of cricket ball a batsman

hits a six

2. Trajectory:The path along which projectile moves is called as trajectory.

3.

Angle of projection:It is defined as it is the angle made by the velocity of

projection with the horizontal at the original point O

Angle of projection

= tan-1(

)

Where= Angle of projection with horizontal at the originH = Maximum height of a projectile

R = Horizontal range of projectile

4.

Time of flight:The time taken by the projectile to reach the ground from

the time it was projected is known as time of flight.

It is given by

T =

Where,T = Time of flight

u = Initial velocity

= Angle of projection, g = Acceleration due to gravity.


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5. Range of projectile:The total horizontal distance covered by a projectile

is called as a range (R).

R = (V

2

sin2) / g (If = 450

C sin2= 1)R = Range of a projectileV = Velocity of projectile

= Angle of projection

6. Maximum Height of projectile:The maximum vertical distance covered

by a projectile from the ground level is called height of a projectile

R = (V2sin2) /2 g

R = Range of a projectile

V = Velocity of projectile

= Angle of projection

g = gravitational acceleration


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[D] CIRCULAR MOTION & PROECTILE MOTION

Q. Define

1. Circular motion

2. Uniform circular motion

3. Centripetal (radial) acceleration

Ans:

1. Circular motion:Circular motion is defined as the motion of a particle

along the circumference of circle.

Ex:a) Moon revolving around the earth

b) Electron revolving around the nucleus of an atom.

2. Uniform circular motion:It is defined as a motion of particle along the

circumference of circle with constant speed.

Ex:a) Moon revolving around the earth

b) Electron revolving around the nucleus of an atom.

3.

Centripetal (radial) acceleration:The acceleration in uniform circular

motion which is directed along the radius & towards the centered of a circle

is called as centripetal acceleration.

Q. Define Centripetal force.Give two examples

Ans: Centripetal force: It is defined as the force acting along the radius

towards the centre of the circular path which keeps the particle in uniform

circular motion.

Ex: Electron revolving around the nucleus - In this case theelectrostatic force of attraction between electron & proton plays the

role of centripetal force.

Moon revolving around the earth - The gravitational force of

attraction between earth and moon plays the important role


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Q. Define Centrifugal force. Give two examples. Write applications of

centrifugal force (circular motion)

Ans: Centrifugal force:It is defined as the force acting on a particle performing

uniform circular motion which is directed away from the centre and alongthe radius of a circular path.

Ex: 1. Person sitting in merry go round or giant wheel, experiences outward

pull.

2.Motor cyclist driving in a artificial death well in a circus experiences

outward pull because of his high speed

Applications of Centrifugal Force:

1.

Centrifugal force used in banking of roads to avoids skidding of speedy

vehicle along a curved road.

2. Centrifuged pump is used to transfer liquid; centrifugal force is used in

this process.

3. A cream separator is used in the dairy works on the principle of centri-

fugal force.

4. Centrifugal blower is used to blow air.

5. Centrifugal force can be used to generate artificial gravity for rotating

space stations

6. Centrifuges are used in science & industry to separate substances.

Q. Differentiate between Centripetal force & Centrifugal force.

Ans:

Centripetal force Centrifugal force

1 It is the force acting on a particlein uniform circular motion which is

along the radius & towards thecentre of circular path

It is the force acting on particleperforming uniform circular motion

which is along the radius & away fromthe circular path

2 This is a real force (has physical

existence)

This is an imaginary (pseuelo) force.

3 This force is acting towards the

centre

This force acting away from the centre


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CHAPTER 2

NON-DESTRUCTIVE TESTING OF MATERIALS

Q. Define ultrasonic waves & states its frequency range.

Ans: Definition:

The ultrasonics are the sound waves having frequency more than 20 kHz.

The frequency range of UV- waves is more than 20 kHz.

Q. State the range for infransonic, sonic & ultrasonic waves

Ans: Type of wave Range of frequency

1.

Infrasonic Less than 20HZ

2. Sonic Between 20 HZ to 20 kHz

3. Ultrasonic More than 20 kHz

Q. State the properties of Ultrasonic waves.(Any Four)

Ans: Properties of Ultrasonic Waves:

1.

Frequency of these sound waves is more than 20khz2.

Ultrasonic waves has shorten wavelength

3.

Ultrasonic sound waves carry high amount of sound energy

4. The speed of propagation of ultrasonic waves increases with increase in

frequency

5. Ultrasonic waves show negligible diffraction

6.

Ultrasonic waves travel over long distance without considerable loss

7.

Ultrasonic waves undergo reflection and refraction at the separation of

two media

8.

If ultrasonic waves are passed through a fluid, then temperature of the

fluid increases

9.

Ultrasonic waves travel with constant speed through a homogeneous

material

10.

Ultrasonic waves possess certain vibrations which are used as good

massage action in case of muscular pain


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Q. State engineering, scientific and medical applications of ultrasonic

waves.

Ans: Applications of ultrasonic waves:

a)

Engineering Applications:1.

Ultrasonic waves are used for cleaning or washing of delicate parts of

machine like watches, electronic components etc.

2.

Ultrasonic waves are used in Non Destructive Testing to detect flaw in

material without damaging.

3. Ultrasonic waves are used in drilling holes in the metal.

4. Ultrasonic waves are used in cutting metal parts in industry.

b)

Scientific Applications:

1.

Ultrasonic waves are used to determine the depth of sea.

2. Ultrasonic waves are used in sound navigation and ranging (SONAR.)

Because of high directionality, the ultrasonic waves are used to locate

obstacle and to determine their distance in sea.

3. Ultrasonic waves are used to produce some alloys like lead aluminum

which cannot be produced by conventional techniques.

c)

Medical Applications:

Ultrasonic waves are used by doctors to monitor the development of

unborn babies, diagnosis of heart disease and other disorders in patient

Q. Explain the production of ultrasonic waves using piezoelectric method.

Ans: Principle: When the electric field is applied across the crystal its dimension

changes and when alternating PD is applied across crystal then the crystal

sets into elastic vibrations

Piezoelectric effect:

When mechanical pressure is applied; the opposite charges are developed on

the parallel faces of certain crystals like quartz, tourmaline, barium titanate

and Rochelle salt etc.


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It results in generation of potential difference across their opposite faces.

This phenomenon is called as piezoelectric effect

Method:

1.

A chip of piezoelectric crystal like quartz is placed between two plates as

shown in fig.

2. A suitable Hartely oscillator is connected across it.

3.

The electric oscillations along the electric axis produce mechanical

vibrations along the mechanical axis.

4. Then the frequency of oscillator is increased.

5. At a particular frequency of oscillator the oscillator frequency becomes

equal to natural frequency of vibration of crystal. Then the crystal thrown

into resonance vibrations & ultrasound is given out (produced)

Q. What is meant by non destructive testing (NDT)?

Ans: Non destructive testing is the method of examining the materials for

internal as well as surface discontinuities without the destruction of

material.


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Q. State the advantage of non destructive testing (NDT) methods.

Ans: Advantage of NDT:

1. Material can be used for its intended purpose after testing.

2.

It is used for quality control tool, because material can be tested for allpossible parameters.

3.

It is used for quality assurance tool; because after testing material one can

check its suitability at a particular place.

4. The testing of material is possible before its manufacturing.

5. The testing is possible after manufacturing.

6. The testing of material is possible during servicing of components

7.

Hundred percent examination of material is possible.

Q. State the limitations (Disadvantages) of Non- destructive testing. (NDT)

Ans: Limitations of NDT:

1. Minimum two methods for complete examination of the material are

required. With only one method, testing for all parameters of materials is

not possible.

2.

Trained and certified persons are authorised to conduct the test (level I,

II, and III) as per American Society for Non-destructive Testing (ASNT).

3. Cost of equipments is high and thus testing charges are more as

compared to destructive testing.

4. Qualitative testing is possible; however, quantitative testing is difficult.

5.

NDT interpretations are relative. One should know the standard results

first.

Q. Name the popular NDT methods used in mechanical industry.Ans:

1. Visual testing (optical testing) (VT)

2. Liquid penetrate testing (LPT)

3. Eddy current testing (ET)

4.

Radiography testing(RT)


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a. X-ray radiography

b.

Fluoroscopy

c.

Gamma ray radiography

5. Ultrasonic scanning and flaw detector method(UT)

6.

Magnetic particle testing (MPT) 7. Thermography method

Q. Explain the criteria for selection of NDT method OR State any four

factors on which NDT method can be selected OR On which factors

selection of NDT method depends.

Ans: Following parameters are considered while selecting NDT methods

1. Codes or standard requirements

2.

Specifications of the material to be tested, for example, nature ofmaterial, its size & shape.

3. Types of disorders to be detected also depends on nature of disorders

4.

Testing also depends on manufacturing process of material to be tested

5.

It is also depending on the instruments available for testing

6.

Total cost required to test the material

Q. Describe Liquid penetrate testing (LPT) with its

1. Principle

2. Experimental procedure.

Ans: Principle:It works on the principle of capillarity

Experimental procedure:

Following steps are involved in liquid penetrant test or inspection.

1.

Surface penetration: Initially the surface of specimen is cleaned.

Because the presence of flakes, dirt, grease etc on the surface of work

pieace prevents penetrant to slip into the cracks. This give wrong

information


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2. Application of dye penetrant: Suitable fluorescent dye is mixed in

penetrant so that its viscosity remains low. This dye penetrant is applied

evenly on specimen. Due to capillary action the penetrant goes into the

surface open discontinuities. It takes some time. In general case this

dwell time is 20-30 minutes.

3. Removal of excess penetrant: After dwell time is over, the excess

penetrant is removed from the surface carefully.

4.

Application of developer: A thin layer of developer is applied over the

surface. The role of developer is to pull the trapped penetrant out of the

crack this provides good visibility of crack.

5.

Inspection and evaluation of defect: Surface of the specimen is seen

under white light or ultraviolet or laser light. The crack can be visualized

under light.


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6. Post cleaning: After inspection the surface of the specimen is cleaned &

the specimen can be used for its intended purpose.

Q. Write advantages and disadvantages of LPT.

Ans: Advantages of LPT:

1.

Cheaper method

2.

No power required

3. All types of material can be examined

4. It is independent on size and shape

5.

Method is suitable for wide and varied application.

Disadvantages of LPT:

1.

It is used only for porous materials

2.

It is used only for detecting surface defects

3. Its running cost is more

4. It is slow process

Q. State applications of LPT in industry.

Ans: Applications of L.P.T.:

1. Rotor discs and blades are checked by LPT

2.

Aluminum casting and forgings, Pistons and cylinder heads in

automotive industry are checked by LPT.

3.

Moulded plastic parts can be checked.

4.

Bogie frames of railway locomotives can be checked.5. Electrical ceramic parts can be checked.


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Q. State the principle, procedure of ultrasonic testing.

Ans: Principle: When ultrasonics waves are introduced into a material, it gets

reflected, transmitted, scattered from surface or flaw

Procedure:

1. At first, the ultrasonic equipment is calibrated before testing the specimen

by using the standard blocks as per recommendation.

2. Then the ultrasonic signals are generated from the transducer.

3. These generated ultrasonic signals can pass through specimen

4. Then the reflected signals from the crack flow are received and amplified

5. Then the amplified signal is applied to the cathode ray tube ( CRT )

6. CRT displays the details of specimen (material) i.e. the material is OK or

defective

Q. Name different types of ultrasonic testing methods

Ans: Types of ultrasonic testing1. Transmission ultrasonic testing method

2. Pulse echo UT method

Q. State advantages of ultrasonic testing.

Ans: 1. It has high sensitivity and reliability

2. It can be used for testing all types of material

3. The equipment is portable and harmless

4. The result are instantaneous


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Q. State the application of ultrasonic testing.(Any four)

Ans: Application of UT:

1. This method is used to detect flaws in all metals, rubber, tyres, concrete,

wood, composite, plastic.2.

It is used to test casting, forging, welding, rolling, and heat treatment.

3.

It is used to detect crack (flaws) in Air craft.

4.

It is used in tunnel inspection.

5. It is used in bridge inspection

6. It is used to detect surface discontinuities.

7. It is used in monitoring of thermal and atomic power plant

8.

It is used to determine thickness.

Q. If the crack is on the surface of job or near the surface which of thefollowing method is used. Why? UT, LPT, NDT

Ans: If the crack is on the surface of job or near the surface, then LPT method is

useful because.

1. It is cheaper method

2.

No power required;

3.

By using this method all types of material can be examined

Q. What is role of developer in LP testing?

Ans: The role of developer is to pull the trapped penetrant out of the crack; this

provides good visibility of crack


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CHAPTER 3

THERMOCOUPLE

Q. Define E.M.F. (electromotive force). State its S. I. Unit.

Ans: EMF: EMF of a cell is defined as the energy required to circulate a

charge of 1 cm through a circuit.

EMF =

=

E =

Where, W = Work done

S.I.Unit of EMF is volts

Q. Define thermoelectricity.

Ans: The electricity obtained by conversion of heat energy into electrical energy

is called as thermoelectricity & the phenomenon is called thermoelectric

effect & the emf developed is called as thermo e.m.f.

Q. What is thermocouple?

Ans: Thermocouple:

Two different metal wires from thermoelectric series are joined together so

that two junctions are formed such an arrangement is called as

thermocouple.

Q. State Seebecks effect.

Ans: Statement: When two dissimilar metals are joined together, so that two

junctions are formed & if one junction is heated & other is cooled themelectric current flows through it.I.e. emf is generated.

This effect is known as Seebeck effect.

Heat (thermal) energy

> electrical energy


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Q. State Peltier effect.

Ans: Statement:

When electric current flows through a junction of two different metals from

thermoelectric series, heat is generated or absorbed at the junction

depending on the direction of current.This is known as Peltier effect

Q. State & explain joules law. Give its equation.

Ans: Statement:

It states that the amount of heat generated (H) due to the flow of electric

current through a resistance is directly proportional to

1. Squae of the current (I2)

2. The resistance (R)

3. The time for which current flows(t)

Hence, H I2RtH = Constant x I2Rt

H = (

) I2Rt

Where, J = Joules constant

(Mechanical equivalent of heat)

= 4200J/Kcal

= 4.2 J/cal

Also, J =

J = I2Rt/H Put IR = V By ohms law

J = VIt/H Put I = V/R

J = V2t/RH


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Q. Define Neutral temperature & Inversion temperature.

Ans: Neutral temperature: In thermocouple, The temperature at which the emf

is maximum is called as neutral temperature.

Inversion temperature: In thermocouple The temperature at which theemf becomes zero & changes its sign (become negative) is called as

inversion temperature.

Q. Define thermoemf (thermoelectric current). State the factors on which

thermo emf are dependent.

Ans: When one junction of thermocouple is heated & other is cooled i.e.

temperature difference between two junctions is created; then the emf is

developed in the circuit called as thermoemf.The thermoemf depends upon

1. The temperature difference between the two junctions

2. Pairs of metals used in thermocouple

Q. How will you use thermocouple to measure high temperature OR

What is thermo couple? Explain how it is used to measure thermoemf.

Ans: Thermocouple:Two different metal wires from thermoelectric series arejoined together so that two junctions are formed. Such an arrangement is

called thermocouple.

1. Thermocouples are used as a thermometer to measure the temperature

from 00C to neutral temperature.

2.

Thermo emf produced is measured using potentiometer.

3.

Connections are made as shown in circuit diagram.

Part 1:In 1stpart only battery cell of known emf E is taken in circuit and

balancing length L is determined

Potential gradient P.G. = E/L

Then potential gradient (P.G.) is recorded.


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Part 2:Now thermocouple is taken in the circuit. For different temperatures

t1, t2, t3(of hot junction), different balancing length L1, L2, L3are recorded.

Then using formulae, E1 = P.G. x L1

E2= P.G. x L2

E3= P.G. x L3

Different emfs at different temperatures are recorded and the graph is

plotted.


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Q. Give the Seebeck thermoelectric series. What is the relation between

pairs of metals & thermoemf OR How can you increase thermoemf

using different metals? Give one example.

Ans: Seebeck performed number of experiments with couples consisting of

various metals and arranged them in a series such that when any pair ofmetals is used, the current always flows from the higher member(i.e. near to

antimony end) to the lower member (i.e. near bismuth end) of the series,

through the cold junction of the thermocouple.

Seebeck also observed that if the metals are more apart in thermocouple

series, then the emf is maximum and if the metals are closer in series, then

the emf is minimum. The following is the thermoelectric series.

If the metals used in thermocouple are more apart in the thermocouple series,then the thermo emf (thermoelectric current) obtained is more.

E.g. if Sb - Bi thermocouple is used, then it gives maximum emf for given

temperature difference.

If metals used are closer in series, then the emf is less and current is also lessfor given temperature difference.


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Q. Differentiate between Seebeck effect, Peltier effete & Joules effect.

Ans:

Seebeck effect Peltier effect

(Thermoelectric effect)

Joules effect

(Heating effect)

1 When two dissimilarmetals are joined

together so that twojunctions are formed

(thermocouple) and ifone junction is heated

and other is cooled

then electric current

flows through it.

When electric currentflows through a junction

of two metals ofthermocouple, then heat

is generated at one endand heat is absorbed at

the other end.

When electriccurrent flows

through conductor,then it gets heated

2 Emf is developedacross the two

junctions

One junction gets heatedand other gets cooled.

(only) heat isproduced

throughout the

conductor

3 Emf generated is small

in mV.

Heat generated or

absorbed is small

Heat generated is

large

4 This effect is

reversible.

This effect is reversible. This effect is

irreversible.

5 Amount of emf

generated dependsupon pair of metals and

temperature difference.

Amount of heat generated

depends on pair of metalsand current through it.

Amount of heat

generated dependsupon value of

resistance, current,time.

6 E.g. thermocouple E.g. thermocouple E.g. electric

heater, electriciron.


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CHAPTER 4

MODERN PHYSICS

[A] PHOTO-ELECTRICITY

Q. Define Photoelectric effect.

Ans: When light of suitable frequency is incident on metal surface, electrons are

emitted from it. This effect is called as photoelectric effect.

Q. State Plancks hypothesis.

Ans: According to Plancks hypothesis, Emission and absorption of energy can

occur only in discrete amounts of bundles. Planck called this discrete

amount or bundle of energy as quanta or photon. The energy of each photon

isE=hv. So for emitted or absorbed energy,

E = nhv

Where, h is Plancks constant, has a value of 6.626 x 10-34J s

vis the frequency,

n is an integer = 1, 2, 3, ----

Q. What is photon? State the properties of photon

Ans: Photon:It is small pocket or bundle of energy.

The properties of Photon are as follows

1.

It is a mass less particle

2.

It is electrically neutral

3.

It moves with speed of light


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Q. Define a) Stopping Potential b) Photoelectric Work Function c)

Threshold frequency d) Threshold Wavelength.

Ans: a) Stopping Potential: It is reverse or negative potential which reduces

the photoelectric current to zero. It is denoted by Vs.

b) Photoelectric Work Function:It is the energy required to detach the

electron from the metal.

It is given by w0= hv0

c) Threshold frequency:It is the minimum frequency of incident light at

which emission just begins.

It is given by v0= c/v0

d) Threshold wavelength:It is the maximum wavelength of incident light

at which emission just begins

It is given by0=

Q. State the properties (Characteristics) of Photoelectric effect.

Ans: Following are the characteristics of photoelectric effect

1. A metal emits electrons only when the incident (light) radiation has

frequency greater than critical frequency (v0)

2. A photoelectric current is directly proportional to the intensity of light

and independent of frequency


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3. The velocity of photoelectron is directly proportional to the frequency of

light and independent of intensify.

4.

For a given metal surface, stopping potential is directly proportional to

the frequency

5.

This process is instantaneous. i.e. the emission of photoelectrons starts atthe moment light is incident on the metal surface.

Q. Derive Einsteins photoelectric equation.

Ans: 1. According to Plancks Hypothesis, Energy is radiated and also travels in

the form of bundles or quanta known as photons

2. Each photon carries an energy hv

3. When a photon of incident light radiation interacts with an electron inside

an atom, the whole amount of energy is absorbed by the electron.

4. The electron uses part of the incident energy, to make the electron free

from the metal (work function) and the rest part of the energy is converted

into kinetic energy

Thus, hv=0+1

2mv2max

hv= hv0+1

2mv2max

1

2mv2max= hvhv0

1

2mv2max= h (vv0)

Where,1

2mv2maxis the maximum kinetic energy of electrons,

h is Plancks constant,

vis the frequency of incident light,

v0is the threshold frequency


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Q. State the Einsteins Photoelectric Equation and explains the significance

of each term involved in it.

Ans: The Einsteins Photoelectric Equation is given by

1

2mv2max= h (vv0)

Where,1

2mv2maxis the maximum kinetic energy of electrons,

h is Plancks constant,

vis the frequency of incident light,

v0is the threshold frequency

Q. Why electrons are not emitted from the surface of metal plate, if

frequency of incident radiation is less than threshold frequency (v0)

Ans: 1. According to Einsteins Photoelectric Equation

12 m v

2max= h(v-v0)

2. If v< v0, then,1 2 mv2maxbecomes negative

3. But for the emission of photoelectrons the kinetic energy must be positive

4. Thus, due to negative value of kinetic energy the photoelectrons are not

emitted from the surface of metal plate

Q. Define Photoelectric cell.

Ans: A Photocell is an electronic device in which light energy gets converted

into an electrical energy. It is used to produce a current or voltage when

exposed to light or other electromagnetic radiation.


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Q. Explain the construction and working of photoelectric cell.

Ans: Principle:The working principle of photoelectric cell is photoelectric

effect. It is an electronic device which converts light energy into an

electrical energy.

Construction:

1. A photocell is an evacuated tube consisting of two electrodes i.e. cathode

and anode

2.

Cathode is concave in shape called an emitter, made from a material that

emits electrons easily. Whereas, anode is in the form of thin rod as shown

in the fig

3. Cathode is connected to negative terminal of battery. An ammeter is

connected in the circuit to measure the current flowing through the

photoelectric cell

Working:

1. When light is allowed to fall on cathode it emits photoelectrons.

2. The photoelectrons are attaracted by anode


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3. The photoelectric current starts flowing through the circuit and the

milliammeter shows the deflection.

Q. Draw the diagram of Photoelectric cell.

Q. State the applications of photoelectric cell.(Any four)

Ans: The applications of photoelectric cell are

1.

It is used in Burglar alarm

2.

It is used in fire alarm

3. It is used in Lux meter to determine the intensity of light

4. It is used in Exposure mete

5.

It is used in automatic control of traffic signal6.

It is used in automatic counting objects

7.

It is used in automatic shutting and opening doors

8. It is used in reproducing sound in motion pictures

9. It is used in detecting flaws in metals

10.It is used in television sets


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Q. State the principle of light dependent resistor (LDR) or photo resistor.

Ans: Principle: The electrical resistance of LDR decreases as the intensity of

incident light increases.

When light is allowed to fall on this LDR (semiconductor), a photon is

absorbed by the material and electrons from valence banded get excited and

jump into the conduction band and hence conductivity of the material

increases or resistivity of the material decreases.

Q. State the applications of LDR.

Ans: Application of LDR:

1. It is used for detecting ships and air crafts by the radiations given out.

2. It is used as flame, smoke and burglar detectors.

3. It is used as a automatic lighting controls for street light.

4. It is used in Camera for exposure control.

5. It is used in Xerox machine- to controls the density of toner.

Q. Draw the symbol of LDR.

Ans:


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[B] X-RAYS

Q. What are X-rays?

Ans: X ray is electromagnetic radiations of short wavelength ranging from 0.01

A0to 100A0.

1 A.U.= 1 Angstron unit = 10-10metre.

Q. State the properties of X-rays.(Any four)

Ans: Properties of X-rays:

1.

X ray are the electromagnetic radiations of very short wavelength

2. X rays travels with speed of light

3. X rays are electrically neutral

4. X rays can ionises gas

5. X rays can damage or kill living cells

6.

X rays are invisible

7.

X rays can affect photographic plate8. X rays are not deflected by electric or magnetic field

Q. State the formula for minimum wavelength of X-rays also state the

meaning of symbols used in it

=/. Where, = minimum wavelength of x raysh= planks constant, V= applied voltage

C = velocity of light, e =charge of electron


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Q. Explain the production of X-rays using Coolidge (Modern) X-ray tube.

Ans: X ray is produced whenever fast moving electrons strike a high atomic

weight solid (tungsten) in vacuum.

1.

The Coolidge X-ray tube is shown in figure

2. It consists of a highly evacuated hard glass tube containing cathode (K)

and the target (T) attached to anode (A).

Working: 1.When the cathode is heated by electric current it produces

electrons due to thermionic emission.

2. The beam of electron is then focussed on the anode (Target).

3. The electrons from cathode are then accelerated by applying high voltage

between cathode and anode using step up transformer.

4. When these fast moving electrons are suddenely stopped by tungestenanode, they loses their kinetic energy and X rays are produced from the

target.

5. Some amount of kinetic energy is converted to large amount of heat.


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Q. State the applications of X-rays.

Ans: Applications of X-rays:

A) Industrial [Engineering] applications

1. X rays are used to detect defects within metals, machine parts, and

castings etc.

2. X rays are used to detect manufacturing defect in rubber tyres or tennis

ball in quality control.

3.

X rays are used to detect cracks in the wall

4.

X rays are used to detect the cracks in the body of aeroplane or motor car

5.

X rays are use to distinguish real diamond from duplicate one

6. X rays are used to detect smuggling gold at airport and ship yard

B) Scientific Research applications:

1. X rays are used to investigate the structure of the atom.

2. X rays are used for analyzing the structure of complex organic molecules.

3. X rays are used in determining the atomic number and identification of

various chemical elements.

C) Medical Applications:

1. X rays are used in detecting fractures in bones

2. X rays are used to cure skin diseases and destroy tumors.

3. X rays are used to detect bullet position inside the body.

4. X rays are used to cure diseases like cancer


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[C] LASER

Q. State the full form of LASER.

Ans: The full form of Laser is Light Amplification by Stimulated Emission of

Radiation.

Q. State the four properties of LASER.

Ans: The properties of LASER are-

1. Laser source is tremendously Intense source of light.

2. Laser source is a Monochromaticsource of light.

3.

Laser source is a Coherentsource of light.

4.

Laser source is a highly Directionalsource of light.

Q. Differentiate between Stimulated and Spontaneous Emission of light.

Ans:

Stimulated emission Spontaneous emission

1 This process can be controlled fromoutside

This process cannot be controlled fromoutside

2 Multiplication of photons takesplace

Multiplication of photons does not takeplace

3 All the emitted photons move in

same direction

All the emitted photons move randomly

4 It results in monochromatic light It results in non monochromatic light

5 The intensity of emitted light does

not decrease with distance from the

source

The intensity of emitted light decreases

with distance from the source

6 It results in highly intense light It results in low intense light


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Q. What is Population inversion of light ?

Ans: The process of increasing the number of atoms in higher energy level, more

than that of ground state is called as population inversion of light

Q. What is optical pumping? Explain Optical pumping.

Ans: Optical pumping: The process of raising the atoms from lower energy state

to higher excited state using light medium is called as optical pumping.

Let E1, E2 and E3 are energy levels and N1, N2 and N3 are respectiveconcentrations of atoms. The atoms in level E1are excited to E3by optical

pumping. The concentration N1decreases. The time for which the atoms can

stay in level E3 is very short. They lose some energy and return to energy

level E2. The transition from E3 to E2is rapid and spontaneous. Since level

E2 is metastable state, hence atoms relax here for longer time. The no. of

atoms in E2 increases and when it is greater than level E1 population

inversion takes places. i.e. N2 >> N1


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Q. State any four applications of Lasers.

Ans: The applications of Lasers are as follows:

A)

Industrial or Engineering use:

1. HeNe gas laser is commonalty used to read barcode.

2. High power lasers are used in cutting, drilling, welding

3. Lasers are used to find defect in material

4. It is used for marking, engraving of number plates .Ex. number plate,

name plate

5.

Lasers are used in holography

6. Lasers are used in computer printers

7.

Lasers are used for 3D,Laser scanners

8. Lasers are used in controlled heat treatment

B)Medical uses:

1. It is used to repair the detached retina of human eye.

2. They are used for surgery in medical field.

3. Dentists use LASER for painless drilling in tooth.

4.

They are used for destroying cancer cells in human body.

C)

Communication purpose uses:

1. It is used in fiber optical communication.

2. It can be used for communication between earth and moon due to

unidirectionality and brightness.

3.

It is used to measure the distance between the earth and the satellite.

applied physics notes mehanical by prof. atul waghmare

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