cbse class 12 physics important questions from past exams

Z I E T B H U B A N E S W A R | 116

3-DAY WORKSHOP ON IMPROVEMENT OF RESULTS FOR CLASS XII, ZIET BHUBANESWAR

CLASS XII

QUESTIONS FROM BOARD PAPERS(1 mark)

Name of the chapter: Electrostatics Name of the Topic:

1 Draw the equipotential surfaces corresponding to a uniform electric field.

2 Why two electric field lines do not intersect each other?

3 What is the relation between electric field at a point on the axial line and equatorial linedue to a short electric dipole (a<<<r).

4 Is it necessary that if the electric field is non zero the potential will also be non zero at apoint. Give an example.

5 What is the relation between electric field and potential difference between two pointslying along the electric field in a uniform electric field

WORKSHEET 1-A NAME OF THE CHAPTER: ElectrostaticsName of the Topic:

ANSWERS

1. E

2. As the electric field vector has unique direction at a given point. If the two field linesintersect the E will have two directions which is not possible.

3. Eaxial = 2Eequitorial

4. No. On the equatorial line of a dipole electric potential is zero but electric field is nonzero.

5. V = Ed

( 2 marks

Name of the chapter: Electrostatics

1 Draw electric field lines due to an electric dipole and a pair of equal positive charges.

2 Deduce the expression for the torque acting on a dipole of dipole moment p in thepresence of a uniform electric field E.

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3 Plot the graph showing the variation of electric field as a function of r, for r > R and r <R.

4 A parallel plate capacitor of capacitance C is charged to a potential V. It is then connectedto another uncharged capacitor of same capacitance. Find the ratio of the energy stored inthe combined system to that stored initially in the single capacitor.

5 Derive the expression for the electrostatic energy stored in a capacitor of capacitance Cand having charge ‘Q’.

Answer Key

WORKSHEET 1-B NAME OF THE CHAPTER: ElectrostaticsName of the Topic:

ANSWERS

1.

2.

3.

E

O R r4. Ui = CV2/2 , then



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C’ = C + C = 2C and V’ = V/2Hence, Uf = (2C)(V/2)2/2

= CV2/4 = Ui/25.

( 3 marks )

Name of the chapter: Electrostatics Name of the TopicPrepared by: ZIET GR. IV1 Define electric dipole moment. Is it a scalar or a vector? Derive the expression for the

electric field of a dipole at a point on the equatorial plane of the dipole.

2 Using Gauss’s theorem deduce the expression for electric field due to uniformly

charged spherical conducting shell of radius R at a point (i) outside (ii) inside the

shell

3 Derive the expression for electric field due to an infinite non conducting plane sheet

of charge having surface charge density σ.

4 Derive the expression for electric field due to an infinitely long linear charge

distribution having linear charge density λ.

5 Find the work done to rotate a dipole in a uniform electric field E from orientation ϴ0

to ϴ1.

6(VBQ) Mr. Bose was driving on a highway along fields. When a drizzle starts with lightning

and thunderstorm. He spots a few farmers walking with iron spoke top umbrellas to

avoid getting wet. He stops his car and instructs his co passengers to keep sitting

inside the car. He advises the farmers not to use the umbrella till the lightning

subsides.

a. What are the two human qualities which Mr. Bose exhibited?

b. Why did he advise the farmers not to use the type of umbrella they were using?

c. Why did he advise his co passengers to sit inside the car and not to venture out?



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NAME OF THE CHAPTER: : ElectrostaticsName of the Topic:

ANSWERS

1. It is the product of either charge and distance between the charges. It is a vector quantity.

2. By Gauss’ law

E = 0 inside the spherical shell as total charge enclosed is zero.3. By using Gauss’ law, we have

4.



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5.

6. a. caring attitude, scientific temper, presence of mind.

b. As the handle of the umbrella is conducting and also having pointed end so there is a

chance of passing charge through it.

c. As it is safe inside the car because inside the car E = 0 (electrostatic shielding)

Name of The Chapters- Current electricity 1 mark1 The resistivity of the alloy manganin is nearly independent of temperature. Explain why?

2. How does the internal resistance of a cell affect, when the distance between electrodes

decreases?

3. State the conditions of balanced wheatstone bridge.

4. State the Kirchhoff’s current law.

5. Write the Principle of potentiometer.

Worksheet 1 Name of The Chapters- Current electricity ANSWER(1 mark)

1. Due to less temperature coefficient.

2. Decreases

3. P/Q= R/S

4. Σi = 0

5. Voltage drop across a wire is directly proportional to its length.

Name of The Chapters- Current electricity 2 marks

1 State Kirchoff’s Rules related to current electricity.2. A battery of emf 10V and internal resistance 3Ω is connected to a resistor. If the current in the

circuit is 0.5A, what is the resistance of the resistor? What is the terminal voltage of the battery



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when the circuit is closed?3. Two wires of equal length one of copper and other of manganin have same resistance. Which of

the two wires will be thicker? Justify your answer with the help of suitable formula.

4. How does resistivity of (i) metallic conductor and (ii) semiconductor varies on increasing

temperature.

5. A carbon resistor has the sequence of colour as Blue, Yellow, Brown and Gold. Write is

resistance and tolerance.

Worksheet 1 Name of The Chapters- Current electricity ANSWER(2 mark)

1. (i)Σi = 0, (ii)Σ iR= 0

2. 17 Ω And 8.5 V.

3. MANGANIN, Reason- less temperature coefficient.

4. (i) increases (ii) decreases

5. R=64 x 10 Ω ± 5%.

Name of The Chapters- Current electricity 3 marks

1 State the principle of a potentiometer. Write the formula used to find internal resistance. How canthe sensitivity of a potentiometer be increased?

2. State the principle of meter bridge. Write the mathematical relation used to determine thevalue of an unknown resistance by meter bridge. Why cannot such an arrangement be used formeasuring very low resistance?

3. Establish the relation between Electric current and drift velocity of electrons in a conductor.4. Define resistivity. Write its SI unit. Write a mathematical formula to find resistivity.5. Define conductivity of a conductor and state its SI unit and explain the variation of conductivity

of ionic conductor with temperature.

Worksheet 1Prepared by : GROUP-I

Name of The Chapters- Current electricity ANSWER(3 mark)

1. Voltage drop is directly proportional ti it length of wire.r = (l1/l2-1) RBy using long potentiometer wire.

2. Balanced Wheatstone bridge.R/S = l/100-lThe resistance of metallic strip is not considered.



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3. Volume of conductor= Al ( A= area of cross section, l= length of wire)No. of charge carrier= nAl (n= no. density of charge carrier)Total charge, q= nAlei= q/t= neAvd

4. The resistance offered by a cube of a said material of unit dimension across its two opposite faces.SI unit- ohm-mρ = m/ne2τ

5. Reciprocal of resistivity.

SI unit- mho/m

Increases.

Q.NO.NAME OF THE CHAPTER : MAGNETIC EFFECT OF CURRENT AND MAGNETISM

NAME OF THE TOPIC : MAGNETIC EFFECT OF CURRENT

1 Where on the surface of the earth is the angle of dip; i) 900 ii) 00

2 Under what condition is the force acting on a charge moving through a uniform magnetic field isi) minimum ii) maximum

3 In which orientation the bar magnet will be in stable equilibrium in the magnetic field?4 A long straight wire AB carries a current I a proton ‘p’ travels with a speed ‘v’ parallel to

the wire at a distance ‘d’ as shown in the figureWhat is the force experiencedby the proton I

----d------- proton

5 What do you mean current sensitivity of a MCG?

Q.NO. ANSWER1 1. At the poles

2. At the equator2 (i) When charge particle moves parallel or antiparallel to the magnetic field lines

(ii) When charge particle moves normal to the magnetic field lines

3 When the bar magnet is placed parallel to the magnetic field lines4 F = q vB5 It is defined as the deflection per unit current.



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QUESTIONS FOR 2- MARK



1 (a) Write two characteristic of a material used for making permanent magnets.(b) Why are core of an electromagnet made of ferromagnetic materials?

2 A magnetic needle free to rotate in a vertical plane parallel to the magnetic meridian has itsnorth tip down at 600 with the horizontal. The horizontal component of the earth’s magnetic fieldat the place is known to be 0.4 G. Determine the magnitude earth’s magnetic field at that place.

3 The susceptibility of a magnetic material is -2.6 x 10-5 . Identify the type of magnetic materialand state its two properties.

4 State the principle of working of moving coil galvanometer giving its labeled diagram.

5 A circular coil of ‘N’ turns and radius ‘R’ carries a current ‘I’. It is unwound to make anothercoil of radius ‘R/2’ and current remain the same. Calculate the ratio of magnetic moments of thenew coil and the original coil.

Q.NO. ANSWER1 (a) High coercivity, high retentivity and high permeability.

(b) High permeability and low retentivity2 = = 0.8 G

3 The material is diamagnetic material.Properties :- (i) they are feebly repel by a magnet, (ii) a net magnetic dipole moment isdeveloped in direction opposite to the magnetic field lines.

4 Diagram,Principle :- Whenever a current carrying coil is placed in a radial magnetic field, it alwaysexperienced maximum torque,

τ = NBIA5 M1 : M2 = 1 : 2

QUESTION FOR 3- MARKS



1 Derive the relation expression force between two long parallel current carrying conductors. Use

the expression to define the SI unit of current.

2 Derive the expression for the torque on a rectangular current carrying loop suspended in a



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uniform magnetic field.

3 Using BiotSavart law, derive the expression for the magnetic field due to a circular coil carrying

current at a point along its axis.

4 Using ampere circuital law obtain the expression for the magnetic field due to a long solenoid at

a point inside the solenoid on its axis.

How is the magnetic field inside a given solenoid made strong?

5 Draw a schematic sketch of a cyclotron. Explain giving the essential details of its cvonstruction,

how it is used to accelerate the charge particles?

Q.NO. ANSWER1 =

I1 I2

2 Diagram .Force on opposite side along its length is given by,

F1 = F2 = BIlThese two forces are equal and opposite but acting in two different lines which constitute atorque and the torque is given by,

τ = either force x perpendicular distance between the two forces= BIl x bSinϴ= BIA Sinϴ [ where A = l x b ]

For N number of loops, we can writeτ = NBIA Sinϴ

3Let, dB Cosϴ

R= radius of the circular loop dBr = distance of the point from the circumfe-

rence of the loop dB Sinϴx = distance of point from the centre P

now,

dB =

component dB Sinϴ cancel each other and the net magnetic field at P is given by

B = ∫ Cosϴ = Sinϴ



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= ( )3/2

4Diagram, NCERT text book , page no. 151, fig 4.18.. = ∮ . = ∫ . + ∫ . + ∫ . + ∫ .

= μoI (nl) where n is the number of turns per unit length∫ . + 0 + 0 + 0 = μoI (nl)

Bl = μoI (nl)

B = μonI

ns 5.Fig. 4.8, NCERT text book , page 141

Principle: A positive ion can acquire sufficiently large energy with a small P.D. by making the ionto cross the same E.F. time and again by making use of a strong magnetic field

Construction: A cyclotron consist of pair of hollow metal cylindrical chamber shaped like D andcalled dees.

Both the dees are placed under the pole pieces of a very strong electromagnet.Both dees are connected to the terminals of voltage oscillator of high frequency of order of few million

hertz.Working : Charged ion ejected at the centre is allowed to pass through electric field again and again till itattains a required K.E. however, the strong magnetic field turns the particle on the semi circular path ineach dee, of increasing radii.Finally the charged ion of required K.E. on a semi circular path of certain radius is taken out from the

window with the help of a deflector.



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ONE MARK QUESTIONS

Ch. – EMI AND AC Prepared by - GR -2

1 Two loops of different shapes are moved in a region of uniform magnetic field in thedirections marked by arrows as shown in the figure. What is the direction of the inducedcurrent in each loop?

2 The motion of copper plate is damped when it is allowed to oscillate between the twopoles of a magnet. What is the cause of this damping?

3 A bar magnet is moved in the direction indicated by the arrow between two coils PQand CD. Predict the directions of induced current in each coil.

N S

4 How does the mutual inductance of a pair of coils change when



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(i) Distance between the coils is increased and(ii) Number of turns in the coils is increased?

5 What is the S.I unit of self inductance?

ONE MARK QUESTIONSAnswer Key

1 Abc,dgfe2 Eddy current3 Clockwise in both the coil4 Decreases,ii) increases5 Henry

TWO MARKS QUESTION

Ch. – EMI AND AC

1 A light bulb is rated 100 W for 220 V ac supply of 50 Hz.Calculate

(i)the resistance of the bulb.

(ii)the rms current through the bulb.

2 An alternating voltage given by V=140sin314t is connected across a pure resistor of

50 Ω. Find

(i)the frequency of the source.

(ii)the rms current through the resistor.

3 For a given a.c., i=Imsinωt, show that the average power dissipated in a resistor R over a

complete cycle is ½ im2 R.

4 A series LCR circuit is connected to ac source. Plot a graph to show the variation of

current with frequency of source, explaining the nature of its variation.

5 A rectangle conductor LMNO is placed in a uniform magnetic field of 0.5 T. the field is

directed perpendicular to the plane of the conductor. When the arm MN of length of 20 cm



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is moved towards left with a velocity of 10 m/s, calculate the emf induced in the arm.

Given the resistance of the arm to be 5 (assuming that other arm of negligible resistance)

find the value of the current in the arm?

BL M

l

O N

TWO MARKS QUESTION

Answer Key

1 484ohm, 5/11 A

2 50Hz, 1.4√2 A3 <P> = <i2>

= < im2R sin2ωt>

= im2 R<sin2ωt>

= = im2 R/2

4

5 I= Blv/R =0.5x0.2x10/5 =0.2 A



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THREE MARKS QUESTION

Ch. – EMI AND AC Prepared by - GR -2

1 State the underlying principle of a transformer. How is the large scale transmission of

electric energy over long distances done with the use of transformers?

2 A series LCR circuit is connected to ac source. Using the phasor diagram, derive the

expression for the impedance of the circuit.

3 A wheel with 8 metallic spokes each 50 cm long is rotated with a speed of 120 rev/min in a

plane normal to the horizontal component of the earth’s magnetic field. The Earth’s

magnetic field at the plane is 0.4 G and the angle of dip is 60 .calculate the emf induced

between the axel and the rim of the wheel. How will the value of emf be effcted if the

number of spokes were increased?

4 A voltage V = V0 sin ωt is applied to a series LCR circuit. Under what condition is (i) no

power dissipated even though the current flows through the circuit, (ii) maximum power

dissipated in the circuit?

5 Define the term self-inductance of a solenoid. Obtain the expression for the magnetic

energy stored in an inductor of self-inductance L to build up a current I through it.



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THREE MARKS QUESTIONS

Answer Key

1 Mutual induction. With the help of step up transformer the AC voltage generated at the source isconverted into high voltage AC which can be transmitted to a long distance without much loss ofenergy.

2

3 e=Bl2ω/2= 3.14x10-5 T4 i) When R = 0 ii) When Lω = 1/Ωc5 It may be defined as the magnitude of emf induced in a coil when unit rate of change of current is

maintained in it.

E =∫ =



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One Mark Questions

Name of the Chapter:- ELECTROMAGNETIC WAVEPrepared by:- GROUP VI1 A plane electromagnetic wave travel in vacuum along z-direction. What can

You say about the direction of electric & magnetic field vectors?

2 Arrange the following in descending order of the wavelength.X-rays, radio wave, visible spectrum, infrared light.

3 Which part of electromagnetic spectrum has largest penetrating power?4 Which part of electromagnetic spectrum is used in satellite communication?5 Which part of electromagnetic spectrum is absorbed by ozone layer?

Name of the Chapter:- ELECTROMAGNETIC WAVE ANSWERSPrepared by:- GROUP VI1 Direction of electric and magnetic field vectors are along X and Y direction

respectively.2 Radio wave >Infrared > Visible light >Xray.

3 Gamma rays4 Microwave is used in satellite communication.5 UV rays

Two Mark Questions

Name of the Chapter:- ELECTROMAGNETIC WAVEPrepared by:- GROUP VI1 How are X-rays produced? Write their two important uses.

2 How are microwaves produced? Write their two important uses.

3 Name the electromagnetic radiations having the wavelength range from 1nmto 700nm.Give its two important properties?

4 Write any two characteristics of EM waves.5 1. Identify the following electromagnetic radiation as per the wavelength given

below. Write one application of each.a. 10-12mb. 10-4m



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Name of the Chapter:- ELECTROMAGNETIC WAVE ANSWERSPrepared by:- GROUP VI1 X-rays can be produced by colliding fast moving electron beam on metal

Target.USES: (i)medical diagnosis (ii) study of crystal structure.

2 Microwaves are produced by oscillating current in the vacuum tubes likeklystrons& magnetrons.

USES: (i) In radar communication.(ii) In analysis of molecular and atomic structure.

3 Infrared waves have the wavelength between 1nm to 700nm.APPLICATIONS: (i) In remote control of TV, VCR etc.

(ii) In photography in thin mist.

4 (i) E. M. waves are produced by accelerated charged particles.(ii) E.M. waves do not require any medium for their propagation. These waves can propagate invacuum as well as in a medium. Velocity of em waves in a free space is given by

= = = 3 X 108 m/s

5 Identification:- a. gamma rays use- radiotherapyb. Infrared rays use – haze photography

Three Mark Questions

Name of the Chapter:- ELECTROMAGNETIC WAVEPrepared by:- GROUP VI1 What is meant by transverse nature of electromagnetic wave? Draw a diagram showing the

propagation of an electromagnetic wave along the x direction, indicating clearly the directions of

the oscillating electric and magnetic fields associated with it.

2 Name the constituent radiation of electromagnetic spectrum which is

(a) is used in satellite communication.

(b) is used for studying crystal structure

(c) is similar to the radiations emitted during decay of radioactive nuclei

(d) has its wavelength range between 390 nm & 770 nm

(e) is absorbed by ozone layer.

(f) produces intense heating effect.

3 Identify and name the part of electromagnetic spectrum to which these radiations belong?

1. The treatment of muscular strength



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2. Used by FM radio station for broad casting

3. To detect fractures in bones.

4 Identify the part of the electromagnetic spectrum which is used in

1. Radar system

2. Adjacent to the law frequency end of EM Spectrum

3. Produced in nuclear reaction

5 Name the constituent radiation of em spectrum which is

1. Used for studying crystal structures

2. Similar to the radiations emitted during radioactive decay

3. Has a wavelength between 390 nm and 770 nm

Name of the Chapter:- ELECTROMAGNETIC WAVE ANSWERS

1 E.M. waves are transverse in nature i,e, E & B are perpendicular to each other as well asperpendicular to the direction of propagation of the wave. E & B are related as follows -= or =

2 a) Micro wavesb) X raysc) γ raysd) Visible lighte) Ultraviolet raysf) Ans= Infra red rays

3 1. Infra red radiation2. Radio waves3. X-rays

4 1. Micro waves2. Radio waves3. Gamma rays

5 1. X-rays2. Gamma rays3. Visible light



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CHAPTER: OPTICS

1 Mark question

1 A bi concave lens of µ=1.5 is placed inside a liquid of µ=1.8, Will the lens behave as aconverging or diverging lens.

2 State Brewster’s law?3 In Young’s double slit experiment, write the expression for fringe width.

4 In single slit diffraction pattern write the expression for width of the central bright fringe5 How will the position of the image be affected if the lower half of the mirror’s reflecting

surface is painted black?Answer Key

ANSWERS1. Converging lens

2. µ=tanip

3. Fringe width=D/d

4. Width=2 D/d

5. No effect

TWO MARKS QUESTION

1 Two polaroid are kept in crossed position. When a third polaroid is placed between the two

making 300with the first. Find the intensity of light after third polaroid if intensity of light

falling on 1st is I0.

2 A convex lens of focal length 25cm is kept in contact with a concave lens of focal length

20cm. Find the power of the combination. Will the system be converging or diverging in

nature?

3 Find the expression for magnification of compound microscope when the image is formed

at infinity.

4 When light travels from rarer to denser medium, what happens to the frequency of light.

Does decrease in speed imply a reduction in energy?

5 The far point of a hypermetropic person is 80cm.What is the power of the lens to be used

to enable him to see infinite distance?



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ANSWERS1. I=I0

2. 20D, conversing lens

3. M P = LD/f0fe

4. Remains sameNo, because frequency remains constant

5. P=100/f, 1.25D


1 State Huygens’sprinciple. Verify Snell’s law from it.

2 In Young’s double slit experiment, find the expression for fringe width.

3 Find the expression for width of central maxima in single slit diffraction pattern.

4 Draw a ray diagram of a reflecting telescope. Mention its two advantages over therefracting telescope.

5 Write lens maker formula. A convex lens of focal length 20cm in air is placed in water ofrefractive index 1.3. What will be the focal length of the lens?

Answer Key

ANSWERS1. Principle

Verification2. Path difference=yd/D, Bright fringe=n D/d

Fringe width= D/d3. Path difference= D/d, Central maxime=2 D/d

4. DiagramMechanical support is less, Chromatic aberration is minimized.

5. 1/f=(ng/nm-1)(1/R1-1/R2)65cm



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TOPIC: ATOMS AND NUCLEI

1 MARK QUESTIONS:

QUESTION TOPIC: ATOMS AND NUCLEI1. State the law of radioactive decay.

2. What is the source of energy in nuclear reaction ?

3. The electron in hydrogen atom is initially in the third excited state. What is the maximum

number of spectral lines which can be emitted when it finally moves to the ground state?

4. Define ionization energy.What is its value for hydrogen atom?

5. Write the relationship between half life and decay constant.

ANSWER TOPIC: ATOMS AND NUCLEI1. The number of nuclei undergoing decay per unit time, at any instant, is proportional to the

total number of nuclei in the sample at that instant.

OR,

-dN/dt α N

2. Mass of parent nuclei is greater than mass of daughter nuclei. This difference of mass

appears as the energy.(E=mc2)

3. Six(for 3rd excited state n=4)

4. The energy required to remove the electron from its ground state and send it to ∞ is called

ionization energy.

Its value for hydrogen atom is 13.6 eV.

5. T1/2 =0.693/λ

2 MARKS QUESTIONS:

QUESTION TOPIC: ATOMS AND NUCLEI1. Using Rutherford model of the atom ,derive the expression for the total energy of the

electron in the hydrogen atom. What is the significance of negative energy possessed by

the electron?

2. Using Bohrs postulates of the atomic model , derive the expression for the radius of nth

electron orbit.

3. State Bohrs postulate for Hydrogen atom.



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4. Write any four properties of nuclear force.

5. Briefly explain β decay process.

ANSWER TOPIC: ATOMS AND NUCLEI1. Electron revolves around nucleus and required centripetal force is provided by the

attractive force between electrons and nucleus.

Kze2/r2 =mv2/r

So, mv2 = Kze2/r

And K.E. =1/2 mv2 == 1/2Kze2/r

And U= - Kze2/r

Total energy =KE+U = - Kze2/2r

Negative sign shows that electron is bound to revolve around nucleus.

2. Electron revolves around nucleus and required centripetal force is provided by the

attractive force between electrons and nucleus.

Kze2/r2 =mv2/r

So, mv2 = Kze2/r

By Bohrs postulate

Mvr = nh/2π

V=nh/2 πmr

So r= n2(h2/4πkme2)/z

3. 1. The electron revolve around the nucleus in the circular orbits.

2. Electron can revolve only in those orbits in which the angular momentum of electron is

the integral multiple of h/2π.

3.When electron jumps from one energy level (Ei) to another energy level (Ef) then it

absorbs or releases energy which is equal to (Ef) -(Ei)

4. Strongest ,non central , charge independent ,short range

5. In beta decay a nucleus spontaneously emits an electron (β- decay) or a positron(β+decay)

n→ p+e-1 + ((β- decay)

p→ n+ e+1 + ((β+ decay)



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3 MARKS QUESTIONS:

QUESTION TOPIC: ATOMS AND NUCLEI1. 1)What characteristic property of nuclear force explains the constancy of binding energy

per nucleon in the range of mass number A lying 30<A<170?

2)Show that the density of nucleus over a wide range of nuclei is constant independent of

mass number A.

2. Using Bohr’s postulates of the atomic model , derive the expression for the total energy

of the electro in the stationary state of the H atom.Hence draw the energy level diagram

showing how the line spectrum corresponding to Balmer series occur due to transition

between energy levels.

3. Draw Binding energy per nucleon vs mass number curve .Explain nuclear fission and

fusion on the basis of this curve.

4. For the past some time Arti had been observing some erratic body movement,

unsteadiness and lack of coordination in the activities of her sister Radha ,who also used

to complain of severe headache occasionally. Arti suggested to her parents to get a

medical checkup of Radha. The doctor thoroughly examined Radha and diagnosed that

she has a brain tumour.

a)What ,according to you, are the values displayed by Arti?

b) How can radioisotopes help a doctor to diagnose brain tumour?

5. Define half life of radioactive substance.Explain the law of radioactive decay. Derive a

relation between half-life and decay constant of the radioactive substance.

ANSWER TOPIC: ATOMS AND NUCLEI1. 1)Saturation/short range nature of nuclear forces.

2) R=R0A1/3

So, ρ = mA/4/3π ( R0A1/3) 3

=m/4/3π ( R03) Here ρ is independent of A.

2. Kze2/r2 =mv2/r

So, mv2 = Kze2/r

And K.E. =1/2 mv2 == 1/2Kze2/r

And U= - Kze2/r



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Total energy (E) =KE+U = - Kze2/2r

Here r = n2(h2/4πkme2)/z

Here E = -13.6eV /n2

Draw the energy level diagram for the electron jump from n1 =3,4,5,------- to n2 =2

3. Draw the related graph.

The lighter nuclei combine together (fusion) to form bigger nucleus having higher binding

energy per nucleon and are more stable.Heavier nuclei also gain stability by splitting into

lighter nuclei(fission) .The binding energy per nucleon of the lighter nuclei is larger than

the heavier parent nucleus and hence more stable.

4. a)Arti shows good awareness towards helth and care for her sister.

b)Certain radio isotopes are injected to body and they are absorbed by brain tumour and by

detecting intensity of radiations we can measure location and severity of tumour.

5. Half life is the time in which number of undecayed atoms of radioactive substance in a

sample is reduced to one half of its initial value.

According to this law the number of nuclei undergoing the decay per unit time (dN / dt)is

proportional to the total number of nuclei in the sample(N).

Derivation of N=N0

ONE MARK QUESTIONS

Ch. – Dual Nature of matter and radiation Prepared by - GR -2

1 Define the following termi) Work function ii) Threshold frequency.

2 How does the stopping potential vary if the intensity of incident radiation is increased?

3 Two metals A and B have work functions 2 eV and 4eV respectively. Which of them has smallerthreshold wave length?

4 The stopping potential in an experiment on photoelectric effect is 1.5 V. what is the maximumK.E. of the photoelectrons emitted?

5 Define stopping potential.



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ONE MARK QUESTIONSAnswer Key

1 The minimum energy required to eject an electron from a metal surfaceThe minimum frequency of the incident photon below which emission of photoelectron is notpossible.

2 No change3 B4 1.5eV5 The minimum reverse potential required to make photoelectric current Zero.

TWO MARKS QUESTION

Ch. – Dual Nature of matter and radiation Prepared by - GR -21 The graph shows the variation of stopping potential with frequency of incident radiation for

two photosensitive metals A and B. Which one of the two has Higher value of work-function?Justify your answer.

StoppingPotential A B

(Vo)

O νoνo’

Frequency of incident radiation(ν)

-Wo/e

-Wo’/e

2 U.V. radiations of frequencies υ1 and υ2 are incident on two photosensitive materials havingwork function W1 and W2 (W1 > W2) respectively. The KE of emitted electrons is same in boththe cases. Which one of the two radiations will be of higher frequency?



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3 The following graph shows the variation of stopping potential Vo with the frequency νof the incident radiation for two photosensitive metals X and Y.

Vo X Y

O 0.4 1.0ν (x 1014 s-1)

Which of the metals has larger threshold wavelength? Give reason.

4 Work function for a photosensitive material is 3.2 eV find the threshold frequency.

5 A Photosensitive material is exposed to radiations of two different frequencies (greater than

threshold frequency) but constant intensities. Draw the graph showing the variation of

photoelectric current and applied collector potential.

TWO MARKS QUESTION

Answer Key

1 B, ν0’ > ν0

2 υ1> υ2 , h υ1-w1 = h υ2 –w2

3 X, since it has smaller threshold frequency.4 v = 7.8 X 1014 Hz5

I

v1 v2

V



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Ch. – Dual Nature of matter and radiation Prepared by - GR -2

1 Draw the graph showing the variation of stopping potential with the frequency of incident

radiation in relation to photoelectric effect. i)Deduce the expression for the slope of this

graph using Einstein’s photoelectric equation. ii) what does this slope of this graph represent?

iii) What information can be obtained from the value of the intercept on the potential axis?

2 The frequency of incident radiation on a photocell is doubled for the same intensity what

changes will you observe in i) the K.E. of the electron emitted ii) photoelectric current and iii)

stopping potential? Justify your answer in each case.

3 a) Why is the wave nature of matter not more apparent to our daily observation?

b) Draw a graph to show the variation of K.E of photo electron with the increase of frequency?

(ν >ν0 ) .

c) Green light ejects photo electrons from a given photosensitive surface where as yellow light

does not. What will happen in case of violet and red light? Give reason.

4 Write Einstein’s photoelectric equation and point out any two characteristics properties of

photons on which this equations based.

Briefly explain the three observed features which can be explained by this equation.

5 An electromagnetic wave of wave length λ is incident on a photosensitive surface of negligible

work function. If the d-Broglie wave length is λ1 then prove that =



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Answer Key

1

Vo

v

eVo = hv -hvo

ΔVo/Δv = h/e = slope

Work function

2 According to Einstein`s relation KE will be more than double, no change in photoelectriccurrent, as kinetic energy.

3 Due to large mass of the object and hence very small wavelength in comparison to its

dimension.



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From violet light photoelectrons will be emitted but from red light photoelectrons will not be

emitted. As frequency of violet light is more than green light and frequency of red light is less

than yellow light.

4 hv = hvo+1/2mv2

i) Particle nature of photon.

ii) Energy and momentum of all photons are equal irrespective of intensity of incident

light.

Observed features

I) K.E of photoelectrons is not dependent on intensity.

II) There exist a minimum frequency below which photoelectric effect is not possible.

III) Stopping potential is directly proportional to the frequency of incident radiation.

5



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One Mark Questions

Name of the Chapter:- SEMICONDUCTOR DEVICES

1 How many diodes are used in full wave rectifier?

2 What is the limit in the use of intrinsic semiconductors?

3 Name one dopant used as acceptor in semiconductors?4 Write one advantage of LED over incandescent lamps?5 What is a solar cell?


1 Two

2 In intrinsic semiconductors ne=nh, thus charge carriers are equal.

3 Indium4 Low operational voltage and less power consumptions5 Solar cell is a junction diode which converts solar energy into electricity.

Two Mark Questions

Name of the Chapter:- SEMICONDUCTOR DEVICESPrepared by:- GROUP VI1 Write two important criteria for selection of material for solar cell fabrication?

2 How does the conductivity of a semiconductor changes with rise in temperature?

3 Name one impurity each when added to pure silicon produces (A) n type and (b) p typesemiconductor?

4 For a transistor the value of β =`100, what is the value of α?5 In transistor why the forward bias is smaller than reverse bias?


1 Band gap (from 1.0eV TO 1.8 eV) and High optical absorption.

2 Increases exponentially with temperature.

3 (a)Arsenic and (b) Indium4 α = β/(1+ β),100/101=0.99



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5 To reduce the heating effect in the base region

Three Mark Questions


1 In what way LED and Photo diode must be biased? Name the factors which determines (a)

frequency and (b) Intensity of light emitted by LED?

2 What is the difference between n-type and p-type semiconductors on the basis of energy band?

3 A Photo diode is fabricated from a semiconductor with a band gab of 2.8Ev, can it detect a wave

length 600 nm?

4 Why do we prefer NPN transistor to PNP for faster action?

5 In NPN transistor circuit the collector current is 5Ma, if 95% of electrons emitted reached the

collector, what is the base current?


1 LED is forward biased and Photo diode is reverse biased.(a)band gap(b) doping level

2 In n-type semiconductor the no of free electrons in the conduction band is much more than no of

holes in the valance band, In p-type semiconductor the no of holes in valance band is much more

than no of free electrons in the conduction band

3 E= hC/λ = 0.2Ev, it cannot detect because E is less than Eg.

4 In NPN transistor current conduction is mainly by free electrons whereas in PNP transistor it is

mainly holes, Mobility of electrons is greater than that of holes.

5 IC=95/100 of IE= (100/95) X5Ma =5.26 Ma. Ib= IE- IC =0.25 mA

IV. 1mark questions

Worksheet 1 Name of the chapter: COMMUNICATION SYSTEM

1 What is the limitation of ground wave propagation?

2 Why sky waves propagation is limited to frequencies below 30MHz?3 State one advantage of FM over AM.

4 What is meant by LOS mode of communication?



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5 Define the term ‘attenuation’Name Of The Chapter COMMUNICATION SYSTEM: ANSWERS

1. Not suitable for long distance transmission as energy is absorbed by the ground

2. Frequencies above 30MHz are not reflected by the ionosphere

3. Less noisy

4. Line of sight communication.5. Loss of strength of a signal as it propagates through a medium

2mark questions

Name of the chapter: COMMUNICATION SYSTEM

1 Define modulation. State any two types of modulation.

2 Write the function of i) Transducer ii) Repeater with reference to communication system.

3 Define modulation index. Why is its value kept less than 1?

4 A message signal of frequency 10kHz and peak voltage of 10 volts is used to modulate a

carrier of frequency 1MHz and peak voltage of 20 volts. Determine i)modulation index

ii)the side bands

5 How is AM Wave produced?

Name Of The Chapter COMMUNICATION SYSTEM: ANSWERS

1. The low frequency message is loaded on a high frequency carrier.

Amplitude modulation, Frequency modulation.

2. i) It changes one form of energy into another form ii) it is a combination of repeater and

transmitter.

3. It is the ratio of amplitude of message signal to that of carrier.

It is done so to avoid distortion

4. i) Mod. Index=10/20 = 0.5 ii) 1000+10=1010kHz to1000-10=990kHz.

5. Message signal and carrier wave are applied to a non-linear device, followed by a band

pass filter.



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3mark questions

Name of the chapter: COMMUNICATION SYSTEM

1 What is the need for modulation?

2 What are the different types electromagnetic wave propagation? Give their frequencyrange.

3 Give block diagram of a simple Modulator for obtaining AM signal.

4 Give block diagram of Detector for AM signal.

5 What is demodulation in a communication system? Explain how it is achieved.

Name Of The Chapter COMMUNICATION SYSTEM: ANSWERS

1. i) to avoid intermixing of signals transmitted from different stations

ii)to reduce the size of antenna iii)to increase the power radiated by antennas.

2. i)Ground wave propagation (less than a few MHz)

ii)Sky wave propagation (upto 30 MHz)

iii)Space wave propagation (above 30MHz)

3. See text book page no 525 for block diagram.

4. See text book page no 527 for block diagram.

5. It is the process of recovering the original message signal from the received signal.

The modulated signal is passed through a rectifier. The envelope of this signal is the

message signal. This signal is passed through an envelope detector to get the original

message.



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cbse class 12 physics important questions from past exams

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