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SAMPLE PAPER – 2016-17

XII- PHYSICS

UNIT NAME 1 MARK

2 MARKS

3 MARKS

4 MARKS ( VBQ)

5 MARKS TOTAL

ELECTROSTATICS CURRENT ELECTRICITY 6(2) 4(1) 5(1) 15(4)

MAGNETISM & MAGNETIC EFFECTS OF CURRENTAC & EMI

1(1) 4(2) 6(2) 5(1) 16(6)

EMW &OPTICS 3(3) 6(3) 3(1) 5(1) 17(8)

DUAL NATURE OF MATTER & ATOMS &NUCLEI

1(1) 9(3) 10(4)

SEMICONDUCTOR DEVICESCOMMUNICATION

12(4) 12(4)

TOTAL 5(5) 10(5) 36(12) 4(1) 15(3) 70(26)MAX. MARKS 70 TIME : 3 Hrs

B SESHA SAI/KV, EOI/KATHMANDU/XI/PHY /2017-18

S

XII- PHYSICS Time : 3 Hrs. Max. Marks: 70General Instruction :

(a) All questions are compulsory.

(b) There are 26 questions in total. Questions 1 to 5are very short answer type questions and

carry one mark each.

(c) Questions 6 to 10 carry two marks each, questions 11 to 22 carry three marks each,

question 23 is value based question carry 4 marks and questions 24 to 26 carry five marks

each.

(d) There is not overall choice. However, an internal choice has been provided in one

question of two marks, one question of three marks and all three question of five marks

each weightage. You have to attempt only one of the choices in such questions.

(e) Use of calculators is not permitted. However, you may use log tables if necessary.

(f) You may use the following values of physical constants wherever necessary.

c = 3 × 108 m/s

h = 6.63 × 10–34Js

e = 1.6 × 10–19 C

μ0 = 4π × 10–7 T mA–1

14 π∈= 9 × 109 N m2 C–1

me = 9.1 × 10–31 kg

Mass of Neutrons = 1.675 × 10–27 kg

Mass of proton = 1.673 × 10–27 kg

1.A conducting wire is kept along the E→W direction and is allowed to fall freely. Will an e.m.f be induced in the wire? 2.What physical quantity is same for X-rays of wavelength 10-10 m, red light of wavelength 6800 Å and radio wave of wavelength 500 m? 3.How does focal length of a lens change when red light incident on it is replaced by violet light? Give reason for your answer.4.Which of the following waves can be polarized (i) Heat waves (ii) Sound waves? Give reason to support your answer.


5.Write the relationship between the size of a nucleus and its mass number (A).

6A parallel beam of light of wavelength 600 nm is incident normally on a slit of width ‘a’. If the distance between the slits and the screen is 0.8 m and the distance of 2nd order maximum from the centre of the screen is 15 mm, calculate the width of the slit. OrTwo polaroids ‘A’ and ‘B’ are kept in crossed position. How should a third polaroid ‘C’ be placed between them so that the intensity of polarised light transmitted by polaroid B reduces to 1/8th of the intensity of unpolarised light incident on A?

7. A magnet is quickly moved in the direction indicated by an arrow between two coils C1 and C2 asshown in the figure. What will be the direction of induced current in each coil as seen from themagnet? Justify your answer..

8.A ray of monochromatic light passes from medium 1 to medium 2 . if the angle of incidence in medium is ϴ and corresponding angle of refraction in medium 2 is ϴ/2, which of the two media is optically denser? Give reason.

9. Define the current sensitivity of a galvanometer. Write its S.I. unit. On which factor current sensitivity of galvanometer depends.

10.(a) An em wave is travelling in a medium with a velocity. Draw a sketch showing the propagation of the em wave, indicating the direction of the oscillating electric and magnetic fields.

(b) How are the electric and magnetic fields related to velocity of the em wave?

11.Two concentric metallic spherical shells of radii R and 2R are given charges Q 1 and Q2

respectively. The surface charge densities on the outer surfaces of the shells are equal. Determine the ratio Q1 : Q2.

12. What is meant by term ‘modulation’? Draw a block diagram of a simple modulator forobtaining an AM signal.


13.Derive the expression for the electric field intensity at any point on the equatorial line of an electric dipole ?

14. Calculate the quality factor of a series LCR circuit with L = 2 H, C= 2 F and R = 10 . Mentionthe significance of quality factor in LCR circuit15. Define self-inductance of a coil. Show that energy required to build up the current I in a coil ofself inductance L is given by(½) LI2.

16.Define power of a lens. Write its units. Deduce the relation 1/f = 1/f1 +1/f 2 for two thin lenses kept in contact coaxially.

17.Using Bohr’s postulates, obtain the expression for the total energy of the electron in the stationary states of the hydrogen atom. Hence draw the energy level diagram showing how the line spectra corresponding to Balmer series occur due to transition between energy levels.

18. Mention the important considerations required while fabricating a p-n junction diode to be used as a Light Emitting Diode (LED). What should be the order of band gap of an LED if it is required to emitlight in the visible range?

19.Write two characteristic features observed in photoelectric effect which supports the photon pictures of electromagnetic radiation.

Draw a graph between the frequency of incident radiation (υ) and the maximum kinetic energy of the electrons emitted from the surface of a photosensitive material .

20.The energy of an electron in orbit is given by En=−13 .6

n2eV

. Calculate the energy required to take an electron from ground state to the second excited state.

21.Draw a labelled circuit diagram to show the use of Zener diode as a voltage regulator. Explain how it is used as a voltage regulator.

OR

Draw the circuit diagram of a full wave rectifier using p-n junction diode.Explain its working and show the output, input waveforms.

22.Sketch a graph b/w angle of incident radiation and stopping potential for a given photosensitive material. What information can be drawn from the value of intercept on the potential axis.

23. During a thunderstorm Rohan and his friends were in the middle of a jungle. His friends were standing under a tree. Rohan advised them not to stand there. Rather he suggested they


should move back to the bus. He explained them that they are safe inside the bus due to electrostatic shielding.

(a) What are the values exhibited by Rohan ?

(b) What is electrostatic shielding?

24.(a) Using Gauss law, derive an expression for the electric field intensity at any point outside a uniformly charged thin spherical shell of radius R and charge density C/m2 . Draw the field lines when the charge density of the sphere is (i) positive, (ii) negative.

(b) A uniformly charged conducting sphere of 2.5 m in diameter has a surface charge density of 100 mC/m 2 . Calculate the (i) charge on the sphere (ii) total electric flux passing through the sphere.

OR

(a) Derive an expression for the torque experienced by an electric dipole kept in a uniform electric field.

(b)Calculate the amount of work done in rotating a dipole, of dipole moment 3 108C-m, from its position of stable equilibrium to the position of unstable equilibrium, in a uniform electric field of intensity 104 N/C.

25.(a) Using Biot-Savart’s law, derive an expression for the magnetic field at the centre of a circular coil of radius R, number of turns N, carrying current I.

Derive the expression for force per unit length between two long straight parallel current

carrying conductors. Hence, define one ampere.

ORDraw a schematic diagram of a cyclotron. Explain its underlying principle and working, starting clearly the function of the electric and magnetic fields applied on a charged particle. Deduce an expression for the period of revolution and show that it does not depend on the speed of the charged particle.26.(a) .(a) Draw a labelled ray diagram of a compound microscope.(b) Derive an expression for its magnifying power.(c) Why is objective of a microscope of short aperture and short focal length? Give reason.

OR(a) Describe briefly how a diffraction pattern is obtained on a screen due to a single narrow slit illuminated by a monochromatic source of light. Hence obtain the conditions for the angular width of secondary maxima and secondary minima. (b) Two wavelengths of sodium light of 590 nm and 596 nm are used in turn to study thediffraction taking place at a single slit of aperture 2 ×


10–6m. The distance between the slit and the screen is 1.5 m. Calculate the separation between the positions of first maxima of the diffraction pattern obtained in the two cases.


MARKING SCHEME

1 Yes. 1

2 Velocity 13

4 1

5 i) Heat waves: As heat waves are transverse or electromagnetic in nature.

1

6 Formula ½Calculations Correct ans.

½1½

7 Direction Justification

11

8 Correct medium Reason

11

9 DefinitionUnitFactors

½½1

10 SketchMagnitude of electric and magnetic field

11

11 Correct formulaCorrect answer

11

12 DefDiagram

12

13 Derivation 314 Quality factor formula

CalculationSignificance

111

15 DefinitionCorrect derivation

12

16 DefinitionDerivation

12

17 ExpressionDiagram

21

18 Important considerationBand gap

21


19 CharacteristicsGraph

12

20 FormulaCalculationCorrect answer

½2½

21 DiagramExplanationOrDiagram Working

12

12

22 Graphexplanation

12

23 Valuesexplanation

22

24 ExpressionElectric fieldCharge on sphereTotal fluxOrExpressionnumerical

2111

32

25 Expression(biot savart law)Expression (force/length)DefinitionOrDiagramPrincipleWorkingDerivation for time period

221

1112

26 DiagramExpression(c)reasonOrAngular width of maxima and minimanumerical

221

32


MODEL ANSWERS1.Yes.2.Velocity3.

The increase in refractive index would result in decrease of focal length of lens. Hence, we can say by replacing red light with violet light, decreases the focal length of the lens used.

4. Heat waves: As heat waves are transverse or electromagnetic in nature.

5

6Here, λ = 600 nm = 600 × 10−19 = 6 × 10−7 m D = 0.8 m, x = 15 mm = 1.5 × 10−3 m,n = 2, a = ?

Or


8.Medium (2) is optically denser.Reason - angle of refraction is less than the angle of incidence.

9.Ratio of deflection produced in the galvanometer to the current flowing through it.Current sensitivity Si = ϴ/I =k/nBAS.I. unit of current sensitivity Si is division/ampere or radian/ampere.It depends upon k(restoring torque) , no. of turns, area of a coil , magnetic field.

10. (a) Given that velocity, V = v î and electric field, E along X-axis and magnetic field, B along Z-axis.

The propagation of EM wave is following:

(b) Speed of EM wave can be given as the ratio

11.


12Meaning of Modulation: The original low frequency message/information signal cannot betransmitted over long distances. Therefore, at the transmitter end, information contained inthe low frequency message signal, is susperimposed on a high frequency carrier signal by aprocess known as modulation.

13


14.


15

16.


17.According to Bohr’s postulates, in a hydrogen atom, a single alectron revolves around a nucleus of charge +e. For an electron moving with a uniform speed in a circular orbit os a given radius, the centripetal force is provided by Columb force of attraction between the electron and the nucleus. The gravitational attraction may be neglected as the mass of electron and proton is very small.

So,


where m = mass of electron r = radius of electronic orbit v = velocity of electron.

Again,

From eq.(1), we get,

(i) Kinetic energy of electron,

(ii) Potential energy

Hence, total energy of the electron in the nth orbit


When the electron in a hydrogen atom jumps from higher energy level to the lower energy level, the difference of energies of the two energy levels is emitted as a radiation of particular wavelength. It is called a spectral line.

In H-atom, when an electron jumps from the orbit ni to orbit nf, the wavelength of the emitted radiation is given by,

Where,

R → Rydberg’s constant = 1.09678 ×107 m−1 ForBalmer series, nf = 2 and ni = 3, 4, 5, …

Where, ni = 3, 4, 5, … These spectral lines lie in the visible region.

18. Important consideration in the fabrication of LED:(a) (i) light emitting diode is a heavily doped p-n junction.(ii) The reverse breakdown voltages of LEDs are very low, typically around 5V.(b) The order of band gap of an LED to emit light in the visible range is about 3 eV to 1.8 eV.

19.The two characteristics features observed in photoelectric effect which support the photon pictures of electromagnetic radiation one: a) All photons of light of a particular frequency , or

wavelength , have the same energy and momentum whatever the intensity of radiation may be. The increase in intensity of the radiation implies an increase in the number of photons crossing a given area per second.

(b) Photons are electrically neutral and are not deflected by electric and magnetic fields.


(i) Plank constant is given by slope of the curve i.e. (ii) Work function is the minimum energy required by the electron to escape out of the metal surface thus,Φ = hνHere, ν is the threshold frequency.

20. The energy of nth orbit of the H-atom is given by

=En=−13 .6n2

eV

¿ E2−E1=−13 . 6(2 )2

−(−13 .612 )

¿−3 . 4+13 .6=10 .2eV .

Now energy required to take an electron from ground state to second excited state.

21.


Here the zener diode is joined in reverse bias to functioning d.c. input voltage through a

resistance R of suitable value depending upon the zener voltage and power rating of

zenerdiode used. The constant output voltage is taken across a load resistance RL

connected in parallel with zener diode. When the input d.c. voltage increases beyond a

certain limit, the voltage across zener diode becomes constant equal to zener break down

voltage, but the current through the zener diode circuit rises sharply as the dynamic

resistance of zener diode becomes almost zero after zener break down voltage. Due to

which there is an increase in voltage drop across R since RL is connected in parallel so

the voltage across RL remains same as that of zener break down voltage.

OR

.(a) Circuit diagram

Working ,Output and input waveform (b) Output waveforms of OR and NAND gates

a)

b)


22. E = h = Φo + eVo

23.a. Scientific temper and care and concern b. explanation of electrostatic shielding.

24.


b)

25


26.


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