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MODEL QUESTION PAPER

SUBJECT: PHYSICS

CLASS: XII

TIME: 3HRS M.M.: 70

GENERAL INSTRUCTIONS:

(i) All questions are compulsory.

(ii) There are 26 questions in total. Questions 1 to 5 are very short answer type questions and carry one mark each.

(iii) Questions 6 to 10 carry two marks each, questions 11 to 22 carry three marks each, question 23 carries 4 marks and questions 24 to 26 carry five marks each.

(iv) There is no overall choice.

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

(vi) You may use the following values of physical constants wherever necessary:

c = 3 × m/s

h = 6.63 × Js

e = 1.6 × C

µ = 4π × T m

= 9 × N m²

= 9.1 × kg

SECTION A

1) A ray of light, incident on an equilateral glass prism (µ = √ ) moves parallel to the

base. Find the angle of incidence for this ray.

2) An α- particle and a proton are moving in a plane of a paper in a region where there is a uniform magnetic field vector B directed normal to the plane of the paper. If the

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two particles have equal linear momenta, What will be the ratio of radii of their trajectories in the field?

3) Manganin is used for making standard resistors. Why? 4) What will be the ratio of radii of two nuclei of mass numbers 125 and 27? 5) An electron and an alpha particle have same de Broglie wavelength associated with

them. Which will have greater Kinetic energy?

SECTION B

6) Write the function of each of the following used in communication system: 1. Transducer 2. Repeater

7) Calculate the quality factor of a series LCR circuit with L = 2.0 H, C = 2 µF and R= 10 Ω. Mention the significance of quality factor in LCR circuit.

8) The sequence of decay of radioactive nucleus is as given. α β α α D D1 D2D3 D4.If the nucleon number and atomic number of D2 are 176 and 71 respectively, what are their values of D and D4 .

9) A thin prism of refractive index 1.5 produces an angle of minimum deviation 5 degrees for an incident ray of light from air. Calculate the new angle of minimum deviation when the prism is immersed in a liquid of refractive index 1.25.

10) State briefly the processes involved in the formation of p-n junction explaining clearly how depletion region is formed.

SECTION C

11) Derive an expression for average power in an LCR series circuit connected to a.c. circuit. What is the power dissipated in an a.c. circuit in which voltage and current are given by V = 230 sin(ωt + π/2 ) and I = 10 sinωt ?

12) Write Einstein’s photoelectric equation. If the frequency of incident radiation on a photocell is doubled for the same intensity, what change will you observe in (i) photo electric current (ii) stopping potential.

OR Write two characteristic features observed in photoelectric effect which support the photon picture of electromagnetic radiation. Draw a graph between the frequency of incident radiation ʋ and the maximum kinetic energy of the electrons emitted from the surface of photosensitive material. State clearly how this graph can be used to determine Plank’s constant and work function of the material.



13) State Gauss theorem in electrostatics. Using this theorem, derive the expression for

the electric field intensity at any point outside a uniformly charged thin spherical shell.

14) The galvanometer in each of the two circuits does not show any deflection. Find the ratio (R1/R2) used in these two circuits.

15) State the principle of working of a meter bridge. In the meter bridge experiment, balance point was observed at J with AJ = l.

(i) The values of R and X were doubled and then interchanged. What would be

the new position of balance point? (ii) If the galvanometer and battery are interchanged at the balance position,

how will the balance point get affected?

16) A uniform magnetic field gets modified as shown below, when 2 specimens X&Y are



placed in it?

i) Identify the two specimens X&Y. ii) State the reason for the behavior of the field lines in both cases. iii) Write the sign of magnetic susceptibly for each specimen.

17) Name the parts of electromagnetic spectrum which is

(a) Suitable for radar system used in aircraft navigation (b) Used to treat muscular strain (c) Used as diagnostic tool in medicine.

Arrange their wavelengths in ascending order. 18) A convex lens made up of glass of refractive index 1.5 is dipped, in turn, into (i) a

medium of refractive index 1.65, (ii) a medium of refractive index 1.33. (a) Will it behave as a converging lens or a diverging lens in the two cases. (b) How will its focal length change in the two media.

19) State Huygen’s principle. Using Huygen’s principle draw a diagram showing how a

plane wave gets refracted when it is incident on the surface separating a rarer medium from a denser medium. Hence verify Snell’s law of refraction.

20) What is an AC generator? Give its principle and explain its working with the help of a suitable diagram.

21) Obtain the binding energy per nucleon of the nuclei 26 Fe 56 in units of MeV from the following data:- mH = 1.007825 U mN = 1.008665 Um( 26 Fe 56) = 55.934939 U

22) Name the two basic modes of transmission. Which of these modes is used for telephonic communication? Complete the following block diagram depicting the essential elements of a basic communication system.

SECTION D



23) Harsh was performing an experiment to carry out a project for that he required an ammeter of range 7.5 A. It was not available in the laboratory and in the market. So, he decided to convert a galvanometer into an ammeter of given range. (a) What values do you think are there in Harsh? (b) How could he do that? Explain by showing calculations. (c) How the same galvanometer is converted into voltmeter of given range.

SECTION E

24) Derive the expression for the energy stored in a parallel plate capacitor of capacitance C with air as medium between its plates having charges Q and –Q. Show that this energy can be expressed in terms of electric field as 1/2 ε0E2Ad, where A is the area of each plate and d is the separation between the plates. How will the energy stored in a fully charged capacitor change when the separation between the plates is doubled and the dielectric medium of dielectric constant 4 is introduced between the plates(when battery remains connected).

OR

What is an electric dipole? Deduce an expression for the torque acting on an electric dipole placed in a uniform electric field. Hence define dipole moment. An electric dipole of length 2 cm is placed with its axis making an angle of 60o to a uniform electric field of 105 NC-1. If it experiences a torque of 8√3 Nm, calculate the;

(i) Magnitude of charge on the dipole. (ii) Potential energy of the dipole.

25) (a) What is interference of light?

(b) Using Young’s Double slit experiment show that the dark and the bright fringes of an interference pattern are of same width. OR (a) What is diffraction of light? (b) Draw a graph showing the variation of intensity with angle in a single slit diffraction experiment. Write one feature which distinguishes the observed pattern from the double slit interference pattern. (c) How would the diffraction pattern of a single slit be affected when- (i) the width of the slit is decreased. (ii) monochromatic source of light is replaced by a source of white light.

26) (a) What is a rectifier? What is its principle? (b) With the help of a circuit diagram explain full wave rectification using junction diodes. Draw the input and output wave



forms. (c) The applied input frequency to a rectifier is 50Hz. What will be its output frequency if it is a i) half wave ii) full wave rectifier? OR Draw a circuit diagram of a common emitter transistor amplifier. Explain its

working. Show that the voltage gain, AV, of the amplifier is given by AV=ᵦ

,

where ᵦ is current gain, RLis the load resistance and riis the input resistance of the

transistor.

What is the significance of the negative sign in the expression for the voltage gain?



MODEL ANSWERS

Q1 Ans: = √

/

=√

i= 600

Q2 Ans: Using =qvB

r=p/qB

r α 1/q

rα/rp=1/2

Q.3 Ans: higher resistivity and low temperature coefficient of resistance.

Q.4 R= R0A1/3

1 = (125/27)1/3 = 5/3

Q.5 Ans λ=h/√ Ee/Eα =mα/me

Q.6 a)Tranducer-A device which convert one form of energy into another.

b) Repeater- It receive the signal, amplify it, and transmit the signal.

Q.7 Q = / = 1/10 = 100

It measures the sharpness of resonance

Q.8 180 α 176 β- 176 α 172 α 168 D-------------D1------------D2--------------D3----------D4 72 70 71 69 67 Q.9 δa = A(n-1) =A(1.5-1) = 0.5A 0.5A= 50 A= 100

δ (o)=A(n-1)= 10(.

.- 1) =20

Q.10 Diffusion-Holes diffuse from n side to p side and free electrons diffuse from p side to n side.



The moment charge carrier cross the junction leave behind ionized atoms ie the acceptor on p side and the donors on n side. The space charge region on either side of the junction where there are no free charges is known as depletion region Q.11Average Power Associated with LCR Circuit in Series In an electrical circuit, energy is supplied by the source of EMF, stored by the capacitive and inductive elements and dissipated in resistive elements. Conservation of energy requires that, at any particular time, the rate at which energy is supplied by the source of EMF must equal the rate at which it is stored in the capacitive and inductive elements; plus the rate at which it is dissipated in the resistive element (we assume ideal capacitive and inductive elements have no internal resistances). Let E = Eosin wt. Since current lags in inductive dominated circuit by an angle q. If dw be the work done by a source of EMF e on a charge dq, then

The power

Using equations (1) and (2),

I = Io sin (wt -q)

If this instantaneous power remains constant for small time dt.then

Total work done or energy spent in maintaining current over one full cycle.

The second term is zero over complete cycle i.e.,

Therefore,



Average power=0 Q.12 (a) E= h(ʋ-ʋ0) (b) No change (c) doubled

OR



Q.13

Q.14 4/R1 =6/9 R1=6 6/12= R2/18 R2=9 R1/R2=2/3 Q.15 (a) R/X= l/100-l 2R/2X= l/100-l X/R= 100-l/l



(b) No change Q.16. X = Diamagnetic material and Y = ferromagnetic material

Dipole are arranged in opposite direction of applied magnetic field

Dipole are arranged in the direction of applied magnetic field

Small negative and large positive Q.17 1.Micro wave 2. Infra-red 3. X ray X ray, Infra-red, Micro wave Q.18 diverging lens converging lens Focal length = negative, Focal length = positive



Q.19

Q.20 GENERATOR: Principle AC Dynamo is based on the phenomenon of electromagnetic induction. That is, when the relative orientation between the coil and the magnetic field changes, the flux linked with the coil changes and this induces a current in the coil.

As the armature coil rotates, the angle Q changes continuously. Therefore, the flux linked with the coil changes. Now,



= NBA cos q = NBA cos wt where q is the flux linked with the coil, N is the number of turns in the coil, A is the area enclosed by each three of the coil and B is the strength of the magnetic field.

= - NBA (-sin wt )w E = + NBA w sin wt e = eo sinwt. This is the EMF Supplied by the A.C. generator

Construction Armature -ABCD is the armature coil consisting of a large number of turns of the insulated copper wire wound over a laminated soft iron core I. The coil can be rotated about the central axis. Magnets N and S are the pole pieces of a strong electromagnet in which the armature coil is rotated. Slip rings-R1 and R2 are two hollow metallic rings to which both ends of the armature coil are connected. These rings rotate with the rotation of the coil. Brusheh Brushes B1 and B2 are two flexible metal plates or carbon rods. These brushes are used to pass current from the coil to the external load resistance. Working To start with, suppose the plane of the coil is perpendicular to the plane of the paper in which the magnetic field is applied, with AB at the front and CD at the back, the flux linked with the coil is maximum in this position. As the coil rotates clockwise, AB moves inwards and CD moves outwards. According to Fleming's right hand rule, the current induced in AB is from A to B, and in CD, from C to D. In the external circuit, current flows from B2 to B1. After half of the rotation of the coil, AB is at the back and CD is at the front. Therefore, AB starts moving outwards and CD inwards. The current induced in AB is from B to A, and in CD, from D to C. The current flows from B1 to B2 through the external circuit. We therefore see that the induced current in the external



circuit changes direction after every half rotation of the coil, and hence is alternating in nature

Q.21 DM = (26Mp+30Mn) - M

Mass defect ∆m = 0.528461

B.E = ∆m × 931.5 MeV

= 492.26 MeV

B.E/nucleon

= . /56=8.79 MeV

Q.22 i) Point to point mode

ii) Broadcast mode

Transmitter Medium Receiver

Q.23 (a) Scientific attitude and intelligent, curious

(b) Conversion of galvanometer into ammeter

(c) Conversion of galvanometer into voltmeter.



Q.24

OR



(i) torque = pEsinθ = q(2a) Esinθ

q = = 8 × 10-3 C

(ii) magnetic potential energy = pE cosθ = 8 J

25. (a) The phenomenon of redistribution of light energy in a medium on account of superposition of light waves from two coherent sources is called interference.



(b)



OR

Definition of diffraction: The bending of light from the corner of small obstacles or apertures is called diffraction of light.



26. Definition of rectifier: It is a device which is used to convert AC into DC.



In half wave rectifier the frequency is 50 Hz and in full wave rectifier it is 100 Hz.

OR



ANSWER KEY

SAMPLE PAPER

Q.NO VALUE POINTS MARKS 1. 60 degrees 1 2. r=mv/qb

r1/q r/rp=1/2

½ 1

½

3. Temperature coefficient of resistance is low and high value of resistivity

½ 1

½

4. 5/3 ½

½

5. electron 1 6. Correct definition each 1+1 7 Formula of quality factor

Correct answer=100 significance

1/2 1 1/2

8 D4 A=168, Z=67 D A=180, Z=72

½ ½ ½ ½

9 Correct formula Correct solution

½ 1+ ½

10 Diffusion drift

1 1

11 Correct derivation Power dissipation = 0

2 1

12 (i)Ip no change (ii)Vs doubles

1



OR Each characteristics Correct graph Plank' constant = slope, work function = y- intercept

1

,

1

,

13 1. Statement

2. Correct derivation 1 2

14 From figure (1), =

R1=6Ω

From fig(2) =

R2=4Ω

:. = =

1 1 1

15 Correct principle

New position 100-l

No change

1 1 1

16 X = Diamagnetic material and Y = ferromagnetic material Correct reason Small negative and large positive

1 1 1

17 Corrects names Correct sequence

, ,

, ,

18 Diverging in (i) and converging in (ii)

Correct answers 1+1 1

19 Correct statements Diagram Correct verification

1 1 1

20 Correct name Correct principle Diagram working

½ 1 ½ 1



21 DM=(26Mp+30Mn)-M Mass defect ∆m=0.528461 B.E= ∆m*931.5 MeV =492.26 MeV B.E/nucleon = . /56=8.79 MeV

½ ½ 1 1

22 i)Point to point mode ii)broadcast mode iii)Point to Point Transmitter Medium Receiver

½ + ½ ½ ½ + ½ + ½

23 Any two correct values (b) correct answer (c) correct answer

2 1 1

24 Correct derivation Correct proof Correct answer OR Correct definition Correct derivation Correct definition of dipole moment

(a) Correct answer (b) Correct answer

2

1

1

½ 2 1/2 1 1

25 Definition Interference figure Derivation OR Diffraction –Definition Graph Difference Effect on changes

1 1 3 1 1 1 2

26 Rectifier Principle Figure Explanation Wave forms

1 1 ½ ½ ½ ½



Half wave 50Hz Full wave 100 Hz.

OR Circuit Working Correct derivation Significance of negative sign

½ ½ 1 1 2 1



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