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CBSE Class 12 Board Exam 2020 : Physics (Series 4)

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Series : HMJ/4 SET 1 . Code No. 55/4/1 - - . Roll No. Candidates must write the Code on the title page of the answer-book. (I) - (I) 15 (II) - (II) - - NOTE Please check that this question paper contains 15 printed pages. Code number given on the right hand side of the question paper should be written on the title page of the answer-book by the candidate. (III) Please check that this question paper contains 37 questions. (III) - 37 (IV) (IV) Please write down the Serial Number of the question in the , (V) - 15 (V) - 10.15 10.15 10.30 - - answer-book before attempting it. 15 minute time has been allotted to read this question paper. The question paper will be distributed at 10.15 a.m. From 10.15 a.m. to 10.30 a.m., the students will read the question paper only and will not write any answer on the answer-book during this period. ( ) PHYSICS (Theory) {ZYm [aV g : 3 K Q>o A{YH$V A H$ Time allowed : 3 hours .55/4/1. 308A : 70 Maximum Marks : 70 1 P.T.O. : (i) - , , (ii) - 37 (iii) - 1 20 , 1 (iv) - 21 27 2 (v) - 28 34 3 (vi) - 35 37 5 (vii) , - , - , - - (viii) , , (ix) (x) , c = 3 108 m/s h = 6.63 10 34 Js e = 1.6 10 19 C 0 = 4 10 7 T m A 1 0 = 8.854 10 12 C2 N 1 m 2 1 = 9 109 N m2 C 2 4 0 (me) = 9.1 10 31 kg = 1.675 10 27 kg = 1.673 10 27 kg = 6.023 1023 = 1.38 10 23 JK 1 .55/4/1. 2 General Instructions : Read the following instructions very carefully and strictly follow them : This question paper comprises four sections A, B, C and D. There are 37 questions in the question paper. All questions are compulsory. (iii) Section A : Q. no. 1 to 20 are very short-answer type questions carrying 1 mark each. (iv) Section B : Q. no. 21 to 27 are short-answer type questions carrying 2 marks each. (v) Section C : Q. no. 28 to 34 are long-answer type questions carrying 3 marks each. (vi) Section D : Q. no. 35 to 37 are also long answer type questions carrying 5 marks each. (vii) There is no overall choice in the question paper. However, an internal choice has been provided in two questions of one mark, two questions of two marks, one question of three marks and all the three questions five marks. You have to attempt only one of the choices in such questions. (viii) However, separate instructions are given with each section and question, wherever necessary. (ix) Use of calculators and log tables is not permitted. (x) You may use the following values of physical constants wherever necessary : (i) (ii) c = 3 108 m/s h = 6.63 10 34 Js e = 1.6 10 19 C 0 = 4 10 7 T m A 1 0 = 8.854 10 12 C2 N 1 m 2 1 = 9 109 N m2 C 2 4 0 Mass of electron (me) = 9.1 10 31 kg Mass of neutron = 1.675 10 27 kg Mass of proton = 1.673 10 27 kg Avogadro s number = 6.023 1023 per gram mole Boltzmann constant = 1.38 10 23 JK 1 .55/4/1. 3 P.T.O. 1. : r emf (E) R R V - 1 2. 2R (a) 3. 5. R (c) R 2 (d) R 4 (b) 0I 2R (1 + ) (c) 0I 4 R (d) 0I 1 1 2R r I y-z : (a) r (b) (c) r (d) I - 4 eV 1 eV - (a) 6. (b) 1 , R , I O (a) 4. 2R 2:1 (b) 1:1 (c) 1:2 (d) 4:1 1 1 1 d I d/2 (a) .55/4/1. I (b) 2I (c) 4 4I (d) I/2 1 1. SECTION A Select the most appropriate option from those given below each question. A cell of emf (E) and internal resistance r is connected across a variable external resistance R. The graph of terminal potential difference V as a function of R is 1 2. 3. A uniform wire of resistance 2R is bent in the form of a circle. The effective resistance between the ends of any diameter of the circle is : R R (a) 2R (b) R (c) (d) 2 4 A current I flows through a long straight conductor which is bent into a circular loop of radius R in the middle as shown in the figure. The magnitude of the net magnetic field at point O will be 0I 0I (a) Zero (b) (1 + ) (c) 2R 4 R 4. 5. 6. 1 (d) 0I 1 1 2R A circular loop of radius r, carrying a current I lies in y-z plane with its centre at the origin. The net magnetic flux through the loop is : (a) directly proportional to r (b) zero (c) inversely proportional to r (d) directly proportional to I The kinetic energy of a proton and that of an -particle are 4 eV and 1 eV, respectively. The ratio of the de-Broglie wavelengths associated with them, will be (a) 2:1 (b) 1:1 (c) 1:2 (d) 4:1 A photocell connected in an electrical circuit is placed at a distance d from a source of light. As a result, current I flows in the circuit. What will be the current in the circuit when the distance is reduced to d/2 ? (a) I (b) 2I (c) 4I (d) I/2 .55/4/1. 5 1 1 1 1 P.T.O. 7. 8. 10A 10 cm (a) 2 10 5 T (b) 2 10 5 T (c) 4 10 5 T (d) 4 10 5 T : (a) 2 (b) 0 (c) (d) 4 (a) (b) (c) (d) 10. - - : (a) (b) (c) (d) 1 1 9. 1 1 : : 11. 12. 13. 14. 15. ________ , - , _______ _______ _______ Io _________ 32 15 P 32 16 S + v + _______ 1 1 1 1 1 : 16. ? .55/4/1. 1 6 1 7. 8. A current of 10A is flowing from east to west in a long straight wire kept on a horizontal table. The magnetic field developed at a distance of 10 cm due north on the table is : (a) 2 10 5 T, acting downwards (b) 2 10 5 T, acting upwards (c) 4 10 5 T, acting downwards (d) 4 10 5 T, acting upwards When a wave undergoes reflection at an interface from rarer to denser medium, adhoc change in its phase is : (a) (b) 0 (c) (d) 2 4 Paschen series of atomic spectrum of hydrogen gas lies in : (a) Infrared region (b) Ultraviolet region (c) Visible region (d) Partly in ultraviolet and partly in visible region 10. In the particle scattering experiment, the shape of the trajectory of the scattered particles depend upon : (a) only on impact parameter. (b) only on the source of particles. (c) both impact parameter and source of particles. (d) impact parameter and the screen material of the detector. 1 1 9. Note : Fill in the blanks with appropriate answer. 11. Torque acting on an electric dipole placed in an electric field is maximum when the angle between the electric field and the dipole moment is _______. 12. A proton released from rest in an electric field, will start moving towards a region of _______ potential in the field. 13. To minimize the percentage error in the determination of unknown resistance of a conductor in meter bridge experiment, the balance point is adjusted near _______ of the wire. OR In potentiometer, a long uniform wire is used to _______ potential gradient along the wire. 1 1 1 1 1 1 14. Unpolarised light of intensity Io is incident on two crossed polaroids. The intensity of light transmitted by the combination will be _______. 1 15. Name the particle emitted spontaneously in the following nuclear reaction : 32 32 P S + v + _______ 1 Answer the following : 16. The work done in moving a charge particle between two points in an uniform electric field, does not depend on the path followed by the particle. Why ? 1 15 .55/4/1. 16 7 P.T.O. 17. 18. 19. 20. 21. A.C. ? ? p-n p- n- ? p-n : A B : 1 1 1 1 1 2 22. Lo Ro - 2 Lo (a) l R (b) 2 Lo ? 2 23. V = Vo sin t R L /4 C /4 L, C R (i) (ii) 24. A, B C (i) A (ii) B - (iii) C .55/4/1. 8 2 2 17. Define magnetic declination at a place on earth. 18. An A.C. source with variable frequency is connected to a parallel plate capacitor. How will the displacement current be affected with the decrease in frequency of the source ? 19. An astronomical telescope may be a refracting type or a reflecting type. Which of the two produces image of better quality ? Justify your answer. 20. Can a slab of p-type semi-conductor be physically joined to another n-type semiconductor slab to form p-n junction ? Justify your answer. OR In a p-n junction diode the forward bias resistance is low as compared to the reverse bias resistance. Give reason. 1 1 1 1 1 SECTION : B 21. Find the total charge stored in the network of capacitors connected between A and B as shown in figure : 2 22. A wire of length Lo has a resistance Ro. It is gradually stretched till its length becomes 2 Lo. (a) Plot a graph showing variation of its resistance R with its length l during stretching. (b) What will be its resistance when its length becomes 2 Lo ? 23. A resistor R and an inductor L are connected in series to a source of voltage V = Vo sin t. The voltage is found to lead current in phase by /4. If the inductor is replaced by a capacitor C, the voltage lags behind current in phase by /4. When L, C and R are connected in series with the same source, Find the : (i) average power dissipated and (ii) instantaneous current in the circuit. 24. Light of same wavelength is incident on three photo-sensitive surfaces A, B and C. The following observations are recorded. (i) From surface A, photo electrons are not emitted. (ii) From surface B, photo electrons are just emitted. (iii) From surface C, photo electrons with some kinetic energy are emitted. Compare the threshold frequencies of the three surfaces and justify your answer. OR .55/4/1. 9 2 2 2 P.T.O. ? (i) (ii) 25. p-n - , 26. (a) ? (b) I1 I2 (I1 > I2) V-I (a) (LED) - (b) LED 27. (a) - (b) (i) (ii) - 2 2 2 2 2 r1 r2 Q q (a) (i) (ii) ? (b) - (a) x (E) (b) x E 29. (a) (b) emf 5V 10 m - emf E1 E2 (i) (ii) - 700 cm 100 cm emf 28. .55/4/1. 10 3 3 3 If the frequency of light incident on the cathode of a photo-cell is increased, how will the following be affected ? Justify your answer. (i) Energy of the photo electrons. (ii) Photo current. 2 25. Briefly explain how a potential barrier is set up across a p-n junction as a result of diffusion and drift of the charge carriers. 2 26. (a) (b) (a) (b) 27. (a) (b) Why is a photo diode operated under reverse bias condition ? Draw V-I characteristic curves of photo diode for incident light of intensities I1 and I2 (I1 > I2). OR State the level of doping and biasing condition used in light emitting diode (LED). Write any two advantages of LED over the conventional low power lamps. Explain the formation of energy bands in crystalline solids. Draw the energy band diagrams of (i) a metal and (ii) a semiconductor. SECTION C 28. A hollow conducting sphere of inner radius r1 and outer radius r2 has a charge Q on its surface. A point charge q is also placed at the centre of the sphere. (a) What is the surface charge density on the (i) inner and (ii) outer surface of the sphere ? (b) Use Gauss law of electrostatics to obtain the expression for the electric field at a point lying outside the sphere. OR (a) An infinitely long thin straight wire has a uniform linear charge density . Obtain the expression for the electric field (E) at a point lying at a distance x from the wire, using Gauss law. (b) Show graphically the variation of this electric field E as a function of distance x from the wire. 29. (a) (b) .55/4/1. Explain the principle of working of a potentiometer. In a potentiometer, a standard source of emf 5V and negligible internal resistance maintains a steady current through the potentiometer wire of length 10m. Two primary cells of emf E 1 and E2 are joined together in a series with (i) same polarity and (ii) opposite polarity. The combination is connected to the potentiometer circuit in each case. The balancing length of the wire in the two cases are found to be 700 cm and 100 cm, respectively. Find the values of emf of the two cells. 11 2 2 2 3 3 3 P.T.O. 2H 100 ms 0.5A (i) (ii) emf. 31. ? 32. (a) (b) (c) TV 30. (a) (b) 5 10 2 W m 2 /6 , , (a) (b) (c) 34. P Q, 20 cm P Q P 50 cm , Q : 35. (a) (b) 7 cm 5 2A X-Y (a) , 1A (b) 3A m2 10 cm F 30 0.25 T F F , ? 3 3 3 33. .55/4/1. 12 3 3 5 5 30. (a) (b) Differentiate between self inductance and mutual inductance. The mutual inductance of two coaxial coils is 2H. The current in one coil is changed uniformly from zero to 0.5A in 100 ms. Find the : (i) change in magnetic flux through the other coil. (ii) emf induced in the other coil during the change. 3 31. Explain with the help of a diagram, the working of a step-down transformer. Why is a laminated iron core used in a transformer ? 3 32. Name the electro-magnetic waves with their frequency range, produced in (a) some radioactive decay (b) sparks during electric welding (c) TV remote 3 33. Two coherent light waves of intensity 5 10 2 Wm 2 each super-impose and produce the interference pattern on a screen. At a point where the path difference between the waves is /6, being wavelength of the wave, find the (a) phase difference between the waves. (b) resultant intensity at the point. (c) resultant intensity in terms of the intensity at the maximum. 34. Two objects P and Q when placed at different positions in front of a concave mirror of focal length 20 cm, form real images of equal size. Size of object P is three times size of object Q. If the distance of P is 50 cm from the mirror, find the distance of Q from the mirror. 35. (a) (b) (a) (b) .55/4/1. SECTION-D Show that a current carrying solenoid behaves like a small bar magnet. Obtain the expression for the magnetic field at an external point lying on its axis. A steady current of 2A flows through a circular coil having 5 turns of radius 7 cm. The coil lies in X-Y plane with its centre at the origin. Find the magnitude and direction of the magnetic dipole moment of the coil. OR Derive the expression for the force acting between two long parallel current carrying conductors. Hence, define 1 A current. A bar magnet of dipole moment 3 Am2 rests with its centre on a frictionless pivot. A force F is applied at right angles to the axis of the magnet, 10 cm from the pivot. It is observed that an external magnetic field of 0.25 T is required to hold the magnet in equilibrium at an angle of 30 with the field. Calculate the value of F. How will the equilibrium be effected if F is withdrawn ? 13 3 3 5 5 P.T.O. A m (b) PQ 1.5 ABC 36. (a) (ii) 1.4 , ? (i) 5 (b) 1.5 10 D 50 cm (a) 37. (a) 5 N = No e t 238 (b) - (c) ? 92 U 4.5 109 5 (b) (a) _______________ .55/4/1. 14 5 36. (a) Draw the ray diagram showing refraction of ray of light through a glass prism. Derive the expression for the refractive index of the material of prism in terms of the angle of prism A and angle of minimum deviation m. (b) A ray of light PQ enters an isosceles right angled prism ABC of refractive index 1.5 as shown in figure. (i) Trace the path of the ray through the prism. (ii) What will be the effect on the path of the ray if the refractive index of the prism is 1.4 ? 5 OR (a) Two thin lenses are placed coaxially in contact. Obtain the expression for the focal length of this combination in terms of the focal lengths of the two lenses. (b) A converging lens of refractive index 1.5 has a power of 10 D. When it is completely immersed in a liquid, it behaves as a diverging lens of focal length 50 cm. Find the refractive index of the liquid. 37. (a) (b) 5 Derive the law of radioactive decay N = N o e t The half life of 238 92 U undergoing -decay is 4.5 109 years. Find its mean life. (c) What fraction of the initial mass of a radioactive substance will decay in five half life periods ? 5 OR (a) State the postulates of Bohr s model of hydrogen atom and derive the expression for Bohr radius. (b) Find the ratio of the longest and the shortest wavelengths amongst the spectral lines of Balmer series in the spectrum of hydrogen atom. ____________ .55/4/1. 15 5 P.T.O. .55/4/1. 16

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