NBSE Question Papers Physics for Class 12

Class 12  Code No. 042/12/1 Series NBSE/XII/2023 Candidates must write the Code No. Roll No.  on the title page of the Answer Sheet. l Please check that this question paper contains 8 pages. l Code number given on the right hand side of the question paper should be written on the title page of the Answer Sheet. l Please check that this question paper contains 33 questions. l 15 minutes time has been allotted to read this question paper. PHYSICS Time Allowed: 3 hours Maximum Marks: 70 General Instructions: (i) All questions are compulsory. There are 33 questions in all. (ii) This question paper has five sections: Section A, Section B, Section C, Section D and Section E. (iii) Section A contains four objective and six very short answer questions and four assertion reasoning MCQs of 1 mark each, Section B has two case based questions of 4 marks each, Section C has nine short answer questions of 2 marks each, Section D contains five short answer questions of 3 marks each and section E contains three long answer questions of 5 marks each. (iv) There is no overall choice. However, internal choice is provided. You have to attempt only one of the choices in such questions. NBSE 2023 1 P.T.O.

Section-A All questions are compulsory. In case of internal choices, attempt any one of them. I.  Objective Type Questions (1 Mark) Q1. The electric field at a point is 1 (a) always continuous. (b) continuous if there is charge at that point. (c) discontinuous only if there is a negative charge at that point. (d) discontinuous if there is a charge at that point. Q2. The self inductance L of a long solenoid of length l and area of cross-section A, with a fixed number of turns N increases as 1 (a) l and A increases (b) l decreases and A increases (c) l increases and A decreases (d) Both l and A decreases. Q3. The radius of second orbit of He+ ion will be 1 (a) 1.058 Å (b) 3.023 Å (c) 2.068 Å (d) 4.058 Å Q4. The electrical conductivity of semiconductor increases when electromagnetic radiation of wavelength shorter than 2800 nm is incident on it. The band gap in (eV) for the semiconductor is 1 (a) 0.7 eV (b) 0.5 eV (c) 2.5 eV (d) 1.2 eV II.  Very Short Answer Type Questions (1 Mark) Q5. On which law Kirchhoff’s junction rule is based? 1 OR What is the basic purpose of using a capacitor? Q6. If the rate of change of current of 2 ampere/second induces an emf of 40 mV in the solenoid, what is the self- inductance of this solenoid? 1 OR If the self-inductance of an iron core inductor increases from 0.01 mH to 10 mH on introducing the iron core into it, what is the relative permeability of the core material used? Q7. Must every magnetic configuration have a north pole and a south pole? 1 OR What is the basic difference between electric and magnetic field lines? Q8. An alternating current generator has an internal resistance Rg and an internal reactance Xg. It is used to supply power to a passive load consisting of a resistance Rg and reactance XL. Find the maximum power to be delivered from the generator to load the value of XL. 1 Q9. A short pulse of white light is incident from air to glass slab at normal incidence. After travelling through the slab, which colour to be emerge first? 1 OR When light travel from a rarer to denser medium, the speed decreases. Does this decrease in speed imply a decrease in the energy carried by the light wave? Justify your answer. Q10. Find the wavelength of a photon needed to remove a proton from a nucleus which is bound to the nucleus with 1 MeV energy. 1 For questions numbers 11, 12, 13 and 14, two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below. (a) Both A and R are true and R is the correct explanation of A (b) Both A and R are true but R is NOT the correct explanation of A (c) A is true but R is false (d) A is false and R is also false NBSE 2023 2

Q11. Assertion (A): Critical angle for total internal reflection (T.I.R) at glass-water interface is qc. If mg = 3 and mw = 2 4 , then angle of incidence (i) of a ray in water, so that its angle of refraction in glass qc, will be π . 1 3 2 Reason (R): Total internal reflection takes place if the angle of incidence at the denser-rarer interface is more than the critical angle. Q 12. Assertion (A): In optical fibre, the radius of core is kept small.  1 Reason (R): The smaller radius of the core ensures the fibre should have incident angle more than the critical angle required for total internal reflection. Q13. Assertion (A): In the phenomenon of mutual induction, self induction of each of the coil persists. 1 Reason (R): Self-induction arises when strength of current in one coil changes. In mutual induction, current is changing in both the individual coils. Q 14. Assertion (A): Balmer series lies in the visible region of electromagnetic spectrum. 1 Reason (R): 1 = R ; 1 − 1 E , where n = 3, 4, 5,..... λ 22 n2 Section-B Question 15 and 16 are Case Study based questions and are compulsory. Attempt any 4 sub-parts from each question. Each sub part carries 1 mark. Q 15. Gauss theorem: The total flux through a closed surface, enclosing a volume, in vacuum is, 1 times the net ε0 4 charge, enclosed by the surface. ∫f = →→ = qenclosed E .dS s ε0 Gaussian surface: Any closed surface imagined around the charge distribution, so that Gauss theorem can be conveniently applied to find electric field due to the given charge distribution. Electric field due to infinitely long straight charged wire of linear charge density λ: λ E = 2πε0 r , where r is the perpendicular distance of the observation point from the wire. Electric field due to an infinite plane sheet of charge of surface charge density σ. σ E= 2ε0 1. S.I. unit of electric flux is (a) N2 mC (b) NmC–2 (c) Nm2C–1 (d) Nm–2C 2. Electric flux is a (a) constant quantity (b) vector quantity (c) scalar quantity (d) None of these 3. Two charges of magnitude –2Q and +Q are located at points (a, 0) and (4a, 0) respectively. What is the electric flux due to these charges through a sphere of radius ‘3a’ with its centre at origin? Q – 2Q 3Q – 3Q (a) ε0 (b) ε0 (c) ε0 (d) ε0 4. A charge q is placed at the centre of a cube of side l. What is the electric flux passing through each face of the cube? q q q q 6ε0 ε0 (a) 5ε0 (b) 9ε0 (c) (d) 5. The figure shows three charges +2q, – q and +3q. Two charges +2q and – q are enclosed within a surface ‘S’. What is the electric flux due to this configuration through the surface ‘S’? NBSE 2023 3 P.T.O.

(a) 5q (b) 3q (c) 2q (d) q 2ε0 ε0 ε0 ε0 Q16. Diode as a Half-Wave rectifier: Diode conducts corresponding to positive half cycle and does not conduct during negative half cycle. Hence, ac is converted by diode into unidirectional pulsating dc. This action is known as half- wave rectification. 4 Diode as a Full-Wave rectifier: In the full-wave rectifier, two p-n junction (Here, D1 and D2) diodes, are used. Its principle is that junction diode puts very low resistance in forward bias while very high resistance in reverse bias condition. T he rectified voltage is in the form of pulses of the shape of half sinusoidals. Although it is unidirectional but not steady. To get steady dc output from the pulsating voltage, a capacitor is connected across the output terminals, parallel to load resistance which will filter out the ac ripple and provide a pure dc steady output. NBSE 2023 4

1. The a.c. to be rectified is applied to the (a) primary of a transformer (b) secondary of a transformer (c) between primary and secondary (d) anywhere on the diode 2. The secondary coil of the transformer is tapped in the (a) corner (b) middle (c) upper half (d) lower half 3. During positive half of a.c. cycle diode D1, is in forward biasing and it conducts. At this point the diode D2 will not conduct because (a) being in reverse biasing (b) being in forward biasing. (c) as in high biasing (d) as in low biasing 4. The output of full wave rectifier is (a) pulsating a.c. (b) pulsating d.c. (c) constant a.c. (d) constant d.c. 5. Pulsations can be minimised by using suitable (a) filter circuit (b) CL circuit (c) RC circuit (d) CR circuit Section-C All questions are compulsory. In case of internal choices, attempt any one of them. Q 17. If the potential difference across a capacitor is doubled, what happens to : (i) The charge on the capacitor and (ii) The energy stored in the capacitors. 2 OR Obtain an expression for the equivalent capacitance of a parallel grouping of three capacitors of capacitances C1, C2 and C3. Q 18. Figure shows a rectangular loop PQRS in which the arm PQ is free to move. A uniform magnetic field acts in the direction perpendicular to the plane of the loop. Arm PQ is moved with a velocity v towards the arm RS Assuming that the arms QR, RS and SP have negligible resistances and the moving arm PQ has the resistance r, obtain the expression for (i) the current in the loop (ii) the force and (iii) the power required to move the arm PQ.  2 OR Two identical circular loops, P and Q, each of radius r and carrying equal currents are kept in the parallel planes having a common axis passing through O. The direction of current in P is clockwise and in Q is anti-clockwise as seen from O which is equidistant from the loops P and Q. Find the magnitude of the net magnetic field at O. Q 19. Distinguish between n-type and p-type semiconductor. 2 Q 20. The wire shown in figure below carries a current of 60 A. Find the magnetic field B at P. 2 P.T.O. NBSE 2023 5

Q21. Suggest a suitable method to invert an image (i) Without change in size. (ii) Without change in size and without deviation from its original direction of view. Explain how the same is achieved? 2 OR An equi-convex lens has refractive index 1.5. Write its focal length in terms of radius of curvature R. Q 22. Find the wavelength of electromagnetic waves of frequency 5 × 1019 Hz in free space. Give its two applications. 2 Q 23. Draw the shape of the refracted/reflected wavefront when a plane wavefront is incident on (i) Prism (ii) Convex mirror. Give brief explanation for the construction. 2 Q24. Find the ratio of the de-Broglie wavelength, associated with protons, accelerated through a potential of 128 V and α particles, accelerated through a potential of 64 V. 2 Q25. Draw a plot showing the variation of (i) Electric field (E) and (ii) Electric potential (V) with distance r due to a point charge Q. 2 Section-D All questions are compulsory. In case of internal choices, attempt any one of them. Q26. In a chamber, a uniform magnetic field of 6.5 G (1 G = 10−4 T) is maintained. An electron is shot into the field with a speed of 4.8 × 106 m s−1 normal to the field. Explain why the path of the electron is a circle. Determine the radius of the circular orbit. (e = 1.6 × 10−19 C, me = 9.1 × 10−31 kg) 3 OR (a) A circular coil of 30 turns and radius 8.0 cm, carrying a current of 6.0 A is suspended vertically in a uniform horizontal magnetic field of magnitude 1.0 T. The field lines make an angle of 60° with the normal to the coil. Calculate the magnitude of the counter torque that must be applied to prevent the coil from turning. (b) Would your answer change if the circular coil in (a) were replaced by a planar coil of some irregular shape that encloses the same area? (All other particulars are also unaltered). Q 27. The binding energy per nucleon of deuteron ( 2 H ) and helium nucleus ( 4 He ) is known to be 1.1 MeV and 7 MeV 1 2 respectively. If two deuteron nuclei react to form a single helium nucleus; calculate the energy released. 3 Q28. Draw a plot of potential energy of a pair of nucleons as a function of their separation. Write two important conclusions which you can draw regarding the nature of nuclear forces. 3 Q29. Group the following six nuclides into three pairs of (i) isotones (ii) isotopes and (iii) isobars. 6C12, 2He3, 80Hg198, 1H3, 79Au197, 6C14 3 Q30. (a) Figure shows a cross-section of a ‘light pipe’ made of a glass fibre of refractive index 1.68. The outer covering of the pipe is made of a material of refractive index 1.44. What is the range of the angles of the incident rays with the axis of the pipe for which total reflection inside the pipe take place, as shown in the figure. 3 NBSE 2023 6

(b) What is the answer if there is no outer covering of the pipe? OR Use Huygen’s principle to verify Snell’s law of refraction. Section-E All questions are compulsory. In case of internal choices, attempt any one of them. Q 31. Define relaxation time of the free electrons drifting in a conductor. How is it related to the drift velocity of free electrons? Use this relation to deduce the expression for the electrical resistivity of the material.  5 OR State Gauss Law in electrostatics. Use this law to derive an expression for the electric field due to an infinitely long straight wire of linear charge density l Cm–1. Q32. (i) (a) D raw a labelled ray diagram to show the formation of image in an astronomical telescope for a distant object, (b) Write three distinct advantages of a reflecting type telescope. (ii) A convex lens of focal length 10 cm is placed coaxially 5 cm away from a concave lens of focal length 10 cm. If an object is placed 30 cm in front of the convex lens, find the position of the final image formed by the combined system.  5 OR With the help of ray diagram, show the formation of image of a point object by refraction of light at a spherical surface separating two media of refractive indices n1 and n2 (n2 > n1) respectively. Using this diagram, derive the relation. n2 n1 = n2 – n1 – R vu Write the sign conventions used. What happens to the focal length of convex lens when it is immersed in water? Q33. State the principle of working of a transformer. Can a transformer be used to step up or step down a d c. voltage? Justify your answer. Mention various energy losses in transformer? 5 OR Define working, principle and construction of a moving galvanometer with its labelled diagram. How a galvanometer can be converted into an ammeter and voltmeter. rrr NBSE 2023 7 P.T.O.

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