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CBSE Class 9 Board Exam 2024 : Science

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ST. FRANCIS XAVIER SCHOOL Academic year 2022-2023 Class - XII Pre- Board Examination PHYSICS PAPER 1 (THEORY) (Set 02) Maximum Marks: 70 Time allowed: 3 hr. This paper is divided into four sections A, B, C and D. Answer all questions. Section A consists of one question having sub-parts of one mark each. Section B consists of seven questions of two marks each. Section C consists of nine questions of three marks each, and Section D consists of three questions of five marks each. Internal choices have been provided in two questions each in Section B, Section C and Section D. The intended marks for questions are given in brackets [ ]. All working, including rough work, should be done on the same sheet as and adjacent to the rest of the answer. Answers to sub parts of the same question must be given in one place only. A list of useful physical constants is given at the end of this paper. A simple scientific calculator without a programmable memory may be used for calculations. SECTION A 14 MARKS Question 1 (A) In questions (i) to (vii) below, choose the correct alternative (a), (b), (c) or (d) for each of the questions given below: [7 1=7] (i) For an electric dipole of charges q and -q separates by 2l0 find the work done in moving a charge q through l alone the perpendicular bisector of the dipole a. b. 1 . 2 . 4 0 0 1 4 0 . 0 . c. d. (ii) 4 0 . 2 . For a body centred cubic lattice of side a if the charge at the centre of the cube is q then the electric flux through each surface of the cube is: a. 0 b. 4 c. d. (iii) 1 0 6 0 6 0 2 For a monochromatic ray that passes through an isosceles triangle with minimum deviation must have: a. Angle of incidence equals to angle of prism. b. Each angle of refraction inside the prism is not half the angle of prism. c. Angle of emergence equals to angle of prism. d. Each angle of incidence and emergence equals to (prism angle +minimum deviation angle). Page 1 / 6 (iv) Total energy of an electron in the ground state of hydrogen atom is -13.6 ev. Then the ionization energy of hydrogen atom is a. -13.6ev. b. Infinity. c. Zero. d. 13.6ev. (v) A pure semiconductor has pair of carriers of electricity, electrons and holes; so under external potential difference will give current: a. Like electrolyte. b. Zero. c. Non Ohmic current. d. None of these. (vi) A battery of emf 15 V and internal resistance of 4 is connected to a resistor. If the current in the circuit is 2 A and the circuit is closed. Resistance of the resistor and terminal voltage of the battery will be a. 2.5 , 6 V b. 2.5 , 7 V c. 3.5 , 6 V d. 3.5 , 7 V (vii) Which of the following impurity with which pure silicon should be doped to make an n-type semiconductor? a. phosphorus b. Indium c. boron d. aluminium. (B) Answer the following questions briefly and to the point: [7 1=7] (i) How do the expressions for electric fields just outside a sheet of charge and a charged conductor differ? (ii) Under what condition, does a charged sphere or spherical shell behave as a point charge for the purpose of determining electric field due to it? (iii) Why is the power of the objective lens of a telescope is kept as small as possible? (iv) Name the quantity in electrical circuits which plays the same role as inertia in mechanics. (v) If a convex lens of glass is immersed in water what will be its change in focal length? (vi) When p-n junction diode in reverse bias, what will be the change in depletion region? (vii) What do you mean by pair production and pair annihilation? Section B 14 MARKS Answer all questions Question 2: Finds the electric potential energy when three equal charges(q) are held together at the corners of an equilateral triangle of side a. what will happen if they set free? OR, You are given an air-filled parallel plate capacitor. Two slabs of dielectric constants k1 and k2 have been filled in between the two plates of the capacitor as shown in Fig. What will be the ratio of the capacitance of the capacitors (initial area of plates is A and distance between plates d)? [2] Page 2 / 6 Question 3: [2] In diffraction pattern, find the relation between slit width and wavelength of light if the central maxima spreads over the entire screen. Question 4: [2] A 200 turn coil of self-inductance 20 mH carries a current of 4 mA. Find the magnetic flux linked with the coil. Question 5: [2] Find the angle of emergence of a ray if it is incident at an angle 45 to an equilateral prism of refractive index 2 OR, Define dispersive power of an optical instrument. Write its mathematical formula. Question 6: [2] Using Biot-savart law, find the expression for the magnetic field strength at the centre of a circular coil of having N number of turns, r as the radius and I amount of current is passing through it. Question 7: [2] Draw a circuits diagram for full wave rectifier, also draw the input output graph. Question 8: [2] A meter bridge while measuring an unknown resistance S with a given resistance R=5 , finds the null point at a distance 62.5 cm. find S and also find the position of null point when two such R are joined in parallel to balance S. SECTION C 27 MARKS Question 9: [3] From Gauss theorem, show that the electric field at a distance r(r<R) from the centre of a uniformly charged non-conducting sphere of radius R and charge density will be E= . 3 . what will be the value of E if it is a 0 conducting sphere carrying the same amount of total charge? Question 10: Using Kirchhoff s rules, calculate the current through the 40 and 20 resistors in the given circuit. [3] OR, With a certain cell, the balance point is obtained at 65 cm from the end of a potentiometer wire. With another cell whose emf differs from that of first by 0.1V, the balance point is obtained at 60 cm. Find the emf of each cell. Question 11: [3] Define self-inductance. Show that the self-inductance of the solenoid given above is = 2 , where A is the cross-sectional area of the solenoid. Question 12: [3] 1 1 Derive the formula = for refraction of light at a (concave or convex) spherical surface. Symbols have usual meanings. Question 13: [3] (i) Young s Double slit experiment, show graphically how intensity of light varies with distance. Page 3 / 6 (ii) In Fraunhofer diffraction, how is the angular width of the central bright fringe affected when slit separation is increased? OR In the young s double slit experiment using monochromatic light of wavelength 600nm, 5th bright fringe is at a distance of 0.48 mm from the centre of the pattern. If the screen is at a distance of 80 cm from the plane of the two slits, calculate (i) distance between the slits, (ii) fringe width, i.e., fringe separation. Question 14: [3] When two thin lenses are kept in contact, prove that the combined or effective focal length F is given by 1 1 1 = + 1 2 Where the terms have their usual meaning. Question 15: [3] (i) With reference to photoelectric effect, plot a labelled graph of stopping potential (Vs) versus frequency (f)of the incident radiation. (ii) State how will you use this graph to determine the value of Planck s constant. Question 16: [3] 2 The energy of an electron in the nth orbit of a hydrogen atom is given by En=-13.6/n eV. Now find the wavelength in Angstorm of the spectral line for transmission from -0.544 eV to -3.4 eV. In which spectral series do this line belong? Question 17: [3] (i) Explain depletion region. (ii) Draw a labelled characteristic curve i.e. I - V graph for a semiconductor diode during forward bias. (iii) Show the forward resistance from the graph. SECTION D 15 MARKS Question 18: [5] An alternating emf of 220V is applied to a circuit containing a resistor R having resistance of 160 and a capacitor C in series. The current is found to lead the supply voltage by an angle = tan-1 (3/4). (i) Calculate: (a) The capacitive reactance (b) Impedance of the circuit (c) Current flowing in the circuit (ii) If the frequency of the applied emf is 50 Hz, what is the value of the capacitance of the capacitor C ? OR, (i) A metal rod of length 0.4 m moves steadily out if a uniform magnetic field of 0.2 T with a velocity 10m/s. the resistance of the rod is 5 . Find the value of induced emf developed, magnitude and direction of induced current. Page 4 / 6 (ii) A rectangular loop of sides a and b carrying current I2 is kept at a distance a from an infinity long straight wire carrying current I1 as shown in the figure. Obtain an expression for the resultant force acting on the loop. Question 18: [5] a. Define stopping potential and drive Einstein s photoelectric equation relating the stopping potential with incident frequency. b. The threshold wavelength of a certain metal is 792 nm. What is the maximum kinetic energy of photoelectron emitted by this metal if it is exposed to ultra violet light of wavelength 396 nm? OR, State Bohr s postulate to define stable orbits in hydrogen atom. How does de-Broglie s hypothesis explain the stability of these orbits? The electron in a given Bohr orbit has a total energy of 1.5 eV. Calculate its (i) kinetic energy (ii) potential energy (iii) wavelength of radiation emitted, when this electron makes a transition to the ground state. [Given : Energy in the ground state = 13.6 eV and Rydberg s constant = 1.09 107 m 1] Question 19: [5] Newton s rings is a phenomenon in which an interference pattern is created by the reflection of light between two surfaces: a spherical surface and an adjacent Touching flat surface. It is named after Issac Newton, who investigated the effect in his 1704 treatise Optics. When viewed with monochromatic light, Newton s rings appear as a series of concentric, alternating bright and dark rings centred at the point of contact between the two surfaces. When viewed with white light, it forms a concentric ring pattern of rainbow colors, because the different wavelengths of light interfere at different thickness of the air layer between the surfaces. 1 (i) In an interference experiment the ratio of amplitudes of coherent waves is 1 = 3. The find the ratio of the 2 maximum and minimum intensities of fringes. (ii) Two coherent sources of intensity 10 W m 2 and 25 W m 2 interfere to form fringes. Find the ratio of maximum intensity to minimum intensity. (iii) how fringe pattern is differed when a monochromatic light is changed by a white light? (v) Two slits in Young s double slit experiment have widths in the ratio 81 : 1. Then find the ratio of the amplitudes of light waves. Page 5 / 6 ************************************** Page 6 / 6

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