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Grade: XII PRELIMINARY EXAMINATION 1(2024-25) PHYSICS Maximum Marks: 70 (Candidates are allowed additional 15 minutes for only reading the paper. They must NOT start writing during this time) Maximum Marks: 70 Time Allowed: Three Hours This paper is divided into four sections A, B, C and D. Answer all questions. Section A consists of one question having fourteen 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: (i) The sensitivity of a moving coil galvanometer can be increased by decreasing (a) The number of turns in the coil (b) The area of the coil (c) The magnetic field (d) The couple per unit twist of the suspension [1] (ii) Which among the following materials display higher magnetic susceptibility? (a) Ferromagnetic material (b) Paramagnetic material (c) Diamagnetic material (d) None of these options [1] ISC This paper consists of 8 printed pages. 1 (iii) The SI unit of magnetic flux is (a) Dyne (b) Tesla (c) Weber (d) Ohm [1] (iv) Which of the following did Bohr use to explain his theory? (a) Quantization of angular momentum (b) Conservation of Quantum frequency (c) Conservation of Mass (d) Conservation of Linear Momentum [1] (v) In a semiconductor, what is responsible for conduction? (a) Electrons only (b) Holes only (c) Both electrons and holes (d) Neither electrons nor holes [1] (vi) Assertion: The resolving power of a telescope is more if the diameter of the objective lens is more. Reason: Objective lens of large diameter collects more light. (a) Both Assertion and Reason are true and Reason is the correct explanation for Assertion. (b) Both Assertion and Reason are true but Reason is not the correct explanation for Assertion. (c) Assertion is false and Reason is true. (d) Both Assertion and Reason are false. [1] (vii) Assertion: In Young s double slit experiment if the wavelength of incident monochromatic light is just doubled, the number of bright fringes on the screen will decrease. Reason: Maximum number of bright fringes on the screen is inversely proportional to the wavelength of light used. (a) Both Assertion and Reason are true and Reason is the correct explanation for Assertion. (b) Both Assertion and Reason are true but Reason is not the correct explanation for Assertion. (c) Assertion is false and Reason is true. (d) Both Assertion and Reason are false. [1] 2 (B) (i) (ii) (iii) (iv) Answer the following questions briefly: What is the value of power factor for a pure resistor connected to an A.C. source? What is eddy current in transformer? Working of Optical fibre is based on which phenomena of light? Give the values of X and Y in terms of and the corresponding phase difference. (v) (vi) Under what condition does a junction diode work as an open switch? Two thin lenses of +10 D and 10 D powers are placed in contact with each other. Calculate the focal length of the combination. (vii) Name an electromagnetic wave used for fomentation. SECTION B 14 MARKS Question 2 (i) What is de Broglie hypothesis? (ii) What conclusion can be drawn from Davisson and Germer s experiment? OR Monochromatic light of wavelength 650 nm falls normally on a slit of width 1.3 x 10-4 cm and the resulting Fraunhofer diffraction is obtained on a screen. Find the angular width of the central maxima. Question 3 State how magnetic susceptibility is different for the types of magnetic materials paramagnetic and ferromagnetic materials. Question 4 Determine the electric current that flows in the circuit as shown in the figure below. OR 3 [1] [1] [1] [1] [1] [1] [1] [2] [2] [2] 3V potentiometer used for the determination of internal resistance of a 2.4V cell. The balanced point of the cell in open circuit is 75.8cm. When a resistor of 10.2 is used in the external circuit of the cell the balance point shifts to 68.3cm length of the potentiometer wire. Find the internal resistance of the cell. Question 5 Figure below are two long, parallel wires carrying current in the same direction such that I1 < I2 (i) In which direction will wire I move? (ii) If the direction of the current I2 is reversed, in which direction will the wire I move now? Question 6 (i) The focal length of a double convex lens is equal to the radius of curvature of either surface. What is the refractive index of its material? (ii) What is meant by a thin prism? Question 7 Name the part of electromagnetic spectrum which is suitable for (i) radar systems used in aircraft navigation (ii) treatment of cancer tumours Question 8 Calculate the threshold frequency of photon for photoelectric emission from a metal of work function 0.1eV. SECTION C 27 MARKS 4 [2] [2] [2] [2] Question 9 Obtain the expression for magnetic field due to long current carrying solenoid. Question 10 Figure below shows two resistors R1 and R2 connected to a battery having an emf of 40 V and negligible internal resistance. A voltmeter having a resistance of 300 is used to measure potential difference across R1. Find the reading of the voltmeter. OR Define current density. Obtain the expression between current density in terms of drift velocity. Question 11 A moving coil galvanometer has a coil of resistance 59 . It shows a full-scale deflection for a current of 50 mA. How will you convert it to an ammeter having a range of 0 to 3A? Question 12 The wires which connect the battery of an automobile to its starting motor carry a current of 300A (for a short time)- What is the force per unit length between the wires if they are 70cm long and 1.5cm apart? Is the force attractive or repulsive? Question 13 With the help of a neatly drawn labelled diagram, prove the law of refraction on the basis of Huygen s wave theory. Question 14 An equiconvex lens of glass, having focal length of 10 cm is split into two identical plano-convex lenses each having focal length f1 as shown below, Find the value of f1. Question 15 [3] [3] [3] [3] [3] [3] [3] 5 In Young s double slit experiment, how is interference pattern affected when the following changes are made: (i) Slits are brought closer to each other. (ii) Screen is moved away from the slits. (i) Red coloured light is replaced with blue coloured light. Question 16 (i) Sameer needs a device to examine the moon, which device will you recommend? (ii) Draw a labelled ray diagram of an image formed by this instrument, assuming (a) a small upright object. (b) image lies at least distance of distinct vision. Question 17 Four capacitors, C1, C2, C3 and C4 are connected as shown in figure below. Calculate equivalent capacitance of the circuit between points X and Y. [3] [3] SECTION D 10 MARKS Question 18 [5] (I) (a) A coil having self-inductance of 0.7 H and resistance of 165 is connected to an a.c. source of 275V,50Hz. If =22/7 Calculate: (i) Reactance of the coil (ii) Impedance of the coil (iii) Current flowing through the coil (b) Draw a labelled graph showing variation of impedance of a series LCR circuit with frequency of the a.c. supply. OR (II) (a) The graph here shows a variation of Irms with angular frequency for two different LCR circuits. Which of the two will have: (i) lower power factor. (ii) calculate resonance frequency for the two different LCR circuits. 6 (b) A student was given a frequency generator, a source, an inductor, a capacitor, a resistor and an ammeter. After connecting the circuit correctly, the student tabulated the readings as shown below. (i) What is the resonant frequency of the circuit? (ii) Plot the graph for the given data using proper axes. Question 19 (I) (a) What is meant by pair annihilation? Write a balanced equation for the same. (b) What is the significance of binding energy per nucleon of a nucleus of a radioactive element? (c) Write any one balanced equation representing nuclear fission. (d) Fill in the blank in the given nuclear reaction: (II) OR (a) A graph showing the variation of binding energy per nucleon with mass number is given below. In the graph mark regions where nuclei are: 7 [5] (i) Prone to fusion. (ii) Prone to fission. (iii) Give a reason for your marking. (b) The energy of the thermal neutrons required for the fission of U235 nuclei is of the order of 0.025 eV. However, the neutrons produced in the fission of U235 possess quite large energy as compared to the energy of thermal neutrons. As these neutrons escape the U235 block without causing the fission of U235 nuclei. Due to this, in a nuclear reactor, it is important that the energy of neutrons is absorbed, and some neutrons are also absorbed. Name the substance: (a) used to absorb the energy of the neutrons. (b) write condition when U235 is said to be of critical size. Question 20 (a) Draw a circuit diagram to show how a photo diode is biased. Draw it's characteristic curves for two different illumination intensities. (b) Name the factor which determines intensity of light emitted by LED. (c) With the help of circuit diagram, explain the use of zener diode as a voltage stabilizer. 8 [5]
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