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Scottish High International School Preboard-I Examination XII ISC (2020-21) PHYSICS PAPER 1 (THEORY) (Maximum Marks: 70) (Time allowed: Three hours) (Candidates are allowed additional 15 minutes for only reading the paper. They must NOT start writing during this time.) -----------------------------------------------------------------------------------------------------------All questions are compulsory. This question paper is divided into 4 Sections, A, B, C and D as follows: Section A Question number 1 is of twelve marks. All parts of this question are compulsory. Section B Question numbers 2 to 12 carry 2 marks each with two questions having internal choice. Section C Question numbers 13 to 19 carry 3 marks each with two questions having internal choice. Section D Question numbers 20 to 22 are long-answer type questions and carry 5 marks each. Each question has an internal choice. 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 Answer all questions. Question 1 A. a. Choose the correct alternative a), b), c) or d) for each of the questions given [1 x 5] below: i. A charge placed at a distance from an electric dipole in the end on position experiences a force F. If the distance be doubled, the force will become: a) 2F b) F/2 c) F/4 d) F/8 ii. A short electric dipole (which consists of two-point charges, +q and -q) is place at the centre O and inside a large cube of length L, as shown in figure. The electric flux, emanating through the cube is: L a) 4 b) Zero c) 2 d) 3 b. B. iii. When a charge particle is projected perpendicular to a uniform magnetic field, it describes a circular path in which: a) Its speed remains constant b) its velocity remains constant c) its momentum remains constant d) its kinetic energy remains constant iv. The focal length of a convex lens of glass of refractive index 1.50 is 2 cm. the focal length of lens when immersed in liquid of refractive index is 1.25 cm will be: a) 1 cm b) 2.5 cm c) 4 cm d) 5 cm v. Total energy of an electron in the ground state of hydrogen atom is -13.6 eV. Its total energy, when hydrogen atom in the first excited state, is: a) +13.6 eV b) +3.4 eV c) -3.4 eV d) -54.4eV B. Answer the following questions briefly and to the point. i. ii. iii. iv. v. vi. vii. [1 x 7] If the current sensitivity of a moving coil galvanometer is increased by 20%, its resistance also increases by 1.5 times. How will the voltage sensitivity of galvanometer be affected? What is the importance of radial magnetic field? Calculate dispersive power of glass, accurately up to three decimal places, from the following data: Refractive index of glass for red colour= 1.60 Refractive index of glass for yellow colour= 1.61 Refractive index of glass for violet colour= 1.62 To convert a pure semiconductor into p-type semiconductor, what impurity is added to it? In a nuclear reactor, what is the function of a moderator? State one condition for obtaining sustained interference of light. According to Bohr, Angular momentum of an orbiting electron is quantised . What is meant by this statement? Section B Question 2 Question 3 Question 4 Question 5 Define relaxation time of the free electrons drifting in a conductor. How is it related to drift velocity? A potentiometer is being used to compare two resistances. The balancing length for a standard resistor R= 10 is 58.3 cm, while that for an unknown resistance X is 68.5 cm. determine the value of X. what would you do if you failed to find a null point on the potentiometer wire AB? A galvanometer with a resistance of 75 produces a full-scale deflection with a current of 5 mA. How can this galvanometer be converted into ammeter which has a range of 0-5 A? Calculate the shortest wavelength of light emitted in the Paschen Series of hydrogen spectrum. Which part of the hydrogen spectrum does it belong to? Or Explain the variation of photoelectric current in a photo-tube with (i) voltage across the tube and (ii) intensity of incident radiation, with graphs. [2] [2] [2] [2] Question 6 Question 7 Question 8 Question 9 Question 10 Question 11 Question 12 What is the advantage of compound microscope over simple microscope? Two straight wires A and B of lengths 10 m and 12 m and carrying currents 4 A and 6 A respectively in opposite directions, lie parallel to each other 3 cm apart. Compute the force on a 10 cm long section of the wire B near its centre. A proton and an electron have same kinetic energy. Which one has smaller de-Broglie wavelength and why? A biconvex lens made of a transparent material of refractive index 1.25 is immersed in water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason. Or You are given three lenses L1 L2 and L3 each of focal length 20 cm. An object is kept at 40 cm in front of L1, as shown. The final real image is formed at the focus T of L3. Find the separations between L1, L2 and L3. [2] [2] [2] [2] How much retarding potential is required to stop the emission of photoelectrons, [2] if the work function of the target material is 1.25 eV and the wavelength of the incident light is 4400 A. In the following nuclear reaction, calculate the energy released in MeV: [2] 2 2 3 1 1H + 1H 2He + 0n 2 (Given: Mass of 1H = 2.015 u, mass of 2He3 = 3.017 u and mass of 1 0n = 1.009 u.) i. ii. Which part of the electromagnetic spectrum does the wavelength 10-10 m corresponds to? Name the part of electromagnetic spectrum which is suitable for radar systems used in aircraft navigations. [2] Section C Question 13 Obtain an expression for electric field E at a point in an equatorial position of an electric dipole. Question 14 Using Kirchhoff s law, determine the value of unknown resistance R in the [3] circuit so that the no current flow through 4 resistance. Also, find the potential difference between points A and D. [3] Question 15 Question 16 A network of five capacitors of C is connected to a 100 V supply, as shown in below figure. Determine (a) the equivalent capacitance of the network (b) the charge on each capacitor. [3] Starting with an expression for refraction at a single spherical surface, obtain Lens Maker s Formula. Or For any prism, prove that: [3] n = + Where the terms have their usual meaning. Question 17 [3] i. Define the terms: (a) Potential Barrier (b) Doping ii. Write an advantage of LED. Question 18 In young s double slit experiment, the slits are 0.2 mm apart and the screen is 1.5 m away. The distance between the central bright fringe and fourth dark fringe is 1.8 cm. Find the wavelength of the light used. Or i. Draw a labelled graph to show variation in intensity of diffracted light with angular position in a single slit diffraction experiment. ii. Question 19 [3] In Fraunhofer diffraction, how is the angular width of the central bright fringe affected when slit separation is increased? i. State one property which is common to all electromagnetic waves. ii. Arrange the following electromagnetic waves in increasing order of their frequencies: Visible light, rays, X rays, micro waves, radio waves, infrared radiations and ultraviolet radiations. [3] Section D Question 20 Draw a labelled diagram of an image formed by a refracting telescope with final [5] image formed at infinity. Derive an expression for its magnifying power with the final image at infinity. Or [3] i. Using Huygens s wave theory, derive Snell s law of refraction. ii. State two advantage of a reflecting type telescope over refracting [2] telescope. Question 21 i. An LCR series circuit with100 resistance is connected to an AC [3] source of 200 V and angular frequency 300 rad s-1. When only capacitance is removed, the current lags behind the voltage by 600. When only the inductance is removed, the current leads the voltage by 600. When only the inductance is removed, the current leads the voltage by 600. Calculate the current and the power dissipated in the LCR circuit. ii. Draw a labelled graph showing the variation in impedance Z of a series [2] LCR circuit with frequency f of alternating emf applied to it. What is the minimum value of this impedance? Or i. ii. Question 22 A circuit containing a 80 mH inductor, a 60 F capacitor and a 15 [3] resistor are connected to a 230 V, 50 Hz supply. Obtain the total power absorbed. A coil has an inductance of 4 H. find the emf induced, if a current of [2] 2.7 A in it is stopped within 1.35 s. Explain the use of a p-n junction diode as a rectifier. Draw the circuit diagram [5] of a full wave rectifier and explain its working. Draw the input and output waveforms. Or [2] i. Explain photodiode with the help of diagram. ii. Draw separate energy band diagrams for conductors, [3] semiconductors and insulators and label each of them. Useful Constants and Relations: 1. 2. 3. Planck s constant (h) Rydberg Constant Charge on electron (e) 6 6 10-34 Js 1.09 x 107 m-1 1 6 10-19C
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