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The Assembly of God Church School 125/1 Park Street, Kolkata-700017 Pre Board Examination For ISC 2018 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 in 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.Choose the correct alternative (a), (b), (c) or (d) for each of the questions given below: [5 1] (i) Biot-Savart s law in vector form is: 1|Pa ge (a) = B o 4 d l( I X r ) r3 o 4 = (b) B I (d l X r ) r3 (c) B = o 4 I (r X dl) r3 (d) B = o 4 I (d l X r ) r2 (ii) Which one of the following graphs in the figure represents variation of reactance Xc of a capacitor with frequency f of an a.c supply. (a) (b) (c) (d) (iii) The expression for displacement current is: o d E (a) ID = (b) ID = o 2 dt d E d E (c) ID = 2 o (d) ID = dt dt d E dt (iv) Which one of the following does not support wave nature of light? (a) Interference (b) Diffraction (c) Polarization (d) Photoelectric effect (v) A n-p-n transistor is preferred to a p-n-p transistor because: (a) it can handle large power (b) it is cheap and easily available (c) it involves smaller energy loss (d) in it majority charge carriers are electrons. (B) Answer the following questions briefly and to the point: [7X1] 2|Pa ge (i) What will be the effect on the position of zero deflection in a potentiometer when the e.m.f of the experimental cell be decreased. (ii) Write the formula of equivalent e.m.f and internal resistance of two cells of unequal e.m.f connected in parallel. (iii) The permeability of Bismuth is 0.9983. To which class of magnetic materials, does bismuth belong? (iv) What is the amount of work done by a uniform magnetic field on a moving charge? (v) An equivalence lens of glass having focal length of 10 cm is split into two identical plano convex lenses each having focal length f1 as shown in the figures: Find the value of f1 (vi) Why are halos observed around the sun or moon? (vii) What is the principle of operation of a nuclear reactor? Section B Answer all questions Question 2 What is the difference between thermal velocity and drift velocity of free electrons in metals? Question 3 Figure shows a current loop having two circular segments and joined by two radial lines. [2] [2] Find the magnetic field at the centre O Question 4 Define the terms Magnetic Dip and Magnetic Declination with the help of relevant diagrams. Question 5 Current in a circuit falls steadily from 5.0 A to 0.0 A in 100 ms. If an average e.m.f of 200 V is induced, calculate the self-inductance of the circuit. OR State the underlying principle of a transformer. How is the large scale transmission of electric energy over long distance done with the use of transformer? Question 6 i) What is displacement current due to? ii) Name the constituent radiation of electromagnetic spectrum which is similar to the radiation emitted during the decay of radioactive nuclei. Question 7 A convex lens and a plane mirror are kept at a distance of 10 cm as shown in the diagram. Parallel rays incident on the convex lens after [2] [2] [2] [2] 3|Pa ge reflection from the mirror formed an image at the optical centre. Complete the ray diagram and find out the focal length of the lens. Question 8 [2] Define the term angular dispersion. Draw the path of ray of light passing through prism and mark angular dispersion on it. Question 9 [2] An alpha particle and a proton are accelerated through the same potential difference. Calculate the ratio of linear momenta acquired by the two. OR Draw a graph showing the variation of photoelectric current (I) with anode potential (V) for different intensities of incident radiation. Name the characteristic of the incident radiation that is kept constant in this experiment. Question 10 [2] The energy of the electron in the ground state of hydrogen atom is -13.6 eV. i) What does the negative sign signify? ii) How much energy is required to take an electron in this atom from the ground state to the first excited state? Question 11 [2] Draw a graph sketch showing the variation of binding energy per nucleon with mass number of various nuclei. Label the graph and state the region where the nuclei are most stable. Question 12 [2] Complete the following block diagram depicting the essential elements of a basic communication system and define them. Section C Answer all questions. Question 13 [3] Using Gauss s theorem calculate the electric field intensity due to an infinite plane sheet of charge of uniform surface charge density (at a distance r form the sheet ). Question 14 [3] Deduce an expression for the electric potential due to an electric dipole at any point on its axis. Mention one contrasting feature of electric potential of a dipole at a point as compared to that due to single charge. OR A parallel plate capacitor of plate separation d is charged to a potential difference V. A dielectric slab of thickness d and dielectric constant K is introduced between the plates while the battery remains connected to the plates. Find the ratio of energy stored in the capacitor after and before the dielectric is introduced. Give the physical explanation for this change in stored energy. Question 15 [3] 4|Pa ge As shown in the diagram alongside, a variable rheostat of 2k is used to control the potential difference across a 500 load. i) If the resistance of AB is 500 , what is the potential difference across the load? ii) If the load is removed, what should be the resistance of BC to get 40 V between B and C. Question 16 [3] When two thin lenses of focal lengths f1 and f2 are placed coaxially and in contact, prove 1 1 1 = + that their combined focal length f is given by f f1 f2 OR Derive the relation between the object distance u, image distance v and focal length f for a concave mirror, when it forms a real image of an object of finite size. Question 17 [3] State Brewster s law of polarization of light. How will you identify with the help of an experiment, whether a given beam of light is of polarized light or of unpolarised light ? Question 18 [3] Define the term decay constant. Derive the relation between decay constant and half-life period of a radioactive sample. Question 19 [3] Draw a labelled diagram of a full wave rectifier circuit. Show the input and output wave forms. Section D Answer all questions. Question 20 [5] a) A 25 F capacitor, 0.1 H inductor and 25 resistance are connected in series with an a.c source whose e.m.f is given by V = 310 sin 314t Volt. What is the i) frequency of the a.c? ii) impedance of the circuit? iii) current in the circuit? and iv) phase angle? What additional value of inductance will make the impedance of the circuit minimum? OR b) 1) In a step down transformer having primary to secondary turn ratio 20:1, the input voltage applied is 250 V and output current is 8 A. Assuming 100% efficiency, calculate i) voltage across secondary coil. ii) current in primary coil. iii) power output. 2) An electric lamp which runs at 40 V and consumes 10 A current is connected to a.c mains at 100V, 50 cycles per second. Calculate the inductance of the required choke. Question 21 [5] i) With the help of the ray diagram obtain the expression for the magnifying power of a simple microscope when the image is formed at the least distance of distinct vision (D) ii) A telescope objective of focal length 1m forms a real image of the moon 0.92 cm in 5|Pa ge diameter. Calculate the diameter of the moon taking its mean distance from the earth to be 38X104 km. If the telescope uses an eye piece of 5 cm focal length, what would be the distance between the two lenses for the final image to be formed at infinity. OR State Huygen s principle and explain the phenomenon of reflection on its basis. In Young s double slit experiment, what is meant by fringe width? State two ways to of increasing the fringe width without changing the source of light. Question 22 [5] Draw the input and output characteristic of transistor when connected in common emitter mode. For a common emitter transistor amplifier, the audio signal voltage across the collector resistance of 2K is 2V. If the current amplification factor of the transistor is 100, calculate i) input signal voltage ii) base current and iii) power gain. Given that the value of the base resistance is 1k OR Show how you will obtain an AND gate using only NAND gates. Draw the truth table for this arrangement of gates. What do you mean by i) Analog circuits ii) Integrated circuits Useful Constants and Relations 1. Mass of electron (mp) = 9.1X 10-31 Kg. 2. Charge of an electron (-e) = -1.6 X 10 -19 C. 3. Rydberg constant ( R) = 1.097 X 107 m-1. 4. Plank s constants (h) = 6.62 X 10-34Js. 5. Avogadro s number (N) = 6.02 X 1023 mole-1. 6. Speed of light in vacuum ( c ) = 3.00 X 10 8 ms-1. 7. Permittivity in vacuum ( o )= 8.88 X 10-`12 Fm-1. 8. Permeability of vacuum ( o ) = 4 X 10 -7 Hm-1. 9. Unified atomic mass ( u) = 931 MeV. 6|Pa ge
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