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ISC Class XII Prelims 2026 : Physics

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THE OBERAI SCHOOL OF INTEGRATED STUDIES PRE-BOARD 1 EXAMINATION 2025-26 CLASS: XII SUBJECT: PHYSICS TEACHER: GRM 1. 2. 3. 4. 5. MAX. MARKS: 70 DURATION: 3 hr. Read these instructions first This paper is divided into four sections: A, B, C, and D. 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. 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 x 1] (i) Three cells having emfs 3V, 1V and 4V are connected as shown in Figure 1 below. Their internal resistances are negligible. The equivalent emf between the terminals P and Q is (a) 0V (b) 4V (c) 6V (d) 8V (ii) Magnetic field inside a long current carrying solenoid is Bo. If each of the current (I) and the number of turns (N) in this solenoid are doubled, magnetic field B, inside it becomes: (a) 1 = 0 4 (b) 1 = 0 2 (c) 1 = 0 Page 1 of 12 (d) 1 = 4 0 (iii) Study the data given below. Material Relative Permeability P 0.9999906 Q 1.000021 R 600 S 280,000 The material most suitable to make permanent magnets is: (a) P (b) Q (c) R (d) S (iv) de Broglie wavelength of moving particles does not depend on their: (a) velocity. (b) mass. (c) size. (d) kinetic energy. (v) If the temperature of a block of a semiconducting material is increased, its resistance: (a) decreases. (c) remains constant. (b) increases. (d) first increases and then decreases. (vi) The maximum kinetic energy of an electron in the photoelectric effect is NOT dependent on: (a) Work function of metal (b) Energy of a photon. (c) Color of light. (d) Intensity of photon. (vii) Given below are two statements marked, Assertion and Reason. Read the two statements and choose the correct option. Assertion: If critical angle of glass-air pair ( = 3/2) is 1 , and that of water-air pair ( = 4/3) is 2 , then the critical angle for the water-glass pair will lie between 1 and 2 . Reason: A medium is optically denser if its refractive index is greater. (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. (B) Answer the following questions briefly: [7 x 1] (i) An ideal Moving Coil Galvanometer (MCG) cannot be used as an ammeter to measure the value of current in a given circuit. Why? (ii) GaAs is a semiconductor, and its resistivity depends on temperature. Show graphically how resistivity ( ) of GaAs varies with the temperature (T). Page 2 of 12 (iii) If radius of first Bohr orbit of an electron in hydrogen atom is 0.05nm, calculate the radius of its third orbit. (iv) Why Zenor diode is called a Voltage Regulator? (v) State any one difference in the energy band diagram of a semiconductor and that of an insulator. (vi) Figure 2 below shows an optical pin kept in front of a lens mirror combination. It is found that the final image formed by the lens mirror combination coincides with the object pin. Draw the ray diagram showing the formation of the final image (I). Figure 2 (vii) Represent diagrammatically how the incident planar wavefronts of wavelength pass through an aperture of size d, when d is approximately equal to . Section-B- (14 Marks) Question 2 [2] (i) Find out the magnitude of resistance X in the circuit shown in Figure 3 below when no current flows through the 5 resistor. Figure 3 Page 3 of 12 OR (ii) Two resistors, 1 = 1 and 2 = 2 are connected in parallel. If the power developed in the resistor 1 is 4W, find the power developed in the resistor 2 . Question 3 [2] (a) Three identical point charges, each equal to Q, are situated at the three vertices of an equilateral triangle MNO, having each side equal to l. How much electrostatic potential energy is stored in this system? (b) A closely wound solenoid of 800 turns and an area of cross-section 2.5 10 4 2 carries a current of 3.0 A. What is its associated magnetic moment? Question 4 [2] (i) Two straight infinitely long wires are fixed in space such that the current in the left wire is 2 A and directed out of the plane of the page, and the current in the right wire is 3 A and directed into the plane of the page. In which region(s) is/are there point(s) on the x-axis at which the magnetic field is zero due to these current-carrying wires? Justify your answer. OR (ii) Two Gaussian surfaces, 1 and 2 are shown in Figure 4 below. (a) What is the ratio of electric flux emerging out of these two surfaces? (b) What happens to this ratio if the space occupied by each Gaussian surface is doubled? Question 5 [2] The intensity of monochromatic UV light incident on the photocathode of a photoelectric cell is increased. What is its effect on: Page 4 of 12 (i) photocurrent? (ii) kinetic energy of the most energetic photoelectron? Question 6 [2] (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 [2] In satellite communication, electromagnetic waves are used to transmit signals between Earth and the satellite. (a) Which Electromagnetic Wave is used in Satellite Communication? (b) Write one property of this Electromagnetic Wave common to other Electromagnetic waves in a vacuum? Question 8 [2] Two concave lenses A and B, each of focal length 8.0 cm, are arranged coaxially 16 cm apart as shown in the figure. As object P is placed at a distance of 4.0 cm from lens A. Find the position and nature of the final image formed. Section-C - (27 Marks) Question 9 [3] "A uniformly charged conducting spherical shell for the points outside the shell behaves as if the entire charge of the shell is concentrated at its centre." Show this with the help of a proper diagram and verify this statement. Page 5 of 12 Question 10 [3] (i) The potentiometer has a driver cell D of emf 6V and an internal resistance of 2 , as shown in Figure 4 below. Figure 4 Calculate: (a) The current flowing through the potentiometer wire AB. (b) The potential gradient across the wire AB. OR (ii) Using Kirchhoff s laws of electrical networks, calculate the current 1 , 2 and 3 . Question 11 [3] Using Biot-Savart law, show that magnetic flux density 'B' at the centre of a current-carrying circular coil of radius R is given by: B= where the terms have their usual meaning Page 6 of 12 Question 12 [3] (i) Figure 5 below shows a metallic rod PQ resting on two parallel, thick, and smooth metallic rails CS and DR. There is a uniform magnetic field B, which is perpendicular to the rails. Figure 5 (a) How much magnetic flux is linked to the closed circuit PQRS? (b) If the rod PQ is moved on the rails with a constant velocity v, how much emf will be induced in it? (c) How much power has to be spent by an external agency to maintain the motion of the rod PQ on the rails? Write your answer in terms of B, l, v, and the resistance R of the rod PQ. OR (ii) Using Huygen s wave theory, show that (for refraction of light): = Constant where terms have their usual meaning. You must draw a neat and labelled diagram Question 13 [3] th In Young's double slit experiment, the distance of the 4 bright fringe from the center of the interference pattern is 1.5 mm. The distance between the slits and the screen is 1.5 m and the wavelength of the light used is 500 nm. Calculate the distance between the two slits. Question 14 [3] A compound microscope consists of two convex lenses having focal lengths of 1.5 cm and 5 cm. When an object is kept at a distance of 1.6 cm from the objective, the final image is virtual and lies at a distance of 25 cm from Page 7 of 12 the eyepiece. Calculate the magnifying power of the compound microscope in this setup. Question 15 [3] Gopal was performing a double slit experiment to determine the wavelength of monochromatic light emitted by a laser. The examiner asked him some questions based on that experiment. Gopal answered those questions as per his understanding of the experiment. State whether his answers are correct. If incorrect, write the correct answer. (i) Examiner: What is meant by fringe width? Gopal: It is the distance between the centres of a bright band and the adjacent dark band. (ii) Examiner: What happens to fringe width if blue light is replaced with red light? Gopal: Fringe width increases. (iii) Examiner: What will happen to the interference pattern if one of the slits is closed? Gopal: The screen will be uniformly illuminated. Question 16 [3] (i) Two identical thin convex lenses, each having an optical power of 2D, are kept in contact with each other. Calculate the focal length of the combination. (ii) Figure 6 below shows a ray of monochromatic light incident normally on the first surface AB of an equilateral glass prism ABC, whose adjacent surface AC is covered with a transparent liquid. If the emergent ray grazes the surface AC, calculate the refractive index of the liquid. Figure 6 Page 8 of 12 Question 17 [3] The graph of stopping potential ( 0) versus frequency (f) of incident UV radiation incident on a metallic surface is shown below. Study the graph and answer the questions that follow. (i) Find the value of the work function of the metal. (ii) Determine the value of Planck's constant 'h'. Section-D - (15 Marks) Question 18 [5] (i) (a) A 220V, 50Hz ac source is connected to a coil having a coefficient of self-inductance of 1 H and a resistance of 400 . Calculate: (1) rms value of the current flowing through the coil. (2) What is the phase difference between the supply voltage and the current? (3) What is the power factor for this circuit? (b) Draw a labelled graph showing the variation of impedance (Z) of a series LCR circuit Vs frequency (f) of the ac supply. Mark the resonant frequency as . OR (ii) (a) When the current flowing through a solenoid increases from 0 to 10A in 0.2s, an emf of 20V is induced in it. Calculate the coefficient of self-inductance (L) of the solenoid. Page 9 of 12 (b) An ideal step-up transformer is shown in Figure 7 below: Figure 7 (1) Calculate its turns ratio. (2) Which of the windings: primary or secondary, carries a larger current? Give a reason for your answer. (c) An ac source generates an emf 'e' given by e = 311 Sin (100 t)V Calculate the rms value of the emf generated by the source. Question 19 (i) (a) Find the binding energy per nucleon based on the information given below. Mass (u) 235.04390 1.0073 1.0087 235 92 Mass of neutral Mass of a proton Mass of a neutron (b) Find the energy released during the following fission reaction. 235 92 235 U Kr 143 Ba 1 n 90 + 10 236 92 90 1 36 + 143 56 + 3 0 Mass (u) 235.0439 89.9195 142.9206 1.0087 (c) During Rutherford s gold foil experiment, it was observed that most of the alpha particles did not deflect; however, some showed a deflection of 180 . What hypothesis was made to justify the deflection of an alpha particle by 180 ? OR Page 10 of 12 [5] (ii) (a) Define unified atomic mass unit. (b) Calculate its energy equivalent (c) In an atom X, electrons absorb the energy from an external source. This energy "excites" the electrons from a lower-energy level to a higher-energy level around the nucleus of the atom. When electrons return to the ground state, they emit photons. The figure below is the energy level diagram of atom X with three energy levels. 1 = 0.00eV, 2 = 1.78eV and 3 = 2.95eV. The ground state is considered 0 eV for reference. The transition of electrons takes place between levels 1 and 2 . What wavelength of radiation is needed to excite the atom to energy level E2 from E1? Question 20 Read the following passage and answer the questions that follow. [5] The discovery of semiconducting devices has revolutionised the field of electronics. Earlier, a computer known as first generation computer was as big as a room! The radio sets used to be so bulky that they could not be taken on a picnic. Semiconducting devices, being light and compact, have changed all that. Now, computers have become so small in size that they are called laptops and palm tops. Semiconducting devices, which are widely used in electronic gadgets like a television, a radio, an amplifier, etc, are diodes, transistors, FETs, ICs, logic gates, etc. Diodes are of different types, like light-emitting diodes, photodiodes, Zener diodes, etc. They perform various functions like converting an AC voltage to de voltage, emitting light, etc. (i) State any one advantage of a semiconducting device over an old electronic device. (ii) What energy transformation takes place in an LED? (iii) What is the function of a rectifier? (iv) The Figure below shows a rectifier. Page 11 of 12 In case of the rectifier shown above, draw the graphs to show the waveform of: (a) input voltage. (b) output voltage. ___________________________________ X X X X ___________________________________________ Page 12 of 12

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