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ISC Class XII Prelims 2025 : Physics (St. Patrick's Academy, Bangalore)

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ST. PATRICK S ACADEMY, BENGALURU PRE-BOARD II EXAMINATION- 2024-25 ISC PHYSICS Date : 24/01/2025 (Theory) Time allowed: three hours Max.Marks:70 Answers to this paper must be written on the answer sheets provided separately. You will not be allowed to write during the first 15 minutes. This time is to be spent in reading the question paper. The time given at the head of this paper is the time allowed for writing the answers. This paper contains 06 printed sheets 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) Choose the correct alternative for the following given questions i. A capacitor is charged by a battery. Between the plates, during process of charging: a) only conduction current exists. b) only displacement current exists. c) both displacement current and conduction current exist. d) no current exists. ii. If two electrons are each 1.5 x 10-10 m from a proton, as shown in Figure, magnitude of the net electric force they will exert on the proton is 1 iii. iv. v. vi. vii. a) 1.97 x 10-8 N b) 3.83 x 10-8 N c) 4.63 x 10-8 N d) 2.73 x 10-8 N The dimension of where is the permittivity of free space and E is the electric field, is a) [ML-1T-2] b) [ML2T-2] c) [MLT-1] d) [ML2T-1] The layered lens is made of two kinds of glass. A point source of light is placed on its principal axis. If the reflections from the boundaries between layers are ignored, the lens will form a) no image at all b) two images c) infinite images d) only one image Assertion (A): The de Broglie wavelength of a molecule varies inversely as the square root of temperature. Reason (R): The root mean square velocity of the molecule depends on the temperature. a) Both A and R are true and R is the correct explanation of A. b) Both A and R are true but R is not the correct explanation of A. c) A is true but R is false. d) A is false but R is true. An electric bulb marked 40 W - 200 V is used in a circuit of supply voltage 100 V. Now its power is: a) 10 W b) 20 W c) 40 W d) 100 W Pick the correct statement. 1. The glass rod gives protons to silk when they are rubbed against each other. 2. The glass rod gives electrons to silk when they are rubbed against each other. 3. The glass rod gains protons from silk when they are rubbed against each other. 4. The glass rod gains electrons from silk when they are rubbed against each other. a) Option (1) b) Option (4) c) Option (3) d) Option (2) B) Answer the following questions to the point and brief. i. Write the modified form of amperes circuit law? ii. Write the equation that corresponds to pair annihilation? iii. Sketch the electric field lines for a charge with q > 0 iv. What will be the path of a charged particle moving in a region of crossed (or transverse) uniform electrostatic and magnetic fields with initial velocity zero? v. Name the conservation principle involved in KVL? 2 vi. An electron is moving round the nucleus of a hydrogen atom in a circular orbit of radius r. Find the Coulomb force between the charges. How can you increase the capacitance of a parallel plate capacitor? vii. SECTION B (14 Marks) Question 2 Two crystals C1 and C2, made of pure silicon, are doped with arsenic and aluminium respectively. i. ii. Identify the extrinsic semiconductors so formed. Why is doping of intrinsic semiconductors necessary? Question 3 A uniform magnetic field gets modified as shown below, when two specimens X and Y are placed in it. i. ii. Identify the two specimens X and Y. State the reason for the behaviour of the field lines in X and Y Question 4 Draw a graph showing the variation of binding energy per nucleon with mass number of different nuclei. Question 5 Consider the following electromagnetic waves -rays Microwaves X-rays Radio waves Name the radiation(s) which is/are that doesn t deflect either in electric or magnetic field. Give reason for your answer. Question 6 Distinguish between the Zenor breakdown and Avalanche breakdown process for a semiconductor diode. 3 Question 7 a) Calculate the value of shunt resistance required to convert a 1 mA, 20 galvanometer into an ammeter with a range of 0-50 mA? (Or) b) A long straight wire carrying a current of 30 A is placed in an external uniform magnetic field of 4.0 10-4 T parallel to the current. Find the magnitude of the resultant magnetic field at a point 2.0 cm away from the wire. Question 8 In the following circuits which one of the two diodes is forward biased and which is reverse biased? SECTION C (27 Marks) Question 9 a) One face of a prism of refracting angle 300 and refractive index 1.414 is silvered. At what angle must a ray of light fall on the un silvered surface so that after refraction into the prism and reflection at silvered surface it retraces its path? (Or) b) What is the position and nature of the image formed by lens combination shown in figure. (f1, f2 are focal lengths) Question 10 A beam of monochromatic radiation is incident on a photosensitive surface. Answer the following questions giving reasons. i. ii. Do the emitted photoelectrons have the same kinetic energy? Does the kinetic energy of the emitted electrons depend on the intensity of incident radiation? 4 iii. On what factors does the number of emitted photoelectrons depend Question 11 i. ii. State Ampere's circuital law. Use the above Ampere s Circuit law to derive formula for the magnetic field due to an infinitely long straight current carrying wire. Question 12 The ground state energy of hydrogen atom is -13.6 eV. If an electron makes a transition from an energy level -1.51 eV to - 3.4 eV, then calculate the wavelength of the spectral line emitted and name the series of hydrogen spectrum to which it belongs. Question 13 Using Bohr's total postulates, derive the expression for the radius of the Bohr orbit in the stationary states of hydrogen atom. Question 14 i. ii. Write the relationship between mobility and drift velocity in a current carrying conductor. Two aluminium wires have their lengths in the ratio 2: 3 and radii in the ratio 1: 3. These are connected in parallel across a battery of emf E and of negligible internal resistance. Find the ratio of drift velocities of the electrons in the two wires. Question 15 With the help of a diagram, State the principle of a device which changes a low voltage into a high voltage but does not violate the law of conservation of energy. Give any one reason why the device may not be 100% efficient. Question 16 i. ii. iii. State the condition for resonance to occur in series LCR alternate current circuit. Write an expression for resonant frequency. Draw a plot showing the variation of the peak current (im) with frequency of the A.C, source used. Question 17 a) A small flat search coil of area 5 cm2 with 140 closely wound turns is placed between the poles of a powerful magnet producing magnetic field 0.09 T and then quickly removed out of the field region. Calculate i. Change of magnetic flux through the coil, and ii. Emf induced in the coil. (Or) b) An electron in a hydrogen atom makes transitions from orbits of higher energies to orbits of lower energies. i. When will such transitions result in (a) Lyman (b) Balmer series. ii. Find the ratio of the longest wavelength in Lyman series to the shortest wavelength in Balmer series. 5 SECTION- D (15 Marks) Question 18 a) i. ii. A compound microscope consists of an objective lens of focal length 2 cm and an eyepiece of focal length 6.25 cm separated by a distance of 15 cm. How far from the objective should an object be placed in order to obtain the final image at the least distance of distinct vision (25 cm). a. Calculate the magnifying power of the microscope. b. Calculate the length of the microscope tube. (Or) b) i. Draw graphs showing the variations of inductive reactance and capacitive reactance with the frequency of the applied ac source. Draw the phasor diagram for a series RC circuit connected to an ac source. An alternating voltage of 220 V is applied across a device X, a current of 0.25 A flows, which lag behind the applied voltage in phase by 2 radian. If the same voltage ii. is applied across another device Y, the same current flows but now it is in phase with the applied voltage. a. Name the devices X and Y. b. Calculate the current flowing in the circuit when the same voltage is applied across the series combination of X and Y. Question 19 a) i. Draw a labelled graph showing the variation of intensity of diffracted light woth angular position in a single slit experiment. For observing Fraunhofer diffraction from single slit, what type of wavefront should be incident on the slit In a diffraction pattern due to a single slit, how will the angular width of central maximum change, if a. Orange light is used in place of green light b. the screen is moved closer to the slit c. the slit width is decreased? ii. iii. (Or) b) i. ii. Write the conditions to observe the sustainable interference? In a Young's double experiment, the slits are 1.5 mm apart. When the slits are illuminated by a monochromatic light source and the screen is kept 1 m apart from the slits, width of 10 fringes is measured as 3.93 mm. Calculate a. the wavelength of light used. b. What will be the width of 10 fringes when the distance between the slits and the screen is increased by 0.5 m. The source of light used remains the same. 6 Question 20 Two students X and Y perform an experiment on potentiometer separately using the circuit shown in figure Keeping other things unchanged. i. ii. Sate the principle of potentiometer Guess the purpose or application of connecting the potentiometer of 100 cm in the above circuit? iii. X : increases the value of resistance R Y : decreases the value of resistance S in the set up. How would these changes affect the position of null point in each case and why? _________________________________________________ This paper ends here Vasudaiva Kutumbakam - The world is One family 7

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