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PHYSICS CLASS XII 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.) 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. Answers to sub parts of the same question must be given in one place only. SECTION A-14 MARKS Question 1 (A) In questions (i) to (vii) given below, choose the correct alternative (a), (b), (c) or (d) for each of the questions. (i) At a point A, the electric field is 500 Volt per metre and potential difference is 3000 Volt due to a point charge. The distance between the point charge and A is. (a) 6m (b) 36m (c) 12m (d) 144m (ii) A current of 10 Ampere exist in a wire of cross sectional area 5 millimetre square with a drift velocity 2 millimetre per second. the number of free electrons in each cube metre of the wire is (a) 2 10^25 (b) 10^25 (c) 625 10^25 (d) 2 10^6 (iii) Assertion: The quantity L/R possesses the dimension of time. Reason: In order to reduce the rate of increase of current through a solenoid, we should increase the time constant. (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 true and Reason is false. (d) Assertion is false and Reason is true. (iv) Assertion : A ray of Light is incident from outside on a glass sphere surrounded by air, this ray may suffer total internal reflection at the second interface. Reason : If a ray of light goes from denser to rarer medium It bends away from the normal. (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 true and Reason is false. (d) Assertion is false and Reason is true. (v) Which level of the singly ionised carbon has the same energy as the ground state energy of hydrogen atom (a) 4 (b) 8 (c) 1 (d) 6 (vi) In a nuclear reactor (a) cadmium rods are used to slow down the neutrons. (b)Moderator is used to slow down the neutrons. (c) Coolant is used to slow down the neutrons. (d) Moderator is used to control the neutrons. (vii) In P type of semiconductor germanium is doped with (a) Gallium (b) boron (c) aluminium (d) all of these (B) (i) (ii) Write the value of the critical angle for a material of refractive index 2. What is called displacement current? (iii) What is the length of telescope for normal adjustment? (iv) Is the electrostatic potential necessarily zero at a point where the electric field strength is zero? Give an example to illustrate your answer. (v) Two electric bulbs are rated 220V- 100 W and 220V - 60 W, which one of these has greater resistance and why? (vi) Which physical quantity has a unit Weber per metre square? Is it a scalar or vector quantity? (vii) Define binding energy. SECTION B-14 MARKS (7 2=14) Question 2 In Young s double slit experiment the distance between the first and the 6th dark fringe is 12.5 centimetre and d = 10^-6 D. Then calculate the wavelength of the light used. OR In Young s double slit experiment If the screen moves 10 centimetre towards the slits. Then the fringe width decreases by one millimetre. Calculate the wavelength of the light used if the separation between the slits is 0.02 millimetre. Question 3 An astronomical telescope is adjusted for the normal adjustment. The magnification is 15 and the length of the telescope is 80 centimetre. Then calculate the focal length of its objective and eyepiece lens. Question 4 Double convex lens has radius of curvature 20 centimetre and 30 centimetre respectively. Find its focal length In a liquid of refractive index 5/3. Given refractive index of glass 3/2. OR Two thin lenses of focal length 15 centimetre and 30 centimetre, First one is convex and the second one is concave are placed in contact. Find the power of the combination. Question 5 Appoint object is placed 36 centimetre from a convex mirror. And a plane mirror is placed in between them in such a way that 2 virtual images formed by the 2 mirrors coincide with each other. If the plane mirror is 24 centimetre from the object, then calculate the curvature of the convex mirror. Question 6 Explain the working of a Full wave rectifier with proper labelled diagram. Question 7 (i) Give the direction in which the induced current flows in the wire loop. (ii) State Faradays law of electromagnetic induction. Question 8 Series LCR circuit. Has L = 2 Henry ,C =32 Microfarad and R = 10 Ohm. At what angular frequency of AC will it resonate? Calculate Q value of the circuit. Section C ( 3 9=27) Question 9 Derive the expression of electric field at the End on point due to an electric dipole with proper labelled diagram. Question 10 Derive condition of balanced Wheatstone Bridge with proper labelled diagram. OR Draw a labelled diagram of a potentiometer circuit used to find the EMF of a cell by comparing it with a known cell. Write the working formula. Question 11 Derive the expression for the magnetic field on the axis of a long straight solenoid with proper labelled diagram. Question 12 Derive the lens equation for a convex lens with proper labelled diagram. Question 13 Two capacitors 2 microfarad and 8 microfarad are connected in series with 300 Volt battery. Calculate the I) II) III) Potential difference across each capacitor. Charge stored in each capacitor. Energy stored in each capacitor. OR A capacitor of 10 microfarad charged with 150 volt battery and then the battery is removed and a dielectric medium of K=6 is introduced completely in between the plates calculate the final potential difference across the plates of the capacitor. Question 14 On interchanging the resistance, the balance point of a metre bridge shifts to the left by 10 centimetre. The resistance of the combination is 1Kohm. How much the resistance on the left gap before the interchange? Question 15 A student is performing an experiment to determine the focal length of a convex lens by Displacement method. The examiner gives some instructions to the student. The student responds to each instruction as per his/her understanding of the experiment. State whether the student's response is correct or incorrect. Give a reason for your answer. (i) EXAMINER: Magnify the image. STUDENT: Moves the object pin towards the lens. (ii) EXAMINER: Remove the parallax. STUDENT : Moves the image pin towards the lens. (iii) EXAMINER: Record the focal length. STUDENT : Writes 10.65 cm. Question 16 Prove the Law of Refraction using Huygens principle with proper labelled diagram. Question 17 Derive the expression for path difference from Young's Double Slit Experiment with proper labelled diagram. SECTION D- (3 5=15 ) MARKS Question 18 (a) An alternating voltage 200 V-50 Hz is applied across the inductor 400/ mH and register 30 ohm. Calculate the RMS current, phase difference between current and voltage and average power. (b) Derive the expression for Average current of an alternating supply. OR (a) Derive the expression for The orbital radius of nth Bohr orbit. (b) Photo electrons are emitted from a metal surface when ultraviolet light of wavelength 300 nm is incident on it. The minimum negative potential required to stop the emission is 0.5 Volt. Then calculate the work function of the metal and threshold frequency. Question 19 (a) Derive the expression for The kinetic energy and potential energy of electron In the nth board orbit Of an atom And hence find out the expression for wave number. (c) Find the recoil speed of an hydrogen atom after it emits a photon while it s electron is having a transition from n= 5 to n=1 orbit. R = 1.097 10^ 7 m-1 and m = 1.67 10^ -27 Kg. OR (a) I) What is called hysteresis loop? II) What is Called coercivity ? III) What is called Meissner's effect? (b) Find the longest and shortest wavelength of the Lyman series of hydrogen Spectra. Question 20 A lens is a portion of a transparent medium bounded by two surfaces and one of these surfaces is essentially spherical. It is used to converge or diverge the light incident on it. Power of a lens is the measure of its ability to converge or diverge the light incident on it. Power of a lens depends on the refractive index of the material of lens With respect to the surrounding medium and the radii of curvature of its two surfaces. (Read the above passage and answer the following questions) A) The power of a convex lens (a) (b) (c) (d) Increases when the lens is dipped in water. Increases when the wavelength of incident light decreases. decreases when another convex lens is placed in contact with it. Increases when the lens is cut into two identical le plan convex lenses. B) The focal length of a concave lens is 40 cm. The power of the lens is (a) 0.025 D (b) 2.5 D (c)-0.025 D (d) - 2.5 D C) The focal length of a concave lens ( g = 1.5) in air is 20 cm What should be the refractive index of the surrounding medium so that the lens behaves as a converging lens of focal al length 60 cm? (a) 1.4 (b) 1.8 (c) 1.7 (d) 1.2 D) Beams of red light, blue light, yellow light and violet light are incident on a convex lens, one-by-one. Which one of a con them converges nearest to the lens ? (a) Blue light (b) Violet light (c) Red light (d) Yellow light E) A beam of light coming parallel to the principal axis of a convex lens L of focal length 15 cm is incident on it. Another convex lens L2 of focal length 25 cm is placed coaxially at a distance d from L1. For the final image to be formed at infinity, the value of d should be (a) 10 cm (b) 15 cm (c) 25 cm (d) 40 cm
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