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SRI AUROBINDO INTERNATIONAL SCHOOL QUARTERLY EXAMINATION SEPTEMBER 2018 CLASS: XII MAX. MARKS: 70 SUB: PHYSICS TIME: 3 HRS (THEORY) (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. Question number 1 is of twenty marks, with three subparts, all of which are compulsory. Question numbers 2 to 8 carry 2 marks each with two questions having internal choice. Question numbers 9 to 15 carry 3 marks each with two questions having internal choice. Question numbers 16 to 18 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 simple scientific calculator without a programmable memory may be used for calculations. PART I Answer all questions. Question 1 (A) Attempt the following questions by choosing the correct alternative (i), (ii), (iii) or (iv). [5X1=5] i) The electric intensity, at a point on the axis of a short electric dipole, depends on the distance y of the point from the dipole as a) y b) y3 c) y-1 d) y-3 ii) A resistor is stretched 2% in its length. The percentage change in its resistance would be a) 2% positive b) 4% positive c) 2% negative iii) Five resistors are connected as shown in figure . the effective resistnace across AB 1|Page d) 4% negative iv) Work function of a photoelectric material is 3.3eV. What will be its threshold frequency? a) 8 X 1010 Hz b) 8 X 1014 Hz c) 8 X 1020 Hz d) 8 X 1025Hz v) The doping of the base of transistor is a) same as that of emitter b) same as that of collector c) less than that of emitter or collector d) greater than that of emitter or collector B) Answer the following questions briefly and to the point. 7X1 = 7m i) Can two equipotential surfaces can intersect each other? Give reason. ii) What is the ratio of intensities of electric fields of axial and broad side position? iii) What is the resistance produced by any capacitor in a d.c circuit? iv) In hydrogen atom, the electron moves in an orbit of radius 0.5 A0 with a speed 2.2 X 106 m/s. Find the current? v) A ray of light which is incident in air on the surface of a glass block is deviated through 150 . The angle of incidence in air is 600 . What is the angle of refraction in glass? vi) What are matter waves? vii) The radius of hydrogen nucleus is 1.5 X 10-15 m. What is the radius of helium nucleus? SECTION - B ANSWER ALL QUESTIONS. Question 2) Two identical charges placed at two different origins, compare the ratio of the two electric field intensities due to two charges at distances of r1 and r2. 2m Question 3) Calculate the equivalent capacitance of the combination the points P and Q as shown in fig. 2m Question 4) What is the colour band of resistor having a resistance of 470 ohm and a tolerance of 5%. 2|Page Question 5) Derive an expression for the energy stored in a capacitor of capacitance C, charged to a potential V. OR A charge of 5C is placed at the centre of a circle of radius 10cm, as shown. Calculate the work done in moving a charge of 2C from point A and B on the circumference of the circle. Give reason (2) Question 6) Draw a labeled circuit diagram of a potentiometer to compare emf s of two cells. Write working formula. (2) Question 7) An object is placed in front of a convex mirror of focal length 30cm. The image formed is a quarter size of the object. Find the location of the object. 2m Question 8) a) How does the focal length of a convex lens change with increase in wavelength of incident light? (2) Question 9) Deduce the de-Broglie wave length of electrons accelerated by a potential of V volt. (2) (or) Draw a graph showing the variation of maximum kinetic energy versus frequency of incident light. What is the shape and slope of the graph. Question 10) With reference to the photoelectric effect, What is meant by threshold wavelength and work function. (2) Question 11) Draw a labeled circuit diagram of full wave rectifier with input and output signals. 2m Question 12) Write frequency band for: i) AM broadcast ii) Cellular mobiles signals (2) 3|Page Section C Answer ALL questions Question 13) Define electric field at a point. Explain the statement relative permittivity of water is 81 . (3) Question 14) State Gauss theorem . Draw a graph showing the variation of electric field, as one moves from centre of a charged metal ball to a point on its surface and then to a far off outside point. (3) (OR) Two point charges 10 C are placed 5mm apart , forming an electric dipole. Compute electric field at a point on the axis of the axis of the dipole 15cm away from the centre on a line passing through the centre and normal to the axis of the dipole. Question 15) Calculate the current flowing in each cell in the given circuit, Also calculate the potential difference across the ends of each cell. Question 16) a ) Calculate the critical angle for glass-air interface if a ray of light in air incident on the interface is deviated through 150 when its angle of incidence is 400 (3) (OR) b) The figure shows the positions of object O, two lenses , a plane mirror and final image I coincides with the object. The focal length of convex lens is 20cm, Calculate the focal length of concave lens. 4|Page Question 17) Draw energy level diagrams of hydrogen atom. Show transitions corresponding to I) Lyman ii) Balmer series. In which regions of the spectrum do these series lie? 3m Question 18) State radioactive decay law. Derive the formula N = N0 e- t for radioactive decay. 3m Question 19) Discuss the input and output characteristics of common emitter transistor. 3m SECTION - D ANSWER ALL QUESTIONS Question 20) a) Use Gauss law to show that electric field does not exist inside a charged conductor. ( 2m) b) Two point charges +2e and -2e are situated at a distance of 2.4A0 from each other and constitute an electric dipole. This dipole is placed in a uniform electric field of 4 X 105 v/m. Calculate : i) Electric dipole moment , ii) potential energy of the dipole in equilibrium position iii) work done in rotating the dipole through 1800 from the equilibrium position. (3m) (OR) a)What is the area of the plates of a 2F parallel plate capacitor with plate separation of 0.5cm? Why do ordinary capacitors have capacitances of the order of microfarads? (2m) b) Ohms law may also be stated by the equation j = E . What do the symbols j, and E stand for? Show that the combination of units on the two sides are identical. (2m) Question 21) 5|Page a) Draw a neat labeled diagram of reflecting telescope, write expression for magnifying power. (3m) b) ) Calculate the critical angle for glass and water pair? Given refractive index of glass is 1.62 and that of water is 1.33 2m (OR) a) Starting with expression for refraction at a single spherical surface, Obtain an expression for len s formula. 3m b) Calculate dispersive power of glass, accurately upto three decimal places,from the following data: Refractive index of glass for red colour is 1.60, refractive index of glass for yellow colour is 1.61 and violet is 1.62. (2m) Question 22) a) i) Explain why the electric conductivity of a pure semiconductor increases on heating. ii) A p-n junction diode is connected to a 220V (rms) AC in series with a capacitor. What is the voltage V across the capacitor? iii) Calculate the current gain of a transistor if the current gain is 0.98. (3m) b) A 2N 3298 transistor has a typical dc of 90. If the collector current is equal to 15mA, Calculate (approximate values): i) base current ii) dc (2m) (OR) 6|Page a) Prepare a truth table for the combination of gates shown in figure below. 2m b) Draw circuit diagram of transistor as an amplifier with input and out put signals.. 3m 7|Page
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