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A G C School, Rupaidiha Preliminary Examination 2020-2021 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 into Four 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 Question 1 (A) Choose the correct alternative (a), (b), (c) or (d) for each of the questions given below: (i) Two point charges A and B, having charges +Q and Q respectively, are placed at certain distance apart and force acting between them is F. If 25% charge of A is transferred to B, then force between the charges become: (a) (b) 16 9 4 3 (c) 9 (d) 16 (ii) A conductor is charged to a potential V by imparting a charge q, the variation of q with V is represented by figure: (iii) A 1 m metallic rod is being revolved in a vertical plane about one of its ends with an angular velocity of 5 rad/s. The earth s horizontal magnetic field is 0.2 x 10-4 T. The emf induced across the ends of the rod will be : (a) 5 mV (b) 5 x 10 4 V (c) 50 mV (d) 50 V (iv) The first diffraction minima due to a single slit are observed at 30 for light of wavelength 6500 A . The width of the slit is : (a) 3250 A (b) 1.3 micron (c) 6.5 x 10 4 mm (d) 2.6 x 10 4 cm (v) A p-n junction diode cannot be used: (a) as rectifier (b) for detecting light intensity (c) for obtaining light radiation (d) as an amplifier. (B) Answer the following questions briefly and to the point: (a) Define equipotential surface. (b) An isolated spherical conductor has radius 3 cm. If charge on the sphere is 10 -9C, find the energy density at the surface. (c) How can a moving coil galvanometer be converted into an ammeter? (d) Define flux density. Write its SI unit. (e) Write the expression for power of combination of two lenses placed in contact. (f) What is the shape of fringes in Young s experiment? (g) State the energy equivalent to 1 a.m.u mass. Section B Question 2 [2] Radius of copper wire is decreased by 0.1% by stretching it Find the percentage change in the resistance of the wire. OR Two identical bulbs of 500 W, 220 V are connected to a supply of 110 V (i) in parallel (ii) in series combination. Find in each case, the energy consumed in bulbs per second. Question 3 [2] Write an expression for force on a moving charge in a combined magnetic and electric fields. Question 4 [2] Derive an expression for the magnetic dipole moment of a revolving electron. Question 5 [2] Distance between two long straight wires is 10 cm. 5 A and 15 A current are flowing in the wires in the same direction. Find the magnitude and direction of magnetic field at point P at a distance of 4 cm from the wire carry 5 A current. OR Find the self inductance of a solenoid whose length is 25 cm, cross-sectional area is 4 cm2. And number of turns is 300. If the current is decreasing at the rate of 50 ampere/second in this solenoid, then find induced e.m.f. Question 6 [2] A radio can tune station in the 7.5 MHz to 12 MHz band. What is the corresponding wavelength band? How the radio-waves are being produced artificially? Question 7 [2] A lens of refractive index ( ) and focal length in air f, is placed in a liquid of refractive index ( ), its focal length becomes f . Prove that: = ( 1) ( ) Question 8 [2] What will be the effect on the focal length and nature of the lens when a convex lens of glass is immersed in a liquid of refractive index (i) less than glass (ii) equal to glass? Question 9 [2] (i) State Einstein s photoelectric equation. (ii) State de-Broglie s wavelength of an accelerated electron. Question 10 [2] (i) What should be the minimum energy of photon for pair production? (ii) Give an example to explain Isotones. OR Three particles combine to form carbon nucleus 6C12. If mass of 2He4 nucleus is 4.002603 amu, calculate the energy emitted. Question 11 [2] What is meant by (i) critical size and (ii) critical mass in nuclear chain reaction? Question 12 (i) What is the resistance of a semiconductor at 0 K? (ii) What is the meaning of 1ppm in doping of semiconductor? [2] Section C Question 13 Show that electric potential at any point in the plane of the dipole is given by: 1 cos = . , [3] 4 0 2 2 cos 2 where the terms have their usual meaning. Question 14 [3] 2 Area of plates of air capacitor is 0.20 m and distance between them is 0.01 m. Potential difference between plates is 3000 V. when a sheet of dielectric material of thickness 0.01 m is placed between them; potential difference reduces to 1000 V. Find: (i) the capacitance of the capacitor before inserting the sheet, (ii) the dielectric constant, (iii) the capacitance of capacitor when dielectric is placed. Question 15 In the following figure each resistance of the combination is of 1.0 . Find the equivalent resistance between point A and B. [3] OR Using Kirchhoff s laws, find the current flowing through the 5 resistance wire in the circuit shown below. Question 16 [3] Draw the ray diagram of refracting telescope and write the formula for magnifying power when image is formed at (i) infinity (ii) least distance of distinct vision. OR A beam of light consisting of two wavelengths, 650 nm and 520 nm, is used to obtain interference fringes in a Young s double slit experiment. (a) Find the distance of the third bright fringe on the screen from the central maximum for wavelength 650 nm. (b) What is the least distance from the central maximum where the bright fringes due to both the wavelengths coincide? Distance between slits is 2 mm and distance between screen and slits is 120 cm. Question 17 [3] 1 ( 1) Establish the expression, = , for refraction of light at spherical surface. Question 18 [3] Ionization potential of hydrogen atom is 13.6 V, Calculate: (i) Lowest energy of electron in hydrogen atom. (ii) Rydberg s constant (iii) Smallest wavelength of Layman series (iv) Wavelength of first spectral line of Balmer series ( ) (v) Wavelength of spectral line of Balmer series (vi) Wavelength of series limit of Balmer series. (h = 6.6 x 10-34 J s, c = 3x108 m/s) Question 19 [3] (i) What is the difference between forward and reverse bias of a p-n junction? (ii) Reverse current increases when a light of wavelength 7000 A or less is incident at the p-n junction diode. What is the value of band gap? (iii) How can germanium be made p-type semiconductor? Section D Question 20 [5] (a) (i) In an alternating current circuit, a coil of inductance 1 H, capacitor of capacitance 0.2 F, and a resistance of 3000 are connected to a source of 200 V and 1000/2 cycles per second in series. Calculate: (1) Impedance in the circuit, (2) Phase difference between current and voltage, (3) Potential across coil, capacitor and resistance. (ii) Write the expression for power factor of L-C-R circuit. What is the value of power factor in case of resonance in L-C-R circuit? OR (b) An electrical circuit draws 550W power from a source of 220V, 50 Hz. Power factor of circuit is 0.8 and current is lagging in comparison the potential difference. To make power factor 1.0, a capacitor of which capacity has to be connected in the circuit? Question 21 [5] (a) (i) In a semiconductor diode what is meant by potential barrier? (ii) Draw the circuit diagram of a half wave rectifier using p-n junction diode. Show the input and output wave form. (iii) Differentiate between intrinsic and extrinsic semiconductor. OR (b) To convert alternating current into direct current, draw the necessary circuit diagram and explain its working. Question 22 [5] (a) (b) (i) In a double slit experiment interference fringes are obtained on a screen using monochromatic light. When screen is shifted towards slit by 5 x 10 -2 m fringe width changes by 3 x 10-5 m. If the distance between slits is 10-3 m, calculate the wavelength of light used. (ii) Explain the necessary conditions for sustained interference of light. OR (i) Derive the expression for position of maxima and minima in diffraction pattern formed by a thin slit. (ii) Differentiate between interference and diffraction.
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