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2/3/2011 Subjective Test Paper - Physics - Meritn… Physics 2007 Set 2 Close Subjective Test These are additional set 2 questions. Question 1 ( 1.0 marks) What is the purpose of modulating a signal in transmission? (1) Solution: The purpose of modulating a signal in transmission is (i) to increase the effective power of the signal (ii) to keep the height of antenna or aerial small (iii) not to allow mixing up of signals from different transmitters Question 9 ( 2.0 marks) A cylindrical metallic wire is stretched to increase its length by . Calculate the percentage change in its resistance. (2) Solution: Initial length of cylindrical metallic wire is L. Change in length, ∆l= of L = 0.1L New length, L′ = L + ∆l = L + 0.1L = 1.1L Let A be the initial area of cross-section of the coil and A′ be the new area of cross-section of the coil. The volume of cylindrical metallic wire remains constant. A L = A′ L′ Let R be the initial resistance of the coil and R′ be the new resistance of the coil. Resistivity ρ remains the same in both the cases. Percentage increase in resistance is Therefore, the percentage change in resistance is Question 10 ( 2.0 marks) Distinguish between the terms ‘effective value’ and ‘peak value’ of an alternating current. An alternating current from a source is represented by (2) I = 10sin (314t) Write the corresponding values of meritnation.com/…/28S6$cMa81Ju87b6… 1/5 2/3/2011 Subjective Test Paper - Physics - Meritn… Write the corresponding values of (i) Its ‘effective value’ (ii) Frequency of the source Solution: Effective value of alternating current is the value of direct current, which produces the same heating effect in a given resistor as is produced by the given ac when passed for the same time. Peak current is the maximum value attained by an alternating current in either of its half cycles. Given, I = 10 sin (314 t) Standard equation for current is, I = I0 sin (2πft) Where, I0 = Peak current = 10 A f = Frequency of source t = Time period (i) (ii) Question 12 ( 2.0 marks) A convex lens of refractive index 1.5 has a focal length of 20 cm in air. Calculate the change in its focal length when it is immersed in water of refractive index . (2) Solution: Reflective index of convex lens in air, µ= 1.5 Focal length of convex lens, fa = 20 cm Reflective index of water, µ′= When the lens is in air: Where R1 and R2 are the radius of curvature of the convex lens When the lens is in air: Where, fw is the focal length of the lens, when immersed in water meritnation.com/…/28S6$cMa81Ju87b6… 2/5 2/3/2011 Subjective Test Paper - Physics - Meritn… Question 13 ( 3.0 marks) If the nucleons bound in a nucleus are separated apart from each other, the sum of their masses is greater than the mass of the nucleus. W here does this mass difference come from? Explain briefly. (2) Solution: Protons and neutrons are bound together inside a nucleus by means of strong attractive nuclear forces. Therefore, a definite amount of work is required to be done to break up the nucleus into its constituent particles and to separate them at infinite distance from one another. This work done gets converted into mass in accordance with Einstein’s mass energy relation, E = mc2 . Thus, when the nucleons bound in a nucleus are separated from each other, extra mass is generated from the energy required to separate the nucleons. Because of this extra mass, the sum of the masses of daughter nuclei is greater than the mass of the mother nucleus. Question 14 ( 3.0 marks) The output of 2- input AND gate is fed to a NOT gate. Draw the logic circuit of this combination of gates and write its truth table. (2) Solution: The above gate functions as a 2-input NAND gate. Truth Table A B 0 0 0 1 0 1 0 1 1 0 0 1 1 1 1 0 Question 24 ( 3.0 marks) Radiations of frequency 1015 Hz are incident on tw o photosensitive surfaces A and B. Following observations are recorded: Surface A: No photo-emission takes place. Surface B: Photo-emission takes place but photo-electrons have zero energy. Explain the above observations on the basis of Einstein’s photo-electric equation. How will the observation with surface B change when the wavelength of incident radiations is decreased? (3) Solution: (a) The threshold frequency of surface A is greater than 1015 Hz. Therefore, no photoemission takes place. (b) The threshold frequency of surface is equal to 10 15 Hz. Therefore, photoemission takes place, but photoelectrons have zero kinetic energy. When the wavelength of the incident radiation is decreased, the kinetic energy of photoelectrons emitted from surface B increases. Question 25 ( 5.0 marks) Draw a labelled ray diagram of an astronomical telescope. W rite mathematical expression for its magnifying power. How does the magnifying power get affected on increasing the aperture of the objective lens and why? (3) Solution: meritnation.com/…/28S6$cMa81Ju87b6… 3/5 2/3/2011 Subjective Test Paper - Physics - Meritn… Least distance of distinct vision: Magnifying power, m The magnifying power is not affected on increasing the aperture of the objective. Question 27 ( 5.0 marks) State Gauss’s theorem in electrostatics. Using it, deduce an expression for electric field intensity at a point near a thin infinite plane sheet of electric charge. (3) Solution: Gauss’s theorem states that the total electric flux through a closed surface is 1/ε0 times the charge enclosed by the surface. Electric field intensity on either side of the sheet must be perpendicular to the plane of sheet having same magnitude at all points equidistant from the sheet. Let P be any point at a distance r from the sheet. Let the small area element ds = πr2 , where r is the radius. and are perpendicular on the surface of the cylinder. Therefore, the electric flux is zero. and are parallel on the two cylindrical edges P and Q, which contributes to electric flux. Electric flux over the edges P and Q of the cylinder is: Charge density,σ = ∴q =πr2 σ … (1) From equation (1), meritnation.com/…/28S6$cMa81Ju87b6… 4/5 2/3/2011 Subjective Test Paper - Physics - Meritn… meritnation.com/…/28S6$cMa81Ju87b6… 5/5

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PREVIOUS YEAR PAPERS CBSE BOARD EXAM AIEEE BITSAT ISAT VITEEE IIT-JEE STUDY MATERIAL PHYSICS CLASS XI XII SAMPLE PAPERS KEY SOLUTIONS ANSWERS QUESTIONS

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