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PRE-BOARD EXAMINATIONS 2020-21 PHYSICS SCIENCE Paper-I Full Marks: 80 Time: 15 minutes reading 2 hours writing 30 minutes scanning and making PDF SECTION I (40 Marks) (Attempt all questions from this Section) Question 1 (a) Choose the correct option and give reason: The handle of the door is always near the (free end/hinge) of the door. (2) (b) State the energy changes that take place in the following when they are in use: (i) A photovoltaic cell (ii) A microphone. (2) (c) Where does the position of centre of gravity lie for: (i) Hollow cylinder (ii) Triangular lamina. (2) (d) A woman draws water from a well using a fixed pulley. The mass of the bucket and water together is 6.0 kg. The force applied by the woman is 70 N. Calculate the mechanical advantage. (2) (e) A body of mass 5 kg is moving with a velocity of 10m/s. What will be the ratio of its initial kinetic energy and final kinetic energy, if the mass of the body is doubled and its velocity is halved? (2) Question 2 (a) What should the angle between force and displacement be to get the: (i) Minimum work? (ii) Maximum work? (2) (b) State two advantages of using a right angled prism as a reflector rather than a plane mirror. (2) (c) The diagram shows a point source of light S, a lens L, and a plane mirror M. These are placed such that rays of light, form S return to it after reflection from M. (i) What is the distance OS called? (ii) To which point (left of S, on S, or right of S) will the rays return, if M is moved to the left and brought in contact with L. (2) (d) An object is placed in front of a convex lens such that the image formed has the same size as that of the object. Draw a ray diagram to illustrate this. (2) (e) During refraction of light at the surface of separation of two media, state the nature of media when (i) i > r (ii) r = 90 where i is the angle of incidence and r is the angle of refraction. (2) Protect by PDF Anti-Copy for Android (Update to Pro to remove watermark) Question 3 (a) What is the range of the spectrum of white light: (i) In angstorm ? (ii) In nanometre ? (2) (b) Two strings of a guitar are of lengths 40cm and 20cm, such that they are made from the same material and are of same thickness and under same tension. If the frequency of the longer wire is 250 Hz, find the frequency of smaller one. (2) (c) Why does a wine glass start rattling when a note of some particular frequency is struck by a piano ? (2) (d) The specific heat capacity of substance A is twice that of substance B. Compare the rate of cooling of equal masses of A and B under identical conditions. (2) (e) State why two pieces of ice combine to form a single piece when pressed together for a short time and then released. (2) Question 4 (a) A resistor of resistance R is connected, in turn, across two batteries of voltages V and 2V respectively. What is the ratio of the rate of heat produced in the two cases? (2) (b) Drinks get cooled more quickly by adding pieces of ice than the ice-cold water. Why? (2) (c) Name the material used for (i) Filament of an electric bulb (ii) Heating element of room heater. (2) (d) Mention any two differences between nuclear energy and chemical energy. (2) (e) At what voltage is the electric power from the generating station transmitted? Give reason. (2) SECTION II (40 Marks) (Attempt any four questions from this Section) Question 5 (a) An engine can pump 30,000 litres of water to a vertical height of 45 metres in 10 minutes. Calculate the work done by the machine and its power. Take g=9.8m/s 2 . [Density of water=10 kg/m , 1000 litre=1m ]. (3) (b)(i) Draw a labelled diagram of a block and tackle system of 5 pulleys. Indicate the directions of the load, effort and tension in the string. (ii) Write down the relation between the load and the effort of the pulley system. (3) (c) An object of mass 'm' is allowed to fall freely from point A as shown in the figure. Calculate the total mechanical energy of the object at: (i) Point A (ii) Point B (iii) Point C (iv) State the law which is verified by your calculations in parts (i), (ii), and (iii). (4) Question 6 (a) Name the following electromagnetic radiations: (i) Used for taking photographs in dark (ii) Used for studying atomic arrangement in crystals. (iii) Used for sterilizing purposes. (3) (b) Diagram given below shows a right-angled isosceles prism of refractive index 1.5. An object PQ is placed in front of its base BC. Copy the diagram. (i) Complete the diagram showing the image produced by the prism. (ii) Name an instrument in which such a prism is used. (3) (c)A concave lens forms an erect image of one-third size of the object which is placed at a distance 30 cm in front of the lens. Find: (i) The position of image (ii) The focal length of the lens. (4) Question 7 (a) A prism deviates a monochromatic ray of light through an angle ' ', when the angle of incidence at the surface of the prism is 'i'. (i) Draw a graph showing the variation of ' ' with 'i'. On your graph show the angle of minimum deviation. (ii) What is the relation between the angle of incidence and the angle of emergence when the ray suffers minimum deviation? (3) (b) A man standing in front of a vertical cliff fires a gun. He hears an echo after 3 seconds. On moving closer to the cliff by 82.5 m, he fires again. This time he hears the echo after 2.5 seconds. Calculate (i) The speed of sound (ii) The distance of initial position of the man from the cliff. (3) (c) In the figure below (X) and (Y) are two waves which are represented graphically with displacement time curves. Calculate the ratio of their (i) Velocity (ii) Wavelength (iii) Frequency Also give a reason which produces a shriller sound. Question 8 (a) Which of the two, (4) (i) An electric bulb, marked 100W, 220V or (ii) An electric heater, marked 2000W, 220V, would need a thicker connecting wire? (3) (b) Three ammeters, A, B and C, all of negligible resistance, are connected in an electric circuit as shown in fig. If the ammeter B shows a reading of 0.5 A, what would be the (i) Readings of ammeters A and C? (ii) Total resistance of the circuit? (3) (c)(i)The earth pin, of a three-pin plug, is longer and thicker than its other two pins. Why? (ii)The total electricity consumption of a 60W bulb in the month of April was registered as 9 (commercial) units of electricity. What was the average 'hours per day' use of this lamp? (2+2=4) Question 9 (a) State that feature of the calorimeter which prevents loss of heat by (i) Radiation (ii) Conduction (iii) Convection. (3) (b) A lead bullet travelling at 300 m/s hits a target and is brought to rest by the retarding force. Calculate the rise in the temperature of the bullet. (Assume that no heat is lost to the surrounding.) [Specific heat capacity of lead=0.031 cal/g -1 C -1 .] (3) (c) When naphthalene powder is heated to 90 C and then allowed to cool to room temperature and the temperature is noted after every half a minute in this process, the observations lead to the following graph: (i) Which part of the curve represents melting of naphthalene? (ii) Which part of the curve represents freezing of naphthalene? (iii) What is the freezing point of naphthalene? (iv) Is naphthalene in the solid or liquid state in the region CDE? (4) Question 10 (a) Following rays emitted by a radioactive substance (i) Name the rays which have electric charge. (ii) Name the type of radiations which are most penetrating (iii) Name the type of radiations which travel with the speed of light. (3) (b) When a resistance of 3 ohm is connected across a cell, the current flowing is 0.5 A. On changing the resistance to 7 ohm, the current becomes 0.25 A. Calculate the e.m.t and the internal resistance of the cell.(3) (c)(i) The diagram below shows a thick lead cube having a cavity in the middle. In the cavity is placed some radioactive substance. Copy the diagram and trace the paths of alpha-particles, beta-particles and gamma-radiations as they pass through a powerful electric field. (ii) State what is background radiation? -----------------------THE END----------------------- (4)
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