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Prelim -I, 2020-21 Class: X Duration: 2hrs Subject: Physics Max. Marks: 80 Section I is compulsory. Attempt any four questions from Section II. The intended marks for questions or parts of questions are given in brackets [ ]. SECTION A (40 Marks) Attempt all questions from this Section. Question 1 a) Why can a machine not be 100 % efficient? [2] b) If the power of a motor is 60 KW, at what speed can it raise a load of 14000 N? [2] c) a) Define radioactivity. b) During the emission of beta particle, the __________number remains the same. [2] d) e) A nut is opened by a wrench of length 20 cm. If the least force required is 2 N, find the moment of force needed to loosen the nut. A large number of free electrons are present in metals. Why is there no current in the absence of electric potential across it? [2] [2] Question 2 a) b) c) d) e) What is the relationship between the mechanical advantage and velocity ratio for : a) an ideal machine b) a practical machine On what factors does the position of the Centre of gravity of the body depend? A machine can open a nut by applying a force of 120 N while using the lever handle of length 50 cm. How long handle is required if he wants to open it by applying force of only 40 N a) Name the radiations which are absorbed by greenhouse gases in the earth s atmosphere. b) A radiation X is focused by a particular device on the bulb of a thermometer and mercury in the thermometer shows the rapid increase. Name the radiation X. What is earthling? What is its use? [2] [2] [2] [2] [2] Question 3 a) b) c) What energy changes that take place in the following: a) a photovoltaic cell b) a microphone Define quality of a sound. What does it depend on? Name two factors on which the heat absorbed or given out by a body depends. [2] [2] [2] [2] d) The diagram shows a ray of light incident on an equilateral glass prism placed in the minimum deviation position. Copy the diagram and complete it to show the path of the refracted ray and the emergent ray e) The surface of an empty test tube held obliquely, in a beaker filled with water shines like a [2] mirror. Give reason Question 4 a) b) c) d) e) A bucket contains 8 kg of water at 25 C. 2 kg of water at 80 C is poured into it. Neglecting the heat absorbed by the bucket, calculate the final temperature of the water. Why is the motion of a body moving with a constant speed around a circular path said to be accelerated? Define the following terms : a) Free vibrations b) Forced Vibrations State two points of difference between an image formed by a convex lens and an image formed by a concave lens. State the position of the object in front of the converging lens if: a) it produces a real and same size image of the object b) When the object is in between the first principle focus and the optical centre [2] [2] [2] [2] [2] SECTION B (40 Marks) Attempt any four questions from this Section Question 5 a) [3] b) c) [3] A lens of focal length 20 cm forms an inverted image at a distance 60 cm from the lens. (i) Identify the lens. (ii) How far is the lens present in front of the object? (iii) Calculate the magnification of the image. [4] Question 6 a) (i) Name the system which enables us to locate underwater objects by transmitting ultrasonic waves and detecting the reflecting impulse. (ii) Draw a graph between displacement from mean position and time for a body executing free vibration in a vacuum. b) (i) A pond appears to be 2.7 m deep. If the refractive index of water is 4/3, find the actual depth of the pond. (ii) Define Refractive Index. (i) When a tuning fork [vibrating] is held close to ear, one hears a faint hum. The same [vibrating tuning fork] is held such that its stem is in contact with the table surface, then one hears a loud sound. Explain. c) [3] [3] [4] (ii) A man standing in front of a vertical cliff fires a gun. He hears the echo after 3.5 seconds. On moving closer to the cliff by 84 m, he hears the echo after 3 seconds. Calculate the distance of the cliff from the initial position of the man Question 7 a) A power circuit uses a cable using three different wires. Name them. (i) Name the three wires of the cable. ii) To which of the two wires should the heating element of an electric geyser be connected? iii) To which wire should the metal case of the geyser be connected? (i) What are superconductors? b) [3] [3] (ii) Calculate the current drawn by an appliance rated 110 W, 220 V when connected across 220 V supply. [4] c) The diagram above shows three resistors connected across a cell of e.m.f. 1.8 V and internal resistance r. Calculate: (ii) The internal resistance r. Question 8 a) A pulley system comprises two pulleys, one fixed and the other movable. Draw a labeled diagram of the arrangement and show clearly the directions of all the forces acting on it. b) 6.4 kJ of energy causes a displacement of 64 m in a body in the direction of force in 2.5 seconds. Calculate (i) the force applied (ii) power in horse power (HP). (Take 1 HP = 746 W) 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). [3] [3] [4] Question 9 a) Complete the path of the ray AB through the glass prism in PQR till it emerges out of the prism. Given the critical angle of the o glass as 42 [3] b) [3] c) (i) The diagrams (a) and (b) given below are of a plug and a socket with arrows marked as 1, 2, 3 and 4, 5, 6 respectively on them. Identify and write Live (L) , Neutral (N) and Earth (E) against the correct number. [4] (ii) Why Pin 1 is made longer and broader. (iii) What is the purpose of earthing? Question 10 a) (i) With reference to the direction of action, how does a centripetal force differ from a centrifugal force during uniform circular motion? (ii) Is centrifugal force the force of reaction of centripetal force? (iii) Compare the magnitudes of centripetal and centrifugal force b) Arrange , and rays in ascending order with respect to their a) penetrating power b) ionizing power c) Biological effect c) ( An atomic nucleus A is composed of 84 protons and 128 neutrons. The nucleus A emits an alpha particle and is transformed into a nucleus B (i) What is the composition of B? (ii) The nucleus B emits a beta particle and is transformed into a nucleus C. What is the composition of C? (iii) What is mass number of the nucleus A? (iv) Does the composition of C change if it emits gamma radiations? [3] [3] [4]
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