2021 Physics Prelim Year 4 SJI P2 Section B

15 CANDIDATE NAME CLASS INDEX NUMBER Section B Q10 Q11 Q12E Q12O Section B (30 marks) Answer all questions in this section. Answer only one of two alternative questions in Question 12. 10 Optical fibre is used extensively to transmit internet data around the world, as an estimated 2.5 x 109 Gigabytes of data is created and accessed each day. Fig. 10.1 shows a cross-sectional view of an optical fibre used for such data transmission. Internet data, in the form of light ray travels through the cylindrical glass core, that is surrounded by a layer of cladding. A light ray enters from air into the core at point P. Fig. 10.1 (not to scale) claddingcoreRSnormalPlight ray12⁰airQTU

16 During the design of a particular optical fibre, various materials for the core and cladding are considered. Fig. 10.2 shows a list of materials, and their refractive indices. Fig. 10.2 (a) State what is meant by the term critical angle. ……………………………………………………………………………..…………... ………………………………………………………………………………..………... ..…………………………………….………………………………………………..[1] (b) One of the core materials from Fig. 10.2 is being considered. A ray of light that enters at P with an angle of incidence of 12o travels into this core, and subsequently makes an angle of incidence along surface MN at 82o. Assuming that the cross section of the core RSTU is a rectangle, calculate the refractive index of this core material, and identify the material chosen by referring to Fig. 10.2. core material = ……………………….. [2] (c) Next, material C is being considered for the core, with the same light ray entering P at an angle of incidence of 12o. (i) Calculate the speed of light that travels through C. speed = ……………………….. [2] Core materialRefractive indexA1 . 5 3B1 . 4 9C1 . 4 0D1 . 4 2E1 . 5 6Cladding materialRefractive indexV1 . 4 4W1 . 4 1X1 . 3 6Y1 . 5 2Z1 . 5 0

17 (ii) From Fig. 10.2, select a suitable cladding to be used with C. Using ideas about angle of incidence, critical angle, and refractive index, explain how your selection of cladding allows a ray of light travelling from P to Q to be reflected off RS. …………………………………………………..……………………………... ………………………………………………..………………………………... ……………………………………………………..…………………………... …………………………………….………………..………………………..[2] (iii) A student suggests that the critical angle (θc) of light for C in (c)(ii) can be obtained by applying the formula: where n is the refractive index of material C. Provide a reason why the student’s suggestion cannot be used here. ……………………………………………..…………………………………... ……………………………………………...………………………………..[1] (iv) Two different incident rays, R1 and R2, enter material C at the same point P. R1 has a lower angle of incidence than R2, and both rays reflect off RS. Compare how the time taken for light to travel from the surface RU to ST is affected when R1 or R2 is chosen. Explain your answer. ……………………………………………..…………………………………... ………………………………………………..………………………………... …………………………………………………..……………………………... ………………………………………….………..…………………………..[2] sin θc=1n

18 11 An engineer conducted an experiment to verify the principle of moments using a set- up shown in Fig. 11.1. Fig. 11.1 A 20 g bucket is placed on a uniform metre beam which is pivoted at the 50.0 cm mark. The centre of gravity of the empty bucket is at point G. Water of density 1.0 g/cm3 is released into the bucket at a constant rate of 40 cm3 per minute. In order to balance the beam, a 50 g sliding weight hung on the beam is adjusted accordingly by the engineer. (a) Define the principle of moments. ……………………………………………………………….....……………………….. ……………………………………………………………..……………………………. …………………………………………………………..…………………………… [1] (b) The tap is turned on for 5.0 minutes. During this period of time, the position of the sliding weight is adjusted continuously to keep the beam horizontal. At the end of the 5.0 minutes, the sliding weight is a distance x from the pivot. (i) It is found that the pivot is exerting a 4.0 N force on the beam. Calculate the weight of the beam. weight = ……………………….. [2] (ii) Calculate x. x = ……………………….. [2] G

19 (c) Explain why the set up will not be able to achieve equilibrium, if the bucket holds more than 230 g of water. ………..…………………………………………………………….…………………… …………………..…………………………………………..………………………..[1] (d) The engineer modified the set-up and created a device similar to a Sozu fountain, a type of water fountain displayed in Japanese gardens as shown in Fig. 11.2. It consists of a tube that is pivoted at a point. Its lower end rests against a rock as shown while the upper end has a cavity used to hold water. https://en.wikipedia.org/wiki/Shishi-odoshi Fig. 11.2 Water from the tap flows into the cavity at the upper end of the tube as shown in Fig. 11.3(a). The water that has accumulated eventually causes the tube to rotate and water flows out of the cavity when this happens. Fig. 11.3(a) Fig. 11.3(b) rock

20 (i) Describe how the centre of gravity of the tube and the water changes as water flows into the empty cavity until it finally rotates. ……………………………………………………………...………………….… ………………………………………………………………..………………….. …………………………………………………………………..…………… [2] (ii) In another Sozu fountain which is faulty, after the water has been emptied, the tube remains in the position as shown in Fig. 11.3 (b). State the position of the centre of gravity of the faulty fountain and explain your answer. ………………………………………………………………..………………….. …………………………………………………………………..……………….. ……………………………………………………………………..…………….. ………………………………………………………………………..……… [2] 12E Fig. 12.1 shows a fire detection circuit designed by a student. The bell will ring when the NTC thermistor detects high temperatures. The bell has a resistance of 4.0 Ω and is designed to operate normally at 12.0 V. A magnetic switch which comprises of a switch and a coil is used to close the circuit automatically when there is a fire. Fig. 12.1 (a) Explain why the bell is an output transducer. ……………………………………………………………………………………… …………………………………..……………………………………………….[1]

21 (b) Explain why the bell will not ring when there is no fire. ……………………………………………………………………………………... ……………………………………………………………………………………... ……………………………………………………………………………………... ……………………………………………………………………………………… ……………………………………………………………………………………... ……………………………………………………………………………..…….[3] (c) During a test, the temperature is increased causing the magnetic switch to close. When this happens, the current through the coil is 0.050 A and the potential difference across the coil is 4.0 V. (i) Calculate the resistance of the thermistor at this instant. resistance = ………………….…….. [2] (ii) Determine the total current drawn from the 12.0 V supply. total current = ………………………… [2]

22 (d) The student modified the circuit and connected the bell to two identical filament lamps in parallel as shown in Fig. 12.2(a) and in series as shown in Fig. 12.2(b). Fig. 12.2 (a) After taking some measurements, the student discovered that the resistance of the identical lamp is lowered when it is connected in series to the bell as shown in Fig. 12.2(b). Explain why this occurs. ………………………………………………………………………………………. ………………………………………………………………………………………. ………………………………………………………………………………………. …………………………………………………………………………………… [