Bukit Merah Prelim Ans

BUKIT MERAH SECONDARY SCHOOL SECONDARY 4 EXPRESS PHYSICS Preliminary Examinations 2024 ANS Paper 1 1 A 11 C 21 C 31 B 2 C 12 C 22 D 32 A 3 B 13 C 23 B 33 A 4 D 14 C 24 B 34 D 5 D 15 A 25 C 35 B 6 C 16 D 26 C 36 C 7 C 17 B 27 C 37 B 8 B 18 B 28 C 38 D 9 D 19 A 29 B 39 B 10 D 20 D 30 A 40 B Paper 2 Q/no Ans Marks Allocation 1a Taking moments about the pivot at rope B, sum of anticlockwise moments = sum of clockwise moments Tension A × 1.8 = 60 × 0.6 + 100 × 0.9 Tension A = ଺଴ × ଴.଺ ା ଵ଴଴ × ଴.ଽ ଵ.଼ =𝟕𝟎 𝑵 Tension B = 100 + 60 – 70 = 90 N 1 1 1 1b He should suspend this second bird cage directly below rope A. Taking moments about the pivot at rope A, since there is no perpendicular distance between the pivot and the weight of the second bird cage, there will not be an additional turning effect about A, hence it will not increase the tension in Rope B. i.e. suspending the second bird cage directly below rope A only increases the tension in rope A by 30 N. 1 1 2a P = Patm + hpg = 100000 + (100/1000) x 1000 x 10 = 101 000 Pa 1 1 2b Water level will rise in left arm and drop in right arm (until the water levels on both sides are at the same level). 1 2c When air leaks from the pipe, number of air molecules in the pipe decreases. This causes the frequency of collision between air molecules and walls of the pipe to decrease and the air pressure inside the pipe decrease. 1

Since pressure due to atmosphere and column of water is now higher than air pressure in the pipe, water levels will move as per (b). 1 2d As the pressure of gas is only 1000 Pa, the mercury height difference in the manometer will be very small (7 mm). This reduces the sensitivity of the manometer. 1 3a Radiation 1 3b The shiny material will reflect infrared radiation down to the food which allows it to be cook faster. OR The shiny material reflects infrared radiation away from the outer metal case which does not get hot. 1 4a t = 5.0 min 1 4b Q = mcӨ Q = 0.25 x 4200 x 45 Q = 47250 J = 47000 J (2sf) 1 4c Energy gain from ice at 0 degrees to water at 0 degrees = m (336000) Energy gain from water at 0 degrees to water at 5 degrees Q = mcӨ = m (4200) (5) = m (21000) Total energy loss by tea = total energy gain by ice cubes 47250 = m (336000 + 21000) m = 47250 / 357000 m = 0.132 kg 1 1 4d The molecules near the surface that also have a high enough kinetic energy can escape the surface of the liquid, causing evaporation. The remaining molecules have a lower average kinetic energy than before, therefore it is cooler. 1 1 5a Since object distance = image distance, focal length is half of the object distance. f = ½ x 5 x 4.0 cm = 10 cm 1 5b Rays (through centre and through focal point) Image (inverted, same size) 1 1 5c Image is formed by converging rays on the screen. 1 5d Photocopier 1 6a v = fλ 6100 = (4.0 x 106) λ λ = 1.5 mm (2 s.f.) 1 6b Distance = 3 x 1.525 = 4.6 mm 1 6c v = 2d/t 6100 = (2 x 0.0040) / t t = 1.31 µs 1 1

6d A shorter pulse 13 spaces after the emitted pulse. 1 7a Electrons in metal can are attracted by the glass rod to the inner surface of can, leaving inner surface with induced negative charges, and outer surface with induced positive charges. Electrons flow from the ground to neutralise the positive charges on outer surface. This flow of charges causes the galvanometer to deflect. Galvanometer returns to zero when outer face is neutral. 1 1 1 7b Use a negative charged rod instead or withdraw the positive charged rod 1 8a 12V 1 8bi Total effective resistance = 6 + 4 + 5 = 15Ω V = IR 12 = I (15) I = 0.80 A 1 1 8bii V = IR V = (0.80)(4.0) V = 3.2 V 1 8c Total resistance decreases, I through battery increases (by V=IR), Therefore ammeter reading increase As I across XJ increases, PD at XJ also increase 1 1 8d The voltmeter reading will decrease. 1 9a 1 9b When the temperature is high, the resistance of the thermistor will decrease. The current flowing in the coil increases. This increases the strength of the magnetic field of the coil. The reed switch is magnetised with unlike poles at the two ends and attract each other to close the reed switch. Current flows in the circuit so the warning lamp lights up. 1 or 1 or 1 1 9c As steel rods are hard magnetic material which will retain magnetism even when resistance of thermistor increases and keep the reed switch closed, the device may not be effective in informing the farmer. 1 10a AC current results in a changing magnetic field at transmitter coil. There is a rate of change of magnetic flux linkage with the receiver coil. Voltage is induced at receiver coil and current flows in receiver coil. 1 1 1 N N S S

10b Soft Iron Core concentrates magnetic field at the transmitter coil Greater rate of change of magnetic flux linkage with receiver coil 1 1 10c turns ratio = 3.7/230 = 0.016 1 11a Speed of light in glass = distance / time = 1000 / 5 X 10-6 = 2.0 X 108 m/s 8 8 3.0 10 2.0 10 cn v    1.5 1 1 11b sin sin in r sin351.5 sinX X = 22.5° 1 1 11ci The single pulse of light consists of three rays with different i. The first pulse that exits at t = 10.0 μs follows ray 1 path, 2nd pulse exits at 10.3 μs follows ray 2 path, 3rd pulse exits at 10.8 μs follows ray 3 path. 1 11cii Pulses from 5.0s to 5.1s, 5.1s to 5.2s and 5.4s to 5.5s 1 mark for duration of each pulse 0.1 μs and same brightness 1 mark for correct exit timing 11ciii Time between pulse is 0.9 μs Maximum number of pulses per second = one pulse per 0.9μs = 1/0.9X10-6 = 1.1 x 106 1 1

11civ Using the data, it can be seen that at longer distances (compare fig 11.4 to 11.5) the time between pulses that exit increases or the pulses will overlap Thus longer distance, will lower the rate at which pulse can enter the cable. Note: must link distance to lower transmission rate 1 12a displacement from A = ½ (12)(18) – ½ (12)(6) = 108 – 36 = 72 m 1 1 12b The graph for t = 0 to 12 s shows positive velocity, while that for t = 12 to 24 s shows negative velocity (the velocities in these two periods have different signs). This shows that the sledge was moving in opposite directions in the two periods. The graph in t = 0 to 12 s also has a steeper gradient. This shows that the rate of change of velocity during this period has a greater magnitude. [Answer must cite the difference as observed from the graph, and interpret accordingly.] 1 1 12c At t = 12 s, the acceleration of the sledge (as seen from the gradient of the graph) is not zero. The forces on the sledge are therefore not balanced. 1 12d From B towards A, a = (6 – 0) / (24–12) = 0.50 m s–2 F = ma = (70) (0.50) = 35 N 1 1 12ei Work done against friction = F x d = 25 x 108 --- e.c.f. allowed from 12(a) = 2700 J 1 12eii KE at A = GPE at B + WD against friction ½ (70) (182) = (70) (10) (108 sin θ) + 2700 8640 = 75600 sin θ Θ = 6.6° 1 1 13ai Decrease the resistance of the variable resistance Increase current Increase turn per unit length [Any 2] 1 13aii Wire PQ will move out of the page. The resultant magnetic field created by both PQ and the electromagnet will be such that the region of stronger magnetic field is in the paper while the region of weaker magnetic field is out of the paper. 1 1 NOTE: in this working, we take B towards A as the positive direction. Th