Temasek 2024 Prelim 4E Phy P1 and 2 Ans and Comments

2024 Prelim Pure Physics 4 Exp Answer 1 C 2 D 3 C 4 A 5 C 6 D 7 A 8 D 9 A 10 B 11 C 12 A 13 C 14 B 15 D 16 C 17 C 18 A 19 B 20 B 21 D 22 A 23 D 24 C 25 C 26 B 27 B 28 A 29 D 30 A 31 B 32 B 33 C 34 B 35 D 36 D 37 A 38 D 39 C 40 C

1a Speed increases uniformly by 2.0 m/s every second. OR For every 1s, the speed increases uniformly by 2.0 m/s. Comments: Responses are often not precise. Should not just repeat what’s already given in the question e.g. rate of 2.0 m/s2. [1] 1b Car A – Straight line starting from 0 with a constant gradient (+value) [1] Car B – Horizontal line (-value) [1] Relevant numerical values for both car A (10 m/s at 5 s mark) and B (at - 3.0 m/s) [1] Comment: Car B travels in the opposite direction of Car A, hence Car B’s v-t graph will be negative. [3] 1c From v = u + at, v = 0 + 2t v = 2t Hence, distance = area under the v-t graph distance = ½ (t)(2.0t) t2 = 1000 t = 31.6 s (no ecf) Comment: Only a few students managed to solve this question. [1] [1] Fig 2.1 velocity/m/s t/s -3 10 0 5

2a Correct magnitude of tensions, and directions of tensions and resultant [1] Correct angle between tensions when arranged head to tail (80o) [1] Correct magnitude of resultant (accept 640 to 660 N), and there should be no doubt that the drawn vector is a resultant e.g. double arrows or labelled accordingly [1] Comment: There is a tendency for students to change the directions of the given vectors. Please note only the position of a vector can be shifted, its direction cannot be changed! [3] 2b Gravitational force of (by) balloon acting on the Earth. Comment: The type of force is often not stated. [1] 3a For a body in equilibrium, the sum of clockwise moments about a pivot is equal to the sum of anti-clockwise moment about the same pivot. (No mark if pivot is not mentioned) [1] 3b Taking moments point P, F x 0.1 = 1.8 x 0.02 F = 0.36 N [1] [1] 4a Particles of liquids are held together by relatively strong intermolecular force, and are slightly less closely packed than that of solids. Thus, with small intermolecular distance, there is little space for liquids to be compressed. Comment: No credit if the arrangement of liquid particles is described as that for solids e.g. ‘very’ closely packed, ‘orderly’ arranged. [1] [1] 500 N 500 N 100° 80° Resultant

4b Since pressure is transmitted equally, 𝐹𝐴 𝐴𝐴 = 𝐹𝐵 𝐴𝐵 30 0.010 = 𝐹𝐵 0.50 𝐹𝐵 = 30 0.010 × 0.50 = 1500 𝑁 [1] [1] 4c Since volume displaced is the same, 200 x 0.010 = d x 0.50 d = 4.0 mm OR by equating work done [1] 5a Patm =(160 – 44 – 30 cm) = 86 cm Hg Comment: Not so well done. A few also didn’t read the question carefully and gave answers in Pascal. [1] 5b P = hρg 86 cm Hg = (0.86 m)(13600)(10) = 116960 = 117000 Pa Comment: Common errors include not converting 86 cm to m, and not rounding the final answer to the appropriate s.f. [1] [1] 5c Atmospheric pressure decreases (with increase in altitude). [1] 5d Gas molecules move continuously and randomly, colliding with the walls of the container and the liquid surface of the mercury column. There is an average force exerted by the gas molecules per unit area / frequent collision of particles with liquid surface, causing a gas pressure to be exerted on the surface of mercury column, causing it to decrease in height. Comment: Students need to answer to the context of the question, i.e. using ideas about molecules. [1] [1]

6a Water that is heated by the heating element will expand, become less dense and rise up [1] Cooler water at the top will sink due to a higher density [1] so that it can gain heat from the heater. This is to set up convection currents to heat the water effectively [1]. Comment: Quite well done. Missing word “expand” [3] b The layer of air trapped between the double walls reduces energy transfer via conduction since air is a poor thermal conductor.[1] The silvery surface of the double walls is a poor emitter of infrared radiation, reduces energy transfer through radiation.[1] Comment: Many still using the term “poor conductor of heat” [2] 7 a Electrons from the glass rod are transferred to the silk cloth, leaving the rod with excess positive charges. Comment: excess or more is acceptable. 1 b Negative charges are attracted to the ball up the stem and away from the leaf. As the leaf and the stem now have the same positive charge, they repel each other and hence the leaf rises. Comment: Some are confused with set up stating that the leaf is attracted to the ball” 1 1 8 a To disconnect the appliance from the high potential mains/supply when the switch is open Comment: Poorly done. Many have the idea that current flows and stops at open circuit. 1 b The fuse melts and breaks/opens the circuit when the current passing through the appliance exceeds 5 A. 1 c I = P/V I = 1500/240 = 6.25 A [1] Not suitable as appliance cannot be switched on without melting the fuse. [1] Comment: Using fuse higher than 6.25 A is not accepted. 2 9 a(i) R = V / I = 3.0 / 0.4 = 7.5 Ω 1

Comment: Well done with handful reading the wrong value (ii) Resistance increases 1 b A significant percentage of the energy provided to the filament lamp is transferred away in the form of infrared radiation instead of visible light due to its high resistance 1 10 a The mains supply has an alternating current which induces a changing magnetic field in the core. [1] There is a change in magnetic flux linking the secondary coil. [1] According to Faradays law an alternating voltage is induced in the secondary coil. [1] 3 c 240/40 = Turns Ratio = 6.00 Comment: Wrong answers were given in ratio form or fraction which is not accepted. Many indicated wrong sf. 1 11a Parallel line. Direction from south to north. 1 b 1 c The nails are made out of hard magnetic materials which are hard to demagnetize. Comment: Steel is also accepted. 1 12a It is a random and spontaneous process whereby an unstable nucleus loses energy by emitting radiation 1 b 1 mark each per particle 2 c 1/16 1 Xe β 131 53 I

13 a [1] correct plotting of points (MUST PLOT ALL points in table) (Accept one missing plotted point but the value in the graph must still be correct) [1] melting at 40C for 5 min starting at 2.5 min (horizontal line) [1] boiling at 70C for 10 min starting at time 10 min (horizontal line) Did not mark down for inappropriate/odd scale but marked down for uneven scale 3 b Heat gained by solid B = mc  = 3kg x c x (40-30) oC Heat supplied by oven = P t = 150 W x (2.5 x 60) s [1] either one of the above workings WITH formula 3

Heat gained = Heat supplied 3kg x c x (40-30) oC = 150 W x (2.5 x 60) s [1] c = 750 J/kg oC [1] NO ECF based on graph as answer can be derived from actual question c Latent heat of vaporization = m lv = 3 kg x 30.0 kJ/kg = 90 kJ 1 d The amount of energy transferred per unit mass of a substance to change between the liquid to gas states, at a constant temperature 1 e From the graph, it took a longer period of time for B to boil as compared to when it was melting. [1] Ecf given based on students graph as qn stated refer to graph. As amount of energy provided per second is that same, this means that the total energy transferred to B during boiling was higher. [1] 2 14 a (i) 𝜂 = sin 𝑖 sin 𝑟 =