PRCSS Prelim Ans

2024 4E Physics Prelim Exam Mark Scheme 1 2 3 4 5 6 7 8 9 10 B C B A D C D A A B 11 12 13 14 15 16 17 18 19 20 B D C D B D C A A D 21 22 23 24 25 26 27 28 29 30 A C B C D D A D B B 31 32 33 34 35 36 37 38 39 40 B A C D B C C A C D 1a upward force equals to weight (in magnitude) upward force and weight are opposite in direction / resultant force is zero 1 1 1b F = ma Fup – 250 000(10) = 250 000 (4.0 – 0)/10 Fup – 2 500 000 = 100 000 Fup = 2 600 000 N 1, 1 1 1c Displacement = area under graph = 0.5(15)(4.0) − 0.5(10)(4.0) + 0.5(20)(2.0) = 30 m 1 1 [7] 2a Force per unit area 1 2bi F1/A1 = F2/A2 100/10 = F2/50 F2 = 500 N 1 1 2bii F1d1 = F2d2 100(20) = 500(d2) d2 = 4.0 cm 1 1 2c Pressure will not be transmitted as gas is compressible 1 [6] 3a Metal is a better thermal conductor than glass Energy is transferred out from the boy quicker //reject Energy transferred from metal to surrounding, absorb heat// 1 1 3b Particles at the heated end are vibrating about their fixed positions more vigorously, they collide with neighbouring particles and pass on energy to cause them vibrate about their fixed position also more vigorously. This process continues till the other end. 1 1 3ci 1 3cii Energy transferred to water = energy transferred from ice mc∆θ = mc∆θ + mlf + mc∆θ 150(4.2)(30 – θf) = 50(2.1)(5) + 50(330) + 50(4.2)θf 18900 – 630θf = 525 + 16500 + 210θf 1875 = 840θf 2.2 °C = θf 1,1 1

[8] 4a Disinfection/sterilisation/kill bacteria and viruses/sun-tanning bed/anti- forgery of printed notes 1 4b hinder chemical reaction/ affects cell process/ damage DNA results in cell death, mutations 1 1 4ci 10−8 m 1 4cii v = f λ 300 000 000 = f 10−8 f = 3.0 × 1016 Hz 1 1 [6] 5a Negatively charged 1 5b dirt particles (negatively charged) attract to metal body unlike charges attract (accept like charges repel) (open cover) knock on metal body 1 1 1 [4] 6a //at least 3 field lines with direction shown// 1 6b use a compass 1 6c Iron is magnetic material so the nails get magnetised/ becomes induced magnets Iron nails attracted to the solenoid 1 1 6d No longer attracted to solenoid, (iron) easily demagnetised 1 [5] 7a clockwise 1 7b The current carrying coil experiences forces, in the magnetic field of the magnet / interaction of magnetic field of current-carrying coil and the magnet produces forces on coil forces on each side of the coil are opposite in direction 1 1 7c Soft iron concentrates magnetic field lines Increases the strength of magnetic field 1 1 7d Reverses direction of current in coil every half a revolution 1 [6] 8ai 1 8aii When the magnet (north pole) moves towards the solenoid, the deflection of the galvanometer shows the direction of current in the solenoid Using the right-hand grip rule, the polarity of the solenoid facing the magnet is found to be (north pole) opposing the movement of the magnet. 1 1 1 8bi Coil experiences a changing magnetic field linking to it 1 8bii Electrical contact between the carbon brush and the coil / allow the coil to be rotated without being twisted 1 8biii 1 Amplitude 1 period [8]

9a 1 9b Total internal reflection is the complete reflection of a light ray in an optically denser medium at the boundary with an optically less dense medium. 1 9c The volume has to be 0.16 For 3.5 carat of garnet, v = m/ρ = 3.5(200/1000) / 7.0 = 0.10 cm3 (not equal to 0.16 cm3) For 2.8 carat of topaz, v = m/ρ = 2.8(200/1000) / 3.5 = 0.16 cm3 For 1.6 carat of opal, v = m/ρ = 1.6(200/1000) / 2.0 = 0.16 cm3 The critical angle is lesser than 45° for TIR to occur For topaz, C = sin−1(1/n) = sin−1(1/1.61) = 38.4° For opal, C = sin−1(1/n) = sin−1(1/1.37) = 46.9° 1 (v = m/ρ) 1 (C<45°) 1 (C=sin−1(1/n) 1 (topaz) 9di Critical angle is the angle of incidence in an optically denser medium for which the angle of refraction is the optically less dense medium is 90°. 1 i r

9dii 2 (light rays) 1 (normal) 9diii More light is reflected within the diamond and go towards the top surface of the diamond / less light is refracted out of the diamond 1 [11] 10a Yes Count rate did not drop after paper implies there may be beta-particles and gamma-particles emitted but not alpha particles. Count rate dropped after aluminium confirms existence of beta particles 1 1 1 10b Random process 1 10c Background radiation 1 10di 6 half lives 6 × 1600 = 9600 years 1 1 10dii Axes label with units Curve with correct half-lives 1 1 [9] 11ai 0 Ω 1 11aii 9.0 (R2 / (R2 + 1000)) = 6.0 3R2 = 2R2 + 2000 R2 = 2000 Ω 1 1 11aiii 1 (LDR circuit symbol) 1 circuit 11bi increase resistance to increase the heating effect (accept increase resistance) 1 11bii P = VI 45 = 240 I I = 0.19 A 1 1 11biii Cost = E × unit cost = Pt × unit cost = 100(45/1000)(2)30 = 270 cents 1 1 [10] 55° 15°

12ai There is clockwise moment due to the 2N weight about the pivot Q, Since the steelyard is balanced, the weight of the steelyard must be between PQ to produce anticlockwise moment. 1 1 12aii Moment = fd = 2.0(10) = 20 Ncm 1 12aiii acwm = cwm w(20) + 20 = 2.0(60) //award 1 mark if the 20 N cm is missing) w = 5.0 N 1 1 12aiv 1 12bi Centre of gravity is an imaginary point where the entire weight of the object seems to act. 1 12bii 1 12biii Clockwise moment due to weight / weight acts out of base area 1 12biv No change, centre of gravity remains at the same point. (accept increase in mass causes higher inertia, thus more stable) 1 [10] 0 1 2 3 4 5 weight normal force