bartley 4E5N Sci Phy Prelim Paper 2 2020 Ans

ANSWER SCHEME 1 Section A [Minus 1 mark for whole paper for wrong significant figures] 1 Conceptually-correct scale diagram [M1] Scale is at least 1 cm represent 20 N [B1] Resultant force = 53-57 N [A1] 51-53 ​ 0 ​ from the 70 N force [A1] 2 (a) a = 10 m/s ​ 2 [B1] (b) 900 N [B1] (c)(i) F = 1350 – 900 = 450 N [B1] Up / upwards (Do not accept just an arrow pointing upwards) [B1] ​ (ii) F = ma 450 = 90 x a [Allow full ECF from (c)(i)] [C1] a = 5.0 m/s ​ 2 [A1] 3 (a)(i) Mass per unit volume [B1] (ii) Mass = 2400 x (15 x 2 x 0.25) [C1] = 18 000 kg [A1] (iii) ​ Weight = 18000 x 10 [Allow ECF from (a)(ii)] = 180 000 N [B1] (b)(i) Pressure = 180 000 N / (15 x 0.25) [Allow ECF from (a)(iii)] [C1] = 48 000 Pa [A1] (ii) When the length of the wall is doubled, the ​ weight/force is doubled along with contact area. [B1] 4 (a) Good insulator of heat or poor conductor of heat [B1] (b) Radiation [B1] Convection [B1] (c) Colder air is denser so it sinks. [B1] Hot air is less dense and it rises. Since the cold sink is lower, the sleeping area will not be too cold / is warmer. [B1] 5 (a) Speed = 15 m / 30 s = 0.50 m/s [B1] (b) λ = v / f = 0.50 / 0.25 [Allow full ECF from (a)] [C1] = 2.0 m [A1] (c) By doubling the frequency, the wavelength is halved. [B1] BSS/2020/Prelim Exam/4E5N Science (Physics/Chemistry)

6 (a) Enlarged/magnified/ larger/bigger than object & [B1] Real/can be formed on a screen ​ (Do not accept not virtual) [B1] (b) 2 correct rays drawn with solid lines and arrows to tip of image. [B1] 2 correct rays drawn with solid lines and arrows to tip of image. [B1] Deduct 1 mark if more than 2 arrows missing from the 4 rays (c) 2 focal points correctly marked out with ​ F ​ on both sides of lens [B1] (d) 15 cm [B1] 7 (a) Some electrons from Perspex XY are transferred to the cloth ​ during rubbing, hence Perspex XY becomes positively charged. [B1] (b) As the Perspex is positively charged, ​ negative charges are induced ​ on top of the pieces of paper that are nearer to the Perspex / ​ The paper is negatively charged [B1] Since ​ unlike charges attract ​ , the paper thus move towards the Perspex by jumping up. [B1] 8 (a) anticlockwise moments = (720 x 2) + (100 x 0.5) [C1] = 1490 Nm [A1] (b) 1490 = F x 1 [C1] F = 1490 N [A1] (c) ½ mv ​ 2 = 720 x 5 [C1] v = 10 m s ​ -1 [A1] 9 (a) The lifeboat crew sees the flash first as ​ light travels faster than sound ​ . [B1] (b)(i) Distance = 330 x 4.7 [C1] = 1550 m (3 sig. fig.) [A1] (ii) The ​ time taken for the flash of light to travel to the lifeboat crew is negligible ​ as the speed of light is very fast compared to that of sound. / The ​ flash reached the lifeboat immediately ​ . [B1] Section B 10 (a) Current is the rate of flow of charge. [B1] (b)(i) 1 / R ​ e ​ = 1/ 6 + 1/6 + 1/6 R ​ e = 2.0 Ω [C1] R ​ T ​ = 2 + 4 = 6.0 Ω [A1] (ii) V = IR 24 = I x 6 [Allow full ECF from (b)(i)] [C1] I = 4.0 A [A1] (iii) P.d. = 4 x 4 [Allow full ECF from (b)(ii)] [C1]

= 16 V [A1] (iv) The lamp becomes ​ dimmer ​ . [B1] The ​ overall resistance of the circuit becomes larger ​ when one of the 6 Ω resistors is removed, resulting in ​ lesser current flowing in the lamp ​ . [B1] (v) Voltmeter drawn correctly on Fig. 10.1, parallel to the lamp. [B1] 11 ​ ​ (a) I = 200 W / 240 V [C1] = 0.833 A [A1] (b) 1.0 A fuse [Allow ECF from (a)] [B1] (c) E = 0.2 kW x 36 h = 7.2 kWh [C1] Cost = 7.2 x 25 = $1.80 [A1] (d) It needs to be placed at the bottom for heat transfer by ​ convection ​ to occur. [B1] The water at the bottom gets heated, expands, becomes ​ less dense and rises. [B1] The water at the top, being cooler ​ , is denser, sinks ​ . This sets up a convection current and cycle repeats itself and the whole water becomes heated eventually. [B1] (e) It is a poor emitter/radiator of radiation and [B1] heat from the water is retained longer / water loses heat at a slower rate [B1] 12 (a) Angle of incidence = 65° Angle of refraction = 30° Refractive index = sin 65 ​ o ​ / sin 30 ​ o [C1] = 1.81 [A1] (b) (i) Critical angle = sin ​ -1 ​ (1/1.5) [C1] = 41.3° [A1] (ii) The angle of incidence is bigger than critical angle [B1] and the light is travelling from an optically denser to less dense medium. [B1] (iii) x = sin ​ -1 ​ (1.5 x sin15°) [C1] = 22.5° [A1] (iv) If the bending is too much, this would make the ​ angle of incidence smaller / less than the critical angle [B1] and ​ total internal reflection could not take place ​ . [B1]