4N 2021 PHY PY Ans Key

Sec 4NA Prelim 2021 Ans Paper 1 1 D 6 B 11 A 16 D 2 C 7 B 12 D 17 B 3 A 8 D 13 C 18 B 4 C 9 C 14 D 19 B 5 A 10 D 15 D 20 A Paper 2 Qns Working Marks 1a ρ = m  v = 55  50 = 1.1 g cm-3 1 1 1b ρ = 55  60 = 0.92 g cm-3 1 1c The uncooked meat ball will be sinking in water as its density is more than 1.0 g/cm3 while the cooked meat ball will be floating in water as its density is less than 1.0 g/cm3. 1 2a B and C 1 2b Can A. Can A has a shiny silver surface that is a poor emitter of infra-red radiation. 1 1 3a Solid to liquid 1 3bi State: Solid Movement: vibrate at fixed position 1 3bii State: liquid Arrangement: randomly arranged OR molecules are further apart from each other compared to solid particles. 1 4a X – Earth – Yellow Green Y – Neutral – Blue Z – Live – Brown 3 correct – 1 m All correct – 2m 2 4b Fuse. It will melt and disconnects/open the circuit when the current flowing is more than the fuse rating. 1 1

5a Increasing speed 1 5b 25 s 1 5c a = v-u/t = 3-0/15 = 0.2 m/s2 1 1 5d Total distance = Area under speed-time graph = ½(15)(3) + (25)(3) + ½(10)(3) = 113 m 1 1 5e W = F x d = 600 x 113 = 67 800 J 1 (e.c.f) 1 6a Points plotted [1] Line drawn [1] 6bi 7 min 1 6bii Energy absorbed is used to raise the internal potential energy of the substance. OR Energy absorbed is used to change the state of the substance, hence there is no change in temperature during the change of state. 1 6ci 1 V 1 6cii R = 5 + 10 = 15 Ω 1 6ciii I = 3/15 = 0.2 A 1 6civ Same reading of 1.5 V each. 1 7ai 0.6 m 1

7aii 5 m 1 7bi P : infra-red radiation Q: ultra-violet radiation 1 7bii All electromagnetic waves are transverse waves. (Accept any other reasonable answers) 1 7ci 2d =vt = 1500 x 0.1 =150 d = 75 m 1 1 7cii λ = v/f =1500/(33 000) = 0.0454 m 1 1