Whitley Prelim Ans

2024 4E Physics Prelim Answers Paper 1 1 C 11 D 21 D 31 C 2 B 12 B 22 B 32 A 3 B 13 A 23 B 33 B 4 C 14 B 24 B 34 B 5 D 15 A 25 A 35 C 6 A 16 D 26 D 36 A 7 A 17 A 27 D 37 A 8 C 18 D 28 A 38 D 9 B 19 A 29 D 39 C 10 D 20 B 30 D 40 B Paper 2 1ai Fill the measuring cylinder with 50cm3 of water. Fully submerge the rock into the measuring cylinder. [1] The difference in reading is the volume of rock Q. [1] 1aii The volume is large and rock R may not be able to fit into the measuring cylinder. [1] 1b Rock S. the density of S is 5.6g/cm3 while P, Q and R is approximately 4.6 g/cm3. [1] 2ai Resultant force is zero. 2aii The object must move a distance. 2aiii Constant velocity 2b F = ma 100 – 85 = 25a a = 0.6 m/s2 2c The friction may not be constant. [1] Hence, the resultant force will change [1] and acceleration is not constant. 2d [1] – curve of decreasing gradient from 10 – 11.5s [1] – constant line from 11.5-12s 3a Density of the ladder is not uniform. 3b Moment = fd = 8 X 10 X 0.90 [1] = 72 Nm clockwise [1]

3c Taking moments about pivot, Sum of clockwise moments = sum of anticlockwise moments 8X10X0.90 + L X 0.54 = 87 X 1.3 L = 76.1 N (upwards) 4a KE = ½ mv2 = ½ (0.400)(2.5)2 = 1.25 J 4bi work done = area under the graph 1.25 = ½ x PQ x 14 PQ = 0.18 m 4bii smooth curve from v = 2.5 at x = 0 to v = 0 at Q curve with increasing gradient 5ai energy transmitted by electromagnetic waves / infra-red rays infra-red or visible < λ < microwaves (scores 2/2 if mentioned infrared in at least one) 5aii1 air is a poor conductor 5aii2 convection not the process as convection current occurs (primarily) upwards / hot air rises 5bi Pt = mlv 120 x 1 = 0.05 x lv lv = 2400 J/g (or 2.4 x 106 J/kg) 5bii work/energy is required to overcome the forces of attraction / to break the bonds of the water molecules kinetic energy / speed of the water molecules does not increase / is constant 6ai compressions and/or rarefactions are closer together OR more compressions and/or rarefactions 6aii layers closer together at compressions layers farther apart at rarefactions 6bi there is always a time difference / time difference is not zero 6bii v = d / t = 1200 / 3.6 = 333 m/s 7a distance between the optical centre and principal focus / focal point 7bi 1. straight line ray from A to B AND lens at intersection of ray and axis. Include label L. 2. ray from A parallel to principal axis, bent at lens to pass through B. F at intersection of ray and axis. Include label F. 3. any third ray from A to B AND bent at lens. 7bii 1. reduced / decreases 2. diminished / smaller 8a The cloth transfers its electrons to the plastic rod during the rubbing process. Hence the plastic rod becomes negatively charged.

8b When the metal rod is held above the plastic rod, the negative charges in the plastic rod will repel the electrons in the metal rod. These electrons will flow from the metal rod to the earth through the hand, leaving behind the positive charges in the metal rod. Therefore the metal rod will become positively charged. 8c Attractive forces between the negatively charged plastic rod and the positively charged metal rod will result in an upward force exerted on the plastic rod. Hence the reading on the balance will decrease. 9ai Resistance = 95 Ω Accept resistance between 95 Ω and 97 Ω. 9aii Current = 20 / (95 + 50) = 0.138 A 9b 50 20 550 150 R R     From Fig. 9.2, when resistance of thermistor is 150 Ω, its temperature is 13 oC. Hence temperature range is 13 oC and above. 9c The alarm bell will be triggered at a temperature range greater or equal to 31 oC. Accept minimum temperature between 30 oC and 31 oC. 10a north south B C axl

10b The function of the split-ring commutator is to reverse the direction of the current along side AB every half a revolution. This occurs whenever the commutator changes contact from one brush to the other. The force acting on AB will change to downwards, and this ensures that the coil will always turn in one direction. 10c As the coil turns, the moment decreases from its maximum value to zero. This is because of a decrease in the perpendicular distance from the axle to the line of action of the force. 11a turns ratio of transformer K = 12 / 24 = 0.5 primary coil turns = 375 secondary coil turns = 750 11b Laminated soft iron [1] Because iron gains and loses magnetism easily and the laminated core reduces the loss of power. [1] 11ci I = V/R = 24/40 [1] = 0.60 A [1] 11cii power loss = I2R = 0.602 x 40 [1] = 14.4 W 12a Z = 89, A = 225 12b ¼ = (1/2)(1/2) = (1/2)2 Hence, 2 half-lives or 2 X 15 = 30 days are required. 13Eai n = sin i/sin r = 1/sin C sin C = sin r / sin i = sin 29 / sin 45 C = 43.3 13Eaii i = 90 – 29 = 61 [1] since angle of incidence is larger than critical angle, [1] and light travels from an optically

denser medium to a less dense medium, total internal reflection occurs. [1] 13Eaiii [1] – light ray parallel to AB [1] – light emerges out of prism as shown 13Ebi UV can causer skin cancer if its over-exposed. 13Ebii v = fλ 3.0 X 108 = f(400X10-9) f = 7.5 X 1014 Hz 13Ebiii X-ray and gamma rays 13Oai V = IR = 2 X 9 = 18V 13Oaii V3 = 22 – 18 = 4V I = V/R = 4/3 = 1.3 A 13Oaiii I = 2 – 1.3 = 0.7 A R = V / I = 4/0.7 = 5.7Ω 13Oaiv Overall resistance will decrease. Current through the bulb will increase (0.7A to 1.5A). Brightness increase. 13Obi 1 year = 365 days = 365 X 24 = 8760 h Power = E/t = 508 000 / 8760 = 58 W 13Obii Difference in cost of usage = (697 – 549) X 0.29 X 8 = $343.36 Fridge D would be cheaper to buy.