2022 ANDSS Phy Prelims P2 Ans

1 EXAM ANSWERS Year: 2022 Exam: Prelim Level/Stream: 4 Express Subject: Pure Physics 6091/02 s.f./d.p.: No limit to deduction of s.f. & deduct 1 mark per s.f./d.p mistake Paper 2 Section A Qn Answers Marks Comments 1a Electric charge is a scalar quantity, as it has magnitude but no direction. B1 Both needed for 1 mark 1bi Average velocity is calculated by taking the total displacement over total time while average speed is the total distance travelled over the total time taken. The displacement of the particle is smaller as the straight line distance between X and Y is shorter than the curved path from X to Y which is the distance travelled. B1 B1 1 mark for differentiating velocity and speed 1bii B1 B1 1 mark for both forces correctly labelled 1 mark for the upward arrow larger than downward arrow Electric/electrostatic force weight

2 2a 0.5 x 6.8 x 2 – 0.5x 3.4 x 1.0 = 5.1 m downward C1 A1 A1 Give max 1 mark if added 2nd mark not given if negative sign included Last mark given only if observed from working 2bi A = 6.8 /2.0 = 3.4 m/s2 C1 A1 2bii M = w/g = 15/10 = 1.5 kg Resultant Force = 1.5 x 3.4 = 5.1N Tension = 15 + 5.1 = 20.1 = 20 N (2sf) C1 A1 Give ECF is 2bi is incorrect 2c Initial velocity/acceleration is the same but line will continue with decreasing gradient v will reach zero after 2.0s. B1 Marks are given as long as curve shape is correct 3a Work done = 14.5/100 x 4.7 = 0.68 J C1 A1 3b Since it is slower, there must have been less force applied which means less work done on the trapped air. Or B1

3 More thermal energy is lost to the surroundings as there is more time for the energy to be conducted/radiated away *must mention longer time not just more thermal energy lost 4a The air particles are moving randomly at high speeds. B1 4b 0.1 MPa = 100000 Pa B1 4bii Pressure at 40m underwater = 40x1025x10+100000 = 410 000 + 100000 = 510 000 Pa Based on graph time = 26-28 minutes C1 A1 Maximum 1 mark if student did not add atmospheric pressure and got 30-32 min 5a For an object in equilibrium, the sum of clockwise moments about a pivot is equal to the sum of anti-clockwise moments about the same pivot. B1 B1 5bi Sum of CM = sum of ACM 0.45W + 19 x 1.3 +1.85W = 2.6 x 22 2.3 W + 24.7 = 57.2 2.3 W = 32.5 W = 14.1 = 14 N (2sf) C1 A1 1 mark for correct substitution 5bii F + 22 = 19 +28 F = 25 N upwards B1 Give full ECF

4 6a Brass is a much better conductor of thermal energy than wood. The thermal energy is quickly conducted away by the brass rod and the paper doesn’t reach a high enough temperature to start burning Or the thermal energy cannot be conducted away quickly enough by the wooden rod thus the paper reaches a sufficiently high temperature to start burning C1 A1 1 mark for stating the difference between brass and wood 6b The ice cubes gain thermal energy via radiation as the ice cubes are colder than the surroundings. Some thermal energy is also gained via conduction through the plastic bag from the surrounding air. B1 B1 7a It means that the ratio of the speed of light in vacuum over the speed of light in the medium is 1.55 B1 Accepted speed of light in vacuum divided by speed of light in medium is 1.55 7b C = sin-1(1/1.55) = 40.2° (3sf) C1 A1 7c sin I = opposite/hypotenuse = 7.0 / 8.0 I = 61° *accept 61.0° C1 A1

5 7d C1 A1 8a Particles in the stream of water with the same charge as the charged rod will be repelled away while those of the opposite charge will be attracted. As the force of attraction is stronger than the force of repulsion due to a shorter distance, the water will move towards the charged rod. B1 B1 B1 8bi As the electric field line directions are by convention the same direction as what would happen to a positive test charge, thus B is positively charged, while A is negatively charged. C1 B1 8bii The field lines for both cannot intersect with one another Or B1 Did not accept both have lines/arrows 8.0 cm 8.0 cm 1.0 cm air plastic 1.0 cm

6 The distance between the field lines indicate the strength of the electric/magnetic field Did not accept force in lieu of field strength 9a The soft iron inside the reed switch will be attracted towards each other. As both strands of soft iron become induced magnets, they will have opposite poles in the middle of the glass which attract each other. B1 B1 9b As the temperature rises, the resistance of the thermistor decreases. Thus the current in the circuit increases. This increases the magnetic field strength of the coil which causes the reed switch to be attracted to each other thus closing the switch. B1 B1 10a When the current flows through the coil, the circular magnetic field around the coil (or CD or AB) interacts with the magnetic field of the permanent magnet. This causes a downward force on CD (or next to South pole) and an upward force on AB (or next to North pole). The resulting clockwise moment causes the coil and the pointer to rotate (or turn) clockwise. B1 B1 B1 10b Both rods will move/accelerate to the right. B1 B1 FLHR must be mentioned in full

7 According to FLHR, as the current flows down, the magnetic field is into the paper, the thumb which represents motion is pointed to the right. No credit for contradicting the direction. Paper 2 Section B Qn Answers Marks Comments 11ai E = Pt = 2000 x 158 = 316000 J (accept 320 000 J) C1 A1 11aii Energy input = 316000 Energy output = 0.75 x 74.5 x 4200+0.01x2260000 = 257275 Efficiency = 257275/316000 = 81% or 81.4% (2sf) B1 B1 1st mark is given only if the energy output working is correct. 2nd mark can be given as long as it is calculated output/input (from 10bi). Credit given for displaying output/input

8 11bi The student should only use the shaved ice once it starts melting but not completely liquid. B1 11bii Energy output = (800-11.2)/1000 *334 000 = 263 459.2 J Efficiency = 263 459.2/316000 = 83% or 83.3% (accept 2 or 3 sf) C1 A1 Accept 82.3% if 320000 is used as the input. 11biii The temperature difference is lower between 0°C and room temperature as compared to 100°C and room temperature. So the rate of thermal energy gain/loss is smaller. B1 Both points needed 11ci Double insulation means that the electrical cables are insulated from the internal components and the internal components are insulated from the external casing. B1 B1 All must be mentioned. 11dii The kettle may not be waterproof which means water can go inside the kettle and cause a short circuit/electrical fire if it comes to contact with the wires. B1 12ai As the coil rotates, the magnetic flux linkage between the coil and the magnets change Or B1 Concept of induced EMF

9 As the coil rotates, it cuts the magnetic field lines between the magnets, an EMF is induced in the coil. Every 180° turn, the induced EMF in the coil reverses direction as the coil moves in the opposite direction. As the induced current flows as the coil is a closed circuit. B1 B1 Concept of alternating Concept of load which turns EMF to current 12aii Slip rings are connected to each side of the coil to maintain electrical contact and ensure induced current flows (to external circuit). B1 12aiii When the transformer is disconnected, the circuit connected to the slip rings is no longer c