assumption 2020 4E5N SciPhy Prelim Ans

Sec 4 Express / 5 Normal Academic Preliminary Examination2020Science (Physics) 5076Answer Keys and Marking Scheme PAPER 1 – 20 M1 2 3 4 5 6 7 8 9 10 B A D D C A C C D C 11 12 13 14 15 16 17 18 19 20 C D A D C A D C B C PAPER 2 SECTION A – 45 M1a diameter of Earth 10 Mmdiameter of an atom 0.1 nmdiameter of human hair 0.1 mm distance between Bukit Panjangand Bugis by car 15 km B2 – all correct, B1 - 2 to 3 correct, B0 – 0 to 1correct B2 1b Taking avernier callipers /micrometer screw gauge,measure the thickness of200 pieces of papers.Then divide by 100 to get theaverage thicknessof1 piece of paper. B2 – all 3 points, B1 – 2 points, B0 – no marks ifjust mention instrumentwithout description of steps B2 2a B1 – correct plotted pointsB1 – smoothness of the graph B2

2b Constant acceleration for the first 6 seconds.Constant velocity from 6 to 12 seconds.Decreasing deceleration from 12 to 22 seconds. B2 – all 3 points, B1 – 2 points, B0 – 0 to 1 point (ecf from 2a) B2 2ci (ecf from 2b) M1 A1 2cii (ecf from 2c) B1 3a For a body in equilibrium, thesum of anticlockwisemoments must beequal to the sum of clockwise momentsabout thesamepivot. *note: must include “same pivot” to award B1 B1 3b M1 A1 3c Move the counterweight toincreasetheperpendiculardistanceto thepivot,So that for thesame moment, F can be reduced. Or Reduce theweightof the pole,So that the total clockwisemomentdecreases, learningto a smaller F. Accept other reasonable answers. B1B1 4a Energy cannot be created or destroyed.However it can be converted from one form to another.Total energy must remain the same. B2 – all 3 points, B1 – 2 points, B0 – 0 to 1 point B2

4bi 1. M1A1 4bi 2. M1 A1 4bii The kinetic energy at Cis less than the gravitationalpotential energyatA because some of the gravitational potential energyisconverted to heat(thermal energy) and/or sound energydue to friction.(or loses energy tosurrounding as heat and/or sound energy) B1 B1 5a Air/plastic/foam is a bad conductor of heat, so heatloss from the water isreduced. Conduction cannot happen through vacuum. B1 B1 5b The plastic cap prevents convection from setting upabove the water. Convection cannot happen in a vacuum. B1 B1 5c The silvered surfaces reflectradiantheat back intothe water, reducing heatloss. Or The silvered surfaces arepoor emitter of radiation,reducing heat loss. B1 6ai 1. 1.25 s B16ai 2. 0.8 Hz (ecf from 6ai 1) B1 6aii The ball will movedownwardto the trough and moveback up again. B16bi Ultraviolet radiation B16bii Might get sunburnt.Might have skin cancer.Ionise the human cells. Accept other reasonable answers. B1 7a The car door isnegatively-chargedas the positively-charged droplets areattractedtothe door. B1B1

7bi An electric field is a region where an electric chargewill experience a force. B17bii B0 if lines touch or cross each other. B1 8a Current is the rate of flow of charges. B18b 0 A because current flows through S1 as it is a lowerresistance wire. OR 0 A because when S1 is closed, short circuit occurred and no current flowsthrough the resistor. B1 8c Total effective resistance will increase. Therefore with the same emf through the circuit, thecurrent will decrease.ThusA2 will be lower. B1 B1 8d M1 M1 A1

PAPER 2 SECTION B – 20 M9ai B1 9aii It is the angle of incidence where the angle of refractionin the optically lessdense medium is 90°. OR It is the angle between the normal and the incidentray in which the refractedray travels along the surface of the block. B1 9aiii B1 9iv B1 9bi Any light ray that shows the location of the F accurately. B1 9bii Focal length is the distance between the optical centreand the focal point. B19biii Projector B1

9c (i) Show that image is equidistant away from the mirroras the object (ii) B2 if – two correct light rays with arrows anddotted lines behind mirror,B1 if – two correct light rays but no arrows– two correct light rays with arrowsbutnotdotted lines behind mirror– two correct light rays with dottedlines behind mirror but no arrows– one correct light ray with arrow anddotted lines behind mirror B1 B2 10ai Longitudinal wave is where the vibration of the particlesis parallel to thedirection of wave motion. B1 10aii When the sourcevibrates, the surrounding air particlesare displaced. Thisproduced aseries of compressions and rarefactionsuntil they reach themicrophones in the form oflongitudinal waves. B2 – all 3 points, B1 – 2 points, B0 – 0 to 1 point B2 10aiii M1 A1 10bi t0 islongerthantbecausethedistancetravelled without cracks islonger. B1B1 10bii M1A1 10biii Sound is less harmful.Sound requires less energy.Sound is convenience.The imaging result using sound is instantaneous.X-ray is dangerous.X-ray requires special equipment / operators.X-ray can only image one at a time. B1

Accept other reasonable answers.11a Record the ammeter reading and voltmeter reading.Repeat the experiment by changing the resistance onthe rheostat.Plot a graph of V against I.The gradient is the resistance of the light bulb. B3 – 4 points, B2 – 3 points, B1 – 2 points, B0 –0 to 1 point B3 11b Y = 120 ΩZ = 90 Ω B1B1 11ci Fuse.It melts andbreak the circuitwhen the current istoo high. B1 11cii Wire A.If the live wire is exposed to the metal casing, theearth wiredirects thecurrent awayfrom the user,causing thefuse to breakand thuspreventing electric shock. B2 – correct wire and 3 points, B1 – correct wireand 2 points, B0 – correctwire but 1 point / B0 if wrong wire is identified B2 11ciii The switch should be connected to thelive wirebecause the electric cookerwill not become live /exposed to highvoltage,thuspreventing user from electric shock. B2 – 3 points, B1 – 2 points, B0 – 0 to 1 point B2