2024 CHS Sec 4 Physics Prelim Answer

2024 CHS SEC 4 PHYSICS PRELIMINARY EXAMINATION Paper 1 1 C 2 D 3 B 4 A 5 D 6 D 7 D 8 B 9 D 10 C 11 A 12 C 13 A 14 A 15 B 16 C 17 B 18 A 19 A 20 D 21 A 22 C 23 C 24 D 25 C 26 D 27 C 28 B 29 D 30 A 31 C 32 A 33 A 34 D 35 D 36 D 37 B 38 C 39 D 40 D Paper 2 Section A S/N Mark Scheme Remarks 1ai ● Ball’s speed is decreasing [M1] at a decreasing rate [A1] OR ● Deceleration [M1] is decreasing [A1] OR ● Acceleration is decreasing [M1] and negative [A1] 1 mark awarded for “ball is decelerating at decreasing rate”. 1aii ● (weight and) air resistance acts (on ball) downwards (or in opposite direction to motion/velocity) (resulting in decreasing speed) [B1] ● As speed (or velocity) decreases, air resistance decreases [B1] ● (downward) resultant force decreases (resulting in decreasing deceleration) [B1] No credit is awarded for 1st mark if the answer mentions that upward force is acting on the ball as it is moving up. 1b 1.750 s [B1] No penalty if answer in 2 d.p. 1c ● graph is below x-axis and gradient decreases smoothly without reaching 0 at 3.50 s. [B1] ● final velocity, v is below x-axis and 0 m / s < v < 25 m / s [B1] No penalty if draw past 3.50 s. 2a ● energy transferred mechanically to gravitational potential (and kinetic) store of load [B1] ● (some) energy is transferred o mechanically due to (work done against) friction (or air resistance) [B1] to the internal store of surrounding air (and load). [B1] OR o by heating (due to temperature difference) [B1] to the internal store of surrounding air (and load). [B1] OR o by propagation of electromagnetic waves / infra-red radiation [B1] to the internal store of surrounding air (and load). [B1] OR o by propagation of mechanical / sound waves [B1] to the kinetic store of surrounding air. [B1] If student writes “store” at least once, BOD all answers without “store”. If student does not write “store” at all, penalise 1 mark. No credit awarded for the method of energy transfer to surrounding air if the energy transfer is not from the motor. 2b Moment = F x d [formula] = 2.5 x 0.20 [B1] = 0.50 Nm (or 50 Ncm) [B1]

2 S/N Mark Scheme Remarks 2c As load is stationary, Upward force due to liquid (F) = Weight of Load M = 2.5 N [B1] F = pA = (pbottom – ptop) x A = [(hbottom – htop) ρ g] x A 2.5 = [0.030 x ρ x 10] x (0.12 x 0.02) [B1] ρ = 3 500 kg m-3 Accept if student calculated density of load M [B1] and concluded that density of load M and the liquid are the same [B1] 3a ● molecules are in constant random motion [B1] ● collide with (inner walls of) piston, exerting a force per unit area on it [B1] 3b ● (when gas is heated), molecules collide more frequently and forcefully with (inner walls of) piston, exerting a larger force / pressure (than that due to atmosphere) [B1] ● hence, a downward force (or pressure) is needed to balance this larger force (or pressure) / keep the piston stationary / result in no net force on piston [B1] 3ci ● wood is a poor conductor/good insulator of heat [B1] 3cii ● prevents hot gas rising out of the cylinder / prevents surrounding cool air sinking into the cylinder / minimising convection currents [B1] 3ciii ● reduce heat transfer to surrounding air by infra-red radiation / silver is a poor emitter of infra-red radiation [B1] Penalise if student writes “absorber” or “absorber and emitter” as question specifically mentions “outer walls”. 4a ● a wave that travels in a direction parallel to the direction of vibration (or oscillation) of particles [B1]

3 S/N Mark Scheme Remarks 4bi 4bii 4biii [B1] for W (arrow must be horizontal) [B1] for R (accept any position that is at the lowest point of first 2 cycles) [B1] for M (accept any point where graph intersects with distance axis) 4c ● Frequency of wave depends on the source of vibration, which remains the same. [M1] ● Hence, claim is incorrect. [A1] Also accepted if students say that the wavelength in the graph is constant, (hence since velocity is constant as the medium is constant), then the frequency is constant. 4d v = f λ f = v / λ = 330 / 3.3 = 100 Hz T = 1/f = 1/100 = 0.01 s = 10 ms As 15 ms corresponds to 1.5 periods later, graph is as drawn below. [B1] for obtaining T

4 S/N Mark Scheme Remarks [B1] for graph shape and correct values of maximum and minimum air pressures 5a satellite: microwaves [B1] terrestrial: radio waves [B1] 5b v = d / t t = d / v = (36 000 x 1000) / (3.0 x 108) [B1] = 0.12 s [B1] Penalise 1 mark for unit conversion error 5ci ● Plastic is an optically denser medium / has a higher refractive index than glass [B1] ● Angle of incidence (in plastic) is larger than critical angle (so total internal reflection occurs) [B1] 5cii ● lighter (than copper wires) ● cheaper to manufacture (than copper wires) ● experience less signal loss ● carry much more information (than copper wires) ● faster rate of data transmission [B1] for any answer 6a live: brown neutral: blue earth: green and yellow [B1] for all three correct answers 6b ● neutral wire connected to the live pin/fuse and live wire connected to the neutral pin [B1] Accept “live and neutral wires are swapped in position”

5 S/N Mark Scheme Remarks 6c ● even when fuse is blown (or switch is off), appliance is live / at high potential [B1] ● user can get electrocuted [B1] 6d ● electrical cables are insulated from (internal components which are in turn insulated from) external casing. [B1] ● user cannot get electrocuted as there is no electrical contact between the user and bare wire(s). [B1] allow e.c.f. from (c) Also accept if students stated that “even if the first layer of insulation is worn out, there is an additional layer of insulation”, or words to that effect. 7a ● copper [B1] 7b ● When magnet falls, there is a change in magnetic flux experienced by the copper tube. [B1] ● By Faraday’s Law, there is an induced electromotive force (e.m.f.) in the tube. [B1] ● As the copper tube forms a closed circuit, there is a current induced in the tube. [B1] Allow e.c.f. from (a), except for last [B1] as plastic is not an electrical conductor 7c ● By Lenz’s Law, induced current creates a magnetic field that opposes the approaching (or departing) pole(s) of the magnet. [B1] ● Magnetic force of repulsion (or attraction) is upwards (opposite to direction of weight), resulting in decreasing (downward) resultant force / acceleration. [B1] Accept “approaching and departing poles” as well when magnet is within the tube. 7d South [B1] 8a ● medical X-rays ● building materials ● waste products from nuclear power stations ● rocks ● radon gas ● cosmic rays from outer space (accept the Sun) ● food high in potassium eg. bananas, carrots, salt [B1] Any other possible answer 8b CPM α 1/d2 CPM = k/d2 k = CPM x d2 When d = 1.0, k = (3 620 – 20) x (1.0)2 = 3 600 When d = 2.0, k = (920 – 20) x (2.0)2 = 3 600 When d = 3.0, k = (420 – 20) x (3.0)2 = 3 600 Since the values of k are the same, the claim is true. [B1] to show k is the same for 2 sets of data [B2] to show k is the same for 3 sets of data 8ci 24 000 years [B1] 8cii Count rate due to plutonium alone = 70 – 20 = 50 Count rate due to plutonium alone after 1 half-life = 200 Count rate after 2 half-lives = 100 Count rate after 3 half-lives = 50 Hence, t = 3 x 24 000 = 72 000 years [B1] No working required e.c.f. from (c)(i) ie. 3 times the answer

6 S/N Mark Scheme Remarks 8d Pu94 239 → U92 235 + He2 4 [B1] for correct atomic numbers [B1] for correct mass numbers 8e ● alpha particles are easily absorbed by paper [B1] ● beta particles are suitable [B1] Do not accept gamma rays as they penetrate through paper regardless of thickness. 8f ● wear lead-lined gloves / suits ● use thick concrete walls and lead-lined doors for rooms in