SASS 2025 Sec 4E Physics Prelim answer P1&2

Qn Answer Remarks 1 1(a)(i) P marked in region t = 0 − 0.8 s or in region 1.6 −2.0 s Q marked between 2.0 and 2.5s 1(a)(ii) Must draw tangent drawn at X eg: speed = (3-0.2)/(1.6-0.4) = 2.3 m/s Accept: 1.9 m/s −2.4 m/s 1(b) v A < vC < vD < vB Did not put a point. Badly done. Did not draw tangent. Simply use 1.7/1 Use A, B, C instead of VA 2 2(a) P × 1.5 2(b)(i) moment of W = 210 × 1.0 = 210 Nm 2(b)(ii) clockwise moment = anti clockwise moment P × 1.5 = 210 P = 210 / 1.5 = 140 N 2(b)(iii) P + Q = 210 140 + Q = 210 Q = 210-140 = 70 N Or Q × 1.5 = 210 × 0.5 ok

Q = 210 × 0.5 / 1.5 Q = 70 N 3 3(a) in a system, energy cannot be created or destroyed. It can only be transferred from one store to another, and the total energy remains constant. 3(b)(i) Chemical potential energy of the girl is transferred to kinetic energy of ball. The KE of the ball is transferred to gravitational Potential Energy when the ball reaches its highest height. When falling, gravitational Potential energy of the ball is transferred to kinetic energy. Kinetic energy of the ball is transferred to thermal energy (or internal energy) when it hits the ground and stops moving 3(b)(ii) a = (v – u) / t a = (12-0)/0.6 = 20 m/s2 F = ma = 4 x 20 = 80 N Not able to quote. Use ‘converted’ instead of ‘transferred’ Many did not state ‘Chemical potential energy’ of girl okay 4 2(a)(i) The steam particles hit/collides with sides of container (or piston) and each hit creates a small force. The average force per unit area gives rise to pressure. 2(a)(ii) When the temperature increases, the particles move faster and have higher kinetic energy. They hit the sides of the wall more frequently and with larger force on each hit. The average force per unit area increases thus pressure increases. 2(b) Pressure difference = 2.1 x 106 – 1.0 x 105 = 2.0 x 106 Pa Resultant force = P  A = 2.0 x 106 x 0.00003 = 60 N Words in bold missing Hit harder omitted by many. Quite a number did not convert 0.30 cm2 to m2 5 (a)(i) object within focal length (less than focal length) (a)(ii) any one: from divergent (or parallel) rays or rays do not meet on a screen / do not converge Some stated wrongly Almost all cannot explain

or rays appear to come from image (b)(i) • ray through optical centre from top of object • ray parallel to axis from top of object to lens and line from junction with lens to top of image • both rays extended back to image and ray parallel to axis correctly refracted through lens to appear to come from top of image (b)(ii) 4.0 cm Arrows not drawn. Rays drawn wrongly. 6 (a) steel (b)(i) Magnetic field lines of magnet are cut by the solenoid and electromotive force is induced. Electromotive force e.m.f. cause current to flow in a closed circuit. (b)(ii) Lenz’s law states that the direction of the induced current is such that its magnetic effect always opposes the change producing it. To repel the downward motion of the Soth pole of the magnet, the magnetic effect of the induced current must produce an upward force ,thus a south pole must be produced at the top end of the solenoid as like poles repel. (b) Current changes direction when magnet oscillates up and down and the magnitude of induced current changes. Ok Lose mark for not explaining clearly why there is a current flow. (which is due to e,m,f in a closed circuit)

7 (a) (b)(i) (b)(ii) At higher current (or voltage), the temperature increases, and resistance increases. (c) Rtotal = (1/40 + 1/60)-1 = 24 Ω I = V/R = 240/ 24 = 10 A Wrong symbol Many cannot give correct graph Many cannot explain fully. 8 9(c)(i) Background radiation is the count that is always present due to the environment / surroundings or not deliberately introduced or not being tested any two from: • rock sources (e.g. benches, building materials, human body, Earth crust) • air sources (e.g. radon) • cosmic sources (e.g. Sun, space) Ok Did not read question carefully and just gave any random- like in hospital even though question said what the teacher measured in the lab. Infra-red radiation cannot be detected by radioactivity detector – only alpla, beta and gamma (which are ioniing

9(c)(ii) use tongs / tweezers / forceps or wear lead-lined clothing / lead-lined gloves or wear goggles or reduce time of exposure or keep distance of separation large 9(c)(iii) (radioactive) emission is a random process and occurs at random intervals. Thus, measurements fluctuate about an average value 9(c)(iv) Must measure a few half-lives and take average. Or take from 2 readings that are at least 2 half-lives apart. e.g. (0, 58) and (138, 14.5) 66.0 s ⩽Half -life ⩽ 73.5 s 9(c)(v) (large half-life because of) slow rate of decay B1 radiation / count rate due to 23492U imperceptible / very much less than that of 234 91Pa or (nearly) all of measured radiation / count rate due to 234 91Pa B1 radiation) can be detected. Lead-lined not given when gloves or protective gear were mentioned. Cannot explained clearly. Only take one half-life thus no full credit given. 9 9(a) Total current = 450 + 34 = 484 mA = 0.484 A P = IV = (0.484)(230) = 111.32 W = 111 W (3s.f) (b) (i) Pstandby =IV = (0.034)(230) = 7.82 W E = Pt = 7.82 x (138 x 60 x 60) = 3.88 x 106 J (ii) 1kWh = 1000 x 60 x 60 = 3.6 x 106 J E= 3.88 x 106/3.6 x 106 Generally okay. Wrong formulae used or wrong values used.

= 1.08 kWh Cost = 1.08 x 0.25 = 27 cents C(i) Both the television and computer have the same amount of current in the standby circuit when the appliances are in standby mode, which means they use energy at the same rate when in the standby mode. Since the television is in the standby mode for more hours per week, unplugging the television saves more energy. (ii) The computer is used for more hours per week, therefore it may be more inconvenient to keep plugging and unplugging the computer Cannot do by many Cannot do by most 10 Metal contains free electrons. These free electrons gain internal energy at the heated end and move at high speed from hot area to cold area. The electrons collide with particles and pass on the energy at the cooler end. b(i) 0 oC b(ii)The heat energy is used to increase the potential energy of the molecules to weaken the intermolecular force and not the KE of the molecules. The particles move further apart to overcome the intermolecular forces. ‘Particles’ was used instead of free electrons. ‘Move further apart’ is missing in many answers. Only state PE has no credit given. (b)(iii) Q = mcΔΘ = 1.5 x 2100 x (15) = 47250 = 47000 J (2 sf) (b)(iv) P = E / t = 47250/ (2 x 60) = 393.75 (W) = 390 W (2sf) (b)(v) water has a higher specific heat capacity Generally, okay except f