AHS 2025 PHY PRELIM P2 MS

1 2025 AHS Physics Prelim Paper 2 Answers 1 (a) (Electric) current, (thermodynamic) temperature, amount of substance (any two) [2] (b) (i) Circle the value for the prefix deci. 10-9 10-6 10-3 10-2 10-1 103 106 109 1012 [1] (ii) G , k , m , µ , n [1] 2 (a) Calculate the displacement of the car from point A at t = 90 s. Displacement = area (+) – area (-) = ½ (50 + 70) (30) - ½ (10) (20) [1] = 1700 m [1] (b) Describe the motion of the car from t = 80 s to t = 90 s. Travel backwards ( to the West ) [1] also accept opposite , (negative not accepted) Accelerating at a constant rate / constant acceleration [1] also accept descriptions using speed or velocity 3 Correct shape with correct angles [1] Correct arrows + labels [1] Magnitude = 69 N +/- 1 N [1]

2 4 (a) State the principle of moments. For an object in equilibrium, [1] The sum of clockwise moments about a pivot is equal to the sum of anticlockwise moments about the same pivot. [1] (b) Determine the force exerted by the worker at point X. Sum of anti-clockwise moments = sum of clockwise moments 20 x 1.3 = F x 0.60 + 16 x 0.85 [1] F = 21 N [1] 5 (a) Liquid B is mercury, which has a density of 13 600 kg / m3. The gravitational field strength g is taken to be 10 N / kg. Determine the density of liquid A. Same atmospheric pressure, hence hA ρA g = hB ρB g [1 algebraic or numerical form] (120 -15 – 20) x ρA = (120 – 25 – 20) x 13 600 ρA = (75 / 85) x 13 600 = 12 000 kg / m3 [1] (b) Without using any additional apparatus, suggest how you can check that there is no air that is trapped at the top of any one of the inverted glass tubes, while it is at the same place. Tilt/ lean the inverted tube sideways/push inverted tube A downwards until the closed end is below 85 cm (liquid A) or 75 cm (liquid B) in height [1] Glass tube fully filled with liquid / no air bubble present [1] Alternative answers. Push the tube down/ add liquids into the troughs/ tilt the tube sideways [1] ensure that the height of the liquid column remains constant[1] 6 (a) (i) Calculate the change in energy in the kinetic store of the skier as he travelled from point T to point B. Change in Ek = Ek at B - Ek at T [1 algebraic or numerical must show difference, not just KE formula] = ½ (75) (302 – 2.02) = 33 600 J or 34 000 J [1]

3 (ii) Hence, determine the height of point T from point B. Change in EP = Change in Ek = 34 000 (75) (10) (h) = 34 000 h = 44.8 m or 45 m [1] (b) Upon reaching point B, the skier digs his skis into the snow and comes to a stop at point P after moving horizontally for 33 m. Calculate the average braking force that acts on the skier between points B and P. F x 33 = ½ (75) (302) F = 1020 N or 1000 N [1] (c) Describe, using ideas about energy stores and transfer pathways, how energy was transferred as the skier moves from point B to point P. Energy in the kinetic store (of skiers) is transferred to the internal store (of the skiers and the ground) [1] mechanically (by friction acting over the distance BP) [1] 7 (a) Define the focal length of a converging lens. The distance between the optical centre (of lens) and the focal point . [1] (b) Describe briefly an experiment to determine the focal length of such a lens. You may include a diagram in your description. Place a lens between a far away/distant object and a screen. (or shown in diagram) [1] also accept object at infinity Adjust the distance between the lens and the screen until a focused/sharp image is formed. the distance between the lens and the screen is the focal length. [1] OR Place a lens between an object and a screen. Move the object and screen until the size of the object and sharp image are the equal/ the same. [1] The distance between object and lens or image and lens is equal to twice the focal length. (hence divide object distance or image distance by 2 to get focal length. [1] OR Place a lens between an object and a screen. Move the screen until a sharp image is produced. Measure the distance between object and image, and the respective sizes of the object and image. [1] By drawing a ray diagram using the details above, identify the focal point where the light ray cuts the principal axis. [1]

4 (c) A small object of height 4.0 cm is placed 5.0 cm away from a thin converging lens of focal length 10.0 cm. Using a suitable scale, draw a ray diagram to determine the size of the image. Suggested scale 1cm:2cm 1 mark for each light ray [2] 1 mark for correct height 8.0 cm +- 0.2 cm (allowance of 1 mm based on scale used) Penalise [maximum 1] for solid image or solid construction lines (d) Other than the location and the magnification of the image, state two other characteristics of the image formed. Virtual, upright [1] each [2]

5 8 (a) p.d.= 16 V[1] (b)(i) Calculate the current in the ammeter. 𝐼 = 𝑉 𝑅 [1 𝑎𝑙𝑔𝑒𝑏𝑟𝑎𝑖𝑐 𝑜𝑟 𝑛𝑢𝑚𝑒𝑟𝑖𝑐𝑎𝑙] 𝐼 = 16 1 + 3 = 4.0 𝐴 [1] (ii) Calculate the potential difference across the 3.0 Ω resistor. 12 𝑉 [1] (iii) The 1.0 Ω resistor is replaced with a different resistor. The potential difference across the 3.0 Ω resistor is now 1.5 V. Calculate the resistance of the new resistor. 1.5 = 3 3+𝑥 × 16 [1] 𝑥 = 29 𝛺 [1] OR New current through 3.0 Ω , I = V / R = 1.5 / 3.0 = 0.50 A [1] R of new resistor = V / I = (16 - 1.5) / 0.50 = 29 Ω [1] 9 (a)Complete Fig. 9.1 by calculating the amount of energy used by each appliance. [1] appliance power rating / W time switched on / hours energy used / kWh TV and computer 1000 2.0 2.0 kettle 2000 0.20 0.40 lights 800 3.0 2.4 water heater 3500 0.20 0.70 air conditioner 1500 6.0 9.0 1 mark for all correct values [1] (b) The cost of electricity is 31 cents per unit. Calculate the cost of the electricity usage for one day. 𝑡𝑜𝑡𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 = 2.0 + 0.4 + 2.4 + 0.7 + 9.0 = 14.5 𝑘𝑊ℎ [1] 𝑐𝑜𝑠𝑡 = 14.5 × 0.31 = $4.50 [1]

6 (c) The wires supplying electric current to the water heater are thicker than those supplying current to the lights. Explain why this is necessary. As the power supplied is higher, the current flowing through the wires are higher. Having wire thick will lower the resistance of the wire to reduce energy / power loss / prevent overheating / melting of the wires. [2 marks for 3 points, 1 mark for 1-2 points] 10 (a) Explain why an e.m.f. is induced in the coil as the turntable rotates. As the turntable rotates, the magnetic flux in/linking the coil changes [1] also accept magnetic field/ lines passing through/linking /cutting the coil changes Due to electromagnetic induction, there is an induced e.m.f.. [1] (b) As one of the magnets approaches the coil, the magnet experiences an opposing force. Explain why. The induced current produces a north pole near the magnet. [1] Like poles repel. [1] Alt. The magnetic effect of the induced current opposes the change inducing it [max 1] (c) The graph below shows how the e.m.f. varies with time when the turntable rotates at a constant speed. On Fig. 10.2, draw the graph for 0 to 30 ms to show how the e.m.f varies with time if the turntable rotates at twice the speed. [2]

7 11 (a) State one other variable that should be kept constant to ensure a fair test. Size (dimensions) of bowl; size of ice cubes; surrounding temperature (any 1) [1] (b) Using the information from Table 11.1, suggest which material is better for insulation and explain your choice. Polystyrene is a better insulator, due to slower rate of energy transfer to ice Use data to justify [1]; example – a