Bowen Prelim P2 Ans

2024 4E Pure Physics Prelim Paper 2 Mark Scheme Note: 1. All numerical answers can be expressed in 2 SF or 3 SF unless stated otherwise in question. 2. Units of physical quantities must be accurate. Section A No. Ans Marks Remarks/Comments 1a Rate of change for displacement / change in displacement per unit time B1 1bi gravitational field strength = gradient of v-t graph = -1.6 Weight = mass x gravitational field strength = 7.5 x 1.6 = 12 N M1 M1 A1 1bii max height = area under v-t graph during first 2.5 s = ½ x 2.5 x 4 = 5.0 m A1 1biii Gradient of v-t graph is equal to the acceleration due to gravity. 1. Gradient of graph drawn = - 10 2. Graph must start from 4.0 m/s B1 B1 2a pressure = height x density x gravitational field strength = 35 x 1000 x 10 = 3.5 x 105 Pa A1 2bi Pressure calculated was only due to water, however the actual pressure will need to include atmospheric pressure which acts on the surface of the water. B1 2bii Using P = F/A, F = P x A = 450 x 103 x 42 = 1.89 x 107 N A1 2c A vector quantity has both magnitude and direction, while a scalar quantity has only magnitude. B1 2d Pressure at P = atmospheric pressure = 1.0 x 105 Pa = 735 mm Hg Pressure at P = Pressure at Q + pressure due to mercury column Pressure of gas = Pressure at Q = 735 – 320 = 415 mm Hg M1 A1 3a Near surface of liquid nitrogen. B1 3b Any of two of the following:  Boiling takes place at a fix temperature (boiling point) and evaporation takes place at any temperature.  Boiling needs a heat source where evaporation does not.  Rate of boiling is faster compared to evaporation.  During boiling, temperature of liquid remains constant but for evaporation, temperature of liquid drops. B1 B1

No. Ans Marks Remarks/Comments 3c Vacuum reduces the amount of energy gain from surrounding because vacuum stop energy transfer via conduction and convection from taking place due to lack of matter. B1 B1 3di 50 g = 0.050 kg energy lost from metal = mass of metal x specific heat capacity of metal x change in temperature = 0.050 x 390 x 103 x (20 – (-196)) = 4.2 x 10 6 J (2 SF) M1 A1 3dii Assuming all energy used to boil nitrogen is from the piece of metal. energy used to boil = mass of nitrogen x latent heat of vaporisation of nitrogen mass of nitrogen = 4.2 x 10 6 / 200 = 21000 g (2SF) or 21 kg A1 4a Microwaves B1 4b (i) X rays (ii) Microwaves (iii) Infra-red Note: B2 for all correct or B1 for 2 of 3 correct B2 4c Ultrasound is a type of sound (or longitudinal) wave, thus it is not an electromagnetic wave. B1 4d (i) Ultrasound: Both source and detector on the same side above the patient’s body. (ii) X-ray: Source and detector opposite of each other of the patient’s body. B1 B1 5a Plastic ruler loses electrons to the cloth and becomes positively charged. B1 5b Droplets are charged positively/negatively when the leave the sprayer. Like charges repel, this causes the droplets to spread out. As charged droplets approached the leaf, the charges on droplets are attracted to the opposite charge (or induced opposite charge) on the leaf. When droplets land on the leaf, they are discharged due to leave being earthed. These helps to allow leaf to be covered with insecticide. B1 B1 B1 6a (i) direction of current is from B to A (ii) Magnetic field drawn from N pole to S pole B1 B1

No. Ans Marks Remarks/Comments 6b When switch is on, current flowing through metal rod AB produces a magnetic field around the rod. This magnetic field interacts with the external magnetic field by the magnet and produces a magnetic force to act on the metal rod. This magnetic force will cause the metal rod to move upwards. Note: student must state the correct direction of movement to be given credit. B1 B1 B1 6c D.C. Motor B1 7ai As tape passes the recording head, the magnetic field produced by the head induced magnetism on the tape. B1 7aii By changing direction of the current in the coil, the direction of magnetic field changes. This allow the sections of the tape to be magnetized in opposite directions. B1 7aiii For the same length of tape, there will be lesser alternate sections and each section will be longer. B1 7b Advantage: It can be easily demagnetized and reuse for new recording Disadvantage: The recorded data can be loss easily over time as it can be easily demagnetized. Accept other logical and suitable answers B1 B1 8a Power input is equal to power output OR Energy is conserved B1 8b The alternating current flowing in the primary coil produces changing magnetic flux. This changing magnetic flux is directed towards the secondary coil through the iron core. With the changing magnetic flux in the secondary coil, it induces an output voltage in the secondary coil. B1 B1 B1 8c N p/Ns = Vp/Vs = 25 000 / 400 000 = 1:16 or 1/16 A1 8d Input power = output power 400 000 x Is = 25 000 x 12 000 Is = 750 A A1 8e P = IV = 750 x 400 000 = 3.0 x 108 W A1 8f Reduce loss of power during transmission of electricity due to lower current used for transmission. B1 9ai Ratio of speed of light in vacuum to speed of light in medium. Or Speed of light in vacuum is 1.3 times the speed of light in medium. B1 9aii Total internal reflection B1

No. Ans Marks Remarks/Comments 9aiii Cladding has lower refractive index compared to core. For total internal reflection to take place, light must travel from optically denser to optically less dense medium. B1 B1 9aiv Distance travelled is longer due to the path that was taken. Speed of light in optical fibre is slower than in air. B1 B1 9bi Total internal reflection occurs. When angle of incidence is 50o, the corresponding angle of refraction is 90o. Therefore, 50o is the critical angle of light in water. When angle of incidence exceeds 50o, total internal reflection takes place. Note: Student must explain why 50o is the critical angle in water and then relate to angle of incidence exceeding critical angle which result in T.I.R. B1 B1 9bii sin c = 1/n n = 1/sin 40o c = 1.6 (2 SF) M1 A1 10a Spontaneous decay mean that it is not possible to predict when a radioactive nucleus will emit radiation. B1 10b B1 B1 10ci Accept one of these 19 or 20 or 21 B1 10cii count rate = count at 3.0 cm – background count = 429 – 19 = 410 The background count used must be the one answer by student in 10ci A1 10ciii 5.0 cm. Count rate per minute decreases to background count from 6.0 cm, thus 5.0 cm is the maximum distance. B1 B1 10di 4 days B1 10dii After 20 days, the isotope has less than 5 % of radioactivity. However, after 5 day, the isotope has 42.5 % of radioactivity. B1 B1 Section B Answer one question. 11ai Force drawn is furthest from the hinge. B1 11aii Matchbox topple and fall over eventually. As cardboard is raised, line of action of the weight of the matchbox will eventually fall out of the base. The weight of the matchbox then produces a clockwise moment to topple the matchbox. B1 B1 B1 Ra88226→ Rn + α 222 86 4 2

11bi  Any two correct forces to be awarded B1  All four correct forces to be awarded B2 B2 11bii Weight of box = 2.5 x 10 = 25 N Taking moment about point X, 25 x 1.5 + 400 x 3.0 = Ty x 4.5 Ty = 275 N Tx + Ty = 400 + 25 Tx = 400 + 25 – 275 = 150 N M1 M1 M1 A1 12a Resistance of X decreases as p.d. increases from 0 V to 2.0 V. B1 12b From 2.2 V to 3.8 V. Gradient of the I against V graph is constant, thus resistance is constant like an ohmic condu