Bedok South 2024 Sec 4EXP PHY PRELIMS P2 MS for students

2024 EXP PHYSICS PRELIMS – Marking Scheme 1ai Pressure due to the mercury column is greater than the atmospheric pressure. This pushes the mercury out of the tube / (resultant) force downwards and mercury flows out of the tube [1] [1] 1aii Vacuum / empty space [1] 1b Atmospheric pressure = h ρ g and h in metres ρ: density of the mercury g: gravitational field strength h: height of the mercury column (above the mercury level in dish or clearly labelled on Fig 1.2) [1] [1] 2a Ray to be drawn. Critical angle marked C (Angle of refraction in this ray diagram). [1] [1] 2b* For angle of incidence = 90° or incident ray to be horizontal, angle of refraction is equal to critical angle & cannot be smaller than that For light to travel to the corner, it needs to travel horizontally before reaching the water. At the corner light refracts, its angle of refraction is equal to critical angle and can’t be smaller due to the formula 𝑛𝑛 = 1 sin 𝐶𝐶. [1] [1] 2c 𝑛𝑛 = 1 sin 𝐶𝐶 = 1 / (sin49) = 1.33 [1] [1]

3a Magnified, inverted, real Marker’s feedback: • inverted is not equivalent to laterally inverted. • Many failed to compare the differences between object and image as they deduce directly by looking the image arrow only. [1] 3b • straight ray from R to top of image • ray crosses principal axis, vertical line drawn & L labelled • parallel ray to principal axis from R to lens refracted to top of image • ray crosses principal axis, marked as F [1] [1] [1] [1] 4a I = Q / t 0.0016 = 7.2 / t t = 4500s [1] [1] 4bi* Reading on ammeter 2 remains unchanged. Potential difference across the resistor N remains the same when LDR resistance changes due to the pd across N is not affected by the LDR [1] [1] 4bii Time taken for 7.2C to pass through ammeter is increased. Resistance of LDR increases, effective resistance of the circuit increases that decreases current flow in ammeter 1. (This is missing in most answers.) [1] [1] 5ai I = V/R = 12 / (20 + 28) = 0.25 A [1] [1]

5aii Magnetic field between the poles of magnet interacts with the magnetic field due to the current in YZ Marker’s feedback: • Many candidates are still confused with the concept of Force acting on current-carrying conductor in a magnetic field vs concept of electromagnetic induction (generating emf using motion of a conductor in a magnetic field). Pls revise Chapter 20 & 21 thoroughly again. • This question is testing on the concept of force acting on a current- carrying conductor in a magnetic field. Applying FLHR. • It has nothing to do with the concept of electromagnetic induction. Hence terms like flux linkage is incorrectly used. [1] 5aiii Force is acting downwards / towards the bottom of the paper Using Fleming’s left hand rule, index finger points to the right while middle finger points from Z to Y. Thumbs points downwards that represents force. Mark