Secondary 4 Pure Physics Practice Paper 5

TuitionGoWhere Practice Paper – Answer Key and Marking Scheme

Subject: Pure Physics Level: Secondary 4 Paper: Practice Paper – Electricity & Magnetism Version: 5 of 5 Total Marks: 60

Section A: Multiple Choice (5 marks)

Explanations:

1. When a positively charged rod is brought near the sphere, electrons are attracted to the near side. Earthing allows electrons to flow from earth to the sphere. When the earth is removed and then the rod, the sphere retains excess electrons and is negatively charged. Answer: B

2. Conventional current flows from the positive terminal (higher potential) to the negative terminal (lower potential) in the external circuit. Answer: B

3. For an ideal transformer: V_s / V_p = N_s / N_p. V_s = 240 × (50/500) = 24 V. Answer: A

4. An RCCB (Residual Current Circuit Breaker) detects any imbalance between live and neutral currents, indicating current leakage to earth. Answer: D

5. By Fleming's Left-Hand Rule, the force on a current-carrying conductor in a magnetic field is perpendicular to both the current direction and the magnetic field direction. Answer: C

Section B: Structured Questions (35 marks)

Question 6 (4 marks)

(a) Place the bar magnet on a sheet of paper. Place the plotting compass near the north pole of the magnet. Mark the positions of both ends of the compass needle. Move the compass so that its south pole is at the previously marked north pole position. Repeat this process, marking dots as you go, to trace a complete field line from the north pole to the south pole. Repeat for several lines around the magnet. [2 marks – 1 for method, 1 for detail about tracing lines]

(b) Diagram should show:

• Field lines emerging from the north pole and entering the south pole [1 mark]
• Arrows on lines pointing from north to south [0.5 mark]
• Lines are closer together near the poles (stronger field) [0.5 mark]

Question 7 (5 marks)

(a) For resistors in parallel: 1/R_total = 1/R₁ + 1/R₂ 1/R_total = 1/4.0 + 1/6.0 = 3/12 + 2/12 = 5/12 R_total = 12/5 = 2.4 Ω [2 marks – 1 for formula, 1 for correct answer with unit]

(b) I = V/R = 12/2.4 = 5.0 A [1 mark]

(c) In parallel, voltage across each resistor = 12 V. I₄ = V/R₄ = 12/4.0 = 3.0 A [2 marks – 1 for method, 1 for correct answer with unit]

Question 8 (6 marks)

(a) P = IV, so I = P/V = 2200/240 = 9.17 A (or 9.2 A) [2 marks – 1 for formula, 1 for answer with unit]

(b) Q = mcΔθ = 1.5 × 4200 × (100 – 25) = 1.5 × 4200 × 75 = 472,500 J [2 marks – 1 for formula, 1 for correct answer with unit]

(c) Efficiency = Useful energy output / Energy input 0.80 = 472,500 / (P × t) 0.80 = 472,500 / (2200 × t) t = 472,500 / (0.80 × 2200) = 472,500 / 1760 = 268.5 s (≈ 269 s or 4 min 29 s) [2 marks – 1 for correct setup, 1 for answer with unit]

Question 9 (5 marks)

(a) The galvanometer needle deflects momentarily in one direction (e.g., to the right). [1 mark]

(b) As the magnet moves into the coil, the magnetic flux (or magnetic field lines) passing through the coil changes. By Faraday's law of electromagnetic induction, a changing magnetic flux induces an e.m.f. across the coil, causing a current to flow. The deflection is momentary because the induced e.m.f. exists only while the flux is changing. [2 marks – 1 for changing flux, 1 for induced e.m.f./current]

(c) The galvanometer needle returns to zero (no deflection). When the magnet is stationary, the magnetic flux through the coil is constant (not changing). No e.m.f. is induced, so no current flows. [2 marks – 1 for observation, 1 for explanation]

Question 10 (5 marks)

(a) Total resistance R_total = 200 + 100 = 300 Ω Circuit current I = V/R_total = 10/300 = 0.0333 A Output voltage across fixed resistor V_out = I × R = 0.0333 × 200 = 6.67 V (or 6.7 V) [3 marks – 1 for total resistance, 1 for current, 1 for output voltage with unit]

(b) When the LDR is covered, its resistance increases (in darkness). The total resistance increases, so the current decreases. The output voltage across the fixed resistor decreases (since V = IR, and I decreases while R is fixed). Application: automatic street lighting (when dark, the voltage across the LDR increases, triggering the light to turn on) or burglar alarm. [2 marks – 1 for explanation, 1 for application]

Question 11 (5 marks)

(a) Step-down transformer. The secondary coil has fewer turns (100) than the primary coil (2000), so the output voltage is lower than the input voltage. [1 mark]

(b) V_s / V_p = N_s / N_p V_s = V_p × (N_s / N_p) = 240 × (100/2000) = 240 × 0.05 = 12 V [2 marks – 1 for formula, 1 for answer with unit]

(c) For 100% efficiency: V_p × I_p = V_s × I_s 240 × I_p = 12 × 0.50 I_p = (12 × 0.50) / 240 = 6.0 / 240 = 0.025 A (or 25 mA) [2 marks – 1 for formula, 1 for answer with unit]

Question 12 (5 marks)

(a) Graph should show:

• Correctly labelled axes (Current/A on y-axis, Voltage/V on x-axis) [0.5 mark]
• Appropriate scales [0.5 mark]
• All points plotted accurately [0.5 mark]
• Smooth curve drawn through points (not straight lines between points) [0.5 mark]

(b) The resistance increases as the voltage increases. (The graph curves towards the voltage axis, showing that current increases at a decreasing rate.) [1 mark]

(c) As current flows through the filament, it heats up. The temperature of the filament increases. In a metal, resistance increases with temperature because the metal ions vibrate more vigorously, causing more frequent collisions with the flowing electrons, impeding their flow. [2 marks – 1 for temperature increase, 1 for explanation of increased resistance]

Question 13 (4 marks)

(a) When current flows through the coil, each side of the coil experiences a force because it is a current-carrying conductor in a magnetic field. By Fleming's Left-Hand Rule, the forces on opposite sides of the coil act in opposite directions (one upward, one downward), creating a turning effect (a couple) that causes the coil to rotate. [2 marks – 1 for forces on sides, 1 for turning effect/couple]

(b) The split-ring commutator reverses the direction of the current in the coil every half-rotation. This ensures that the forces on the sides of the coil always act in the same rotational direction, allowing continuous rotation. [1 mark]

(c) Any one of: increase the current in the coil; use a stronger magnet; increase the number of turns on the coil; wind the coil on a soft iron core. [1 mark]

Question 14 (5 marks)

(a) The fuse contains a thin wire that melts and breaks the circuit if the current exceeds the fuse rating (e.g., 13 A). This prevents overheating of the wires and reduces the risk of electrical fires. [2 marks – 1 for melting/breaking circuit, 1 for preventing overheating/fire]

(b) If a fault occurs (e.g., the live wire touches the metal casing), the casing becomes live. The earth wire provides a low-resistance path for the current to flow to the ground. This large current blows the fuse (or trips the circuit breaker), disconnecting the appliance from the supply and preventing electric shock to the user. [2 marks – 1 for providing path to earth, 1 for blowing fuse/preventing shock]

(c) A circuit breaker can be reset (reused) after it trips, whereas a fuse must be replaced. OR A circuit breaker operates faster than a fuse. [1 mark]

Question 15 (4 marks)

(a) The electroscope leaf diverges (rises). The charged sphere induces an opposite charge on the inner surface of the can and a like charge on the electroscope leaf and stem. Since like charges repel, the leaf diverges. [2 marks – 1 for observation, 1 for explanation]

(b) The leaf collapses (returns to its original position). When the sphere is removed, the induced charges redistribute. The charge on the leaf and stem flows back to neutralise the charge on the can, so the electroscope returns to its neutral state. [2 marks – 1 for observation, 1 for explanation]

Section C: Data-Based and Extended Response (20 marks)

Question 16 (5 marks)

(a) As the number of turns increases, the maximum mass lifted increases. The relationship is non-linear; the mass lifted increases at an increasing rate (or approximately proportional to the square of the number of turns, or the increase becomes greater as N increases). [1 mark – accept any reasonable description of the positive, non-linear relationship]

(b) As the current increases, the maximum mass lifted would increase. The strength of the magnetic field produced by an electromagnet is directly proportional to the current flowing through the coil (for a given number of turns). A stronger magnetic field can exert a greater force on magnetic materials, lifting a larger mass. [2 marks – 1 for stating increase, 1 for explanation linking current to field strength]

(c) The type of core material. Using a soft iron core greatly increases the strength of the electromagnet because iron is a ferromagnetic material that becomes strongly magnetised when placed in the coil's magnetic field, concentrating and strengthening the field. (Accept: increasing the current, or decreasing the length/increasing the cross-sectional area of the core.) [2 marks – 1 for factor, 1 for explanation]

Question 17 (5 marks)

(a) As the coil rotates, the angle between the plane of the coil and the magnetic field lines changes continuously. This causes the magnetic flux (or magnetic field lines) passing through the coil to change. By Faraday's law, a changing magnetic flux induces an e.m.f. across the coil. The magnitude of the induced e.m.f. depends on the rate of change of flux, which varies as the coil rotates. [2 marks – 1 for changing flux, 1 for induced e.m.f.]

(b) Graph should show:

• x-axis labelled "Time" or "t" [0.5 mark]
• y-axis labelled "Induced e.m.f." or "e.m.f." [0.5 mark]
• A sinusoidal curve (sine wave) with positive and negative halves [0.5 mark]
• One complete cycle shown, crossing zero at the start, middle, and end [0.5 mark]

(c) An A.C. generator produces alternating current (current that changes direction periodically), while a D.C. generator produces direct current (current in one direction only) because it uses a split-ring commutator instead of slip rings. [1 mark]

Question 18 (6 marks)

(a) Transmitting electricity at high voltage reduces the current in the transmission lines for the same amount of power (since P = IV). A lower current means less power is dissipated as heat in the transmission lines (since P_loss = I²R). This improves the efficiency of power transmission and reduces energy waste. [2 marks – 1 for lower current, 1 for reduced power loss/I²R]

(b) P = IV, so I = P/V I = 50 × 10⁶ / 400,000 = 125 A [2 marks – 1 for formula/conversion, 1 for answer with unit]

(c) P_loss = I²R = (125)² × 20 = 15,625 × 20 = 312,500 W = 312.5 kW (or 0.3125 MW) [2 marks – 1 for formula, 1 for answer with unit]

Question 19 (6 marks)

(a) When the polythene rod is rubbed with the woollen cloth, electrons are transferred from the wool to the polythene rod. Polythene has a greater affinity for electrons than wool. The rod gains excess electrons and therefore becomes negatively charged. (The wool becomes positively charged due to losing electrons.) [2 marks – 1 for electron transfer, 1 for direction of transfer]

(b) The stream of water bends towards the charged rod. Water molecules are polar (have positive and negative ends). The negatively charged rod attracts the positive ends of the water molecules (or induces a charge separation in the water), causing a net attractive force that deflects the stream towards the rod. [2 marks – 1 for observation, 1 for explanation involving attraction/induction]

(c) Application: Electrostatic precipitators used in chimneys to remove smoke/ash particles from exhaust gases (or electrostatic spray painting, photocopiers). Hazard: Electrostatic discharge can cause sparks that may ignite flammable vapours or cause electric shock (or damage to sensitive electronic components). [2 marks – 1 for application, 1 for hazard]

Question 20 (5 marks)

(a) The plotting compasses form concentric circles around the wire. The compass needles align tangentially to these circles, showing that the magnetic field lines are circular and centred on the wire. The direction of the needles reverses when the compass is placed on the opposite side of the wire. [2 marks – 1 for concentric circles, 1 for tangential alignment]

(b) The right-hand grip rule: Point the thumb of the right hand in the direction of the conventional current. The curled fingers show the direction of the magnetic field lines (the direction the north pole of a compass would point). [1 mark]

(c) The magnetic field becomes stronger (the compass needles would show a stronger alignment, or the field would affect compasses at a greater distance). The strength of the magnetic field around a straight current-carrying wire is directly proportional to the current (B ∝ I). Doubling the current doubles the magnetic field strength at any given distance from the wire. [2 marks – 1 for stating field becomes stronger, 1 for explanation/proportionality]

END OF ANSWER KEY

Total: 60 marks