Secondary 4 Pure Physics Preliminary Examination Paper 4

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TuitionGoWhere Exam Practice (AI)

Subject: Pure Physics
Level: Secondary 4
Paper: Preliminary Examination (Version 4 of 5)
Duration: 1 hour 45 minutes
Total Marks: 60

Name: ___________________________ Class: ___________ Date: ___________

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Instructions to Candidates

1. Answer all questions.
2. Write your answers in the spaces provided.
3. For calculation questions, show all necessary working.
4. Use $g = 10\text{ m/s}^2$ where applicable.

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Section A: Short Answer and Structured Questions

Question 1 (a) State the function of the neutral wire in a three-pin plug system. [1]

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(b) Explain one advantage of using a circuit breaker over a fuse for protecting a high-power appliance. [1]

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Question 2 A small lamp is connected to a $15\text{ V}$ DC power source. The lamp dissipates energy at a rate of $450\text{ mW}$. (a) Calculate the current flowing through the lamp. [1]

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(b) Determine the resistance of the lamp. [1]

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Question 3 A transformer is used to step down the voltage from a $240\text{ V}$ AC mains supply to $12\text{ V}$. The transformer has an efficiency of $80\%$. The secondary current is $4.0\text{ A}$. (a) Calculate the power output of the transformer. [1]

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(b) Calculate the current in the primary coil. [2]

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Question 4 A student performs an experiment with a coil of wire connected to a galvanometer. A negatively charged plastic rod is moved quickly into the coil. (a) State the observation made by the student on the galvanometer. [1]

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(b) Describe what happens to the galvanometer needle when the rod is then removed quickly from the coil. [1]

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Question 5 An ideal transformer has a primary coil with 500 turns and a secondary coil with 25 turns. (a) State whether this is a step-up or step-down transformer. [1]

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(b) If the input voltage is $200\text{ V}$, calculate the output voltage. [1]

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(c) Sketch a graph of the output voltage ($V_s$) against the input voltage ($V_p$) for this ideal transformer. [2] (Space for sketch)

Question 6 A $12\text{ V}$ battery is connected to two resistors, $R_1 = 4\text{ }\Omega$ and $R_2 = 6\text{ }\Omega$, in parallel. (a) Calculate the effective resistance of the circuit. [2]

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(b) Calculate the total current flowing from the battery. [1]

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Question 7 (a) Define the term electromotive force (e.m.f.) of a cell. [1]

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(b) Distinguish between e.m.f. and potential difference. [2]

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Question 8 A conductor of length $0.5\text{ m}$ is placed perpendicular to a uniform magnetic field of flux density $0.2\text{ T}$. A current of $3.0\text{ A}$ flows through the conductor. (a) Calculate the magnitude of the force acting on the conductor. [2]

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(b) State the rule used to determine the direction of this force. [1]

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Question 9 Describe the operation of a D.C. motor, explaining how the split-ring commutator ensures the motor continues to rotate in one direction. [3]

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Question 10 A potential divider circuit consists of a $6\text{ V}$ supply and two resistors, $R_1 = 2\text{ k}\Omega$ and $R_2 = 3\text{ k}\Omega$, connected in series. The output voltage is taken across $R_2$. (a) Calculate the output voltage. [2]

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(b) If $R_2$ is replaced by a light-dependent resistor (LDR), explain how the output voltage changes as the light intensity increases. [2]

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Question 11 (a) State the law of magnetism regarding the poles of a magnet. [1]

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(b) Distinguish between "hard" and "soft" magnetic materials. [2]

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Question 12 An A.C. generator consists of a coil rotating in a magnetic field. (a) State two factors that can be used to increase the peak voltage induced in the coil. [2]

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(b) Explain why the current produced by an A.C. generator is called "alternating". [1]

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Question 13 A $2\text{ kW}$ electric kettle is rated at $240\text{ V}$. (a) Calculate the current flowing through the kettle when in use. [1]

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(b) Calculate the cost of using the kettle for 30 minutes if electricity costs \0.30$ per kWh. [2]

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Question 14 (a) Draw the electric field pattern for two identical positive point charges placed side-by-side. [2] (b) Explain how a charged sphere can be charged by induction. [2]

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Question 15 A transformer has a turns ratio of $1:10$. (a) Calculate the secondary current if the primary current is $5.0\text{ A}$ and the transformer is ideal. [2]

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(b) Explain why the core of a transformer is laminated. [2]

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Answer Key - Pure Physics Preliminary (Version 4)

Question 1 (a) Provides a return path for the current to the supply / Completes the circuit at zero potential. [1] (b) It can be reset without needing replacement / It responds faster to overcurrent. [1]

Question 2 (a) $P = IV \Rightarrow I = P/V = 0.450 / 15 = 0.03\text{ A}$ (or $30\text{ mA}$). [1] (b) $V = IR \Rightarrow R = V/I = 15 / 0.03 = 500\text{ }\Omega$. [1]

Question 3 (a) $P_{out} = V_s I_s = 12 \times 4.0 = 48\text{ W}$. [1] (b) $\eta = P_{out} / P_{in} \Rightarrow 0.8 = 48 / (240 \times I_p)$ $I_p = 48 / (0.8 \times 240) = 48 / 192 = 0.25\text{ A}$. [2]

Question 4 (a) The needle of the galvanometer deflects momentarily. [1] (b) The needle deflects momentarily in the opposite direction. [1]

Question 5 (a) Step-down transformer. [1] (b) $V_s = V_p \times (N_s/N_p) = 200 \times (25/500) = 200 \times 0.05 = 10\text{ V}$. [1] (c) Straight line through the origin. Y-axis: $V_s$, X-axis: $V_p$. Gradient = $0.05$. [2]

Question 6 (a) $1/R = 1/4 + 1/6 = (3+2)/12 = 5/12 \Rightarrow R = 2.4\text{ }\Omega$. [2] (b) $I = V/R = 12 / 2.4 = 5.0\text{ A}$. [1]

Question 7 (a) The energy per unit charge supplied by the source. [1] (b) e.m.f. is the total energy supplied by the source per unit charge, whereas potential difference is the energy converted to other forms per unit charge as it passes through a component. [2]

Question 8 (a) $F = BIl = 0.2 \times 3.0 \times 0.5 = 0.3\text{ N}$. [2] (b) Fleming's Left Hand Rule. [1]

Question 9 Current in the coil creates a magnetic field; the interaction between this field and the permanent magnet's field creates a force (Lorentz force) that rotates the coil. [1] The split-ring commutator reverses the direction of current in the coil every half turn. [1] This ensures the force always acts in the same direction, maintaining continuous rotation. [1]

Question 10 (a) $V_{out} = V_{total} \times [R_2 / (R_1 + R_2)] = 6 \times [3 / (2+3)] = 6 \times 0.6 = 3.6\text{ V}$. [2] (b) As light intensity increases, the resistance of the LDR ($R_2$) decreases. [1] Since $R_2$ is smaller relative to $R_1$, the share of the voltage across $R_2$ decreases, so output voltage decreases. [1]

Question 11 (a) Like poles repel and unlike poles attract. [1] (b) Hard magnetic materials are difficult to magnetize/demagnetize (retain magnetism); soft magnetic materials are easily magnetized/demagnetized. [2]

Question 12 (a) Increase the number of turns in the coil / Increase the strength of the magnetic field / Increase the speed of rotation. (Any two) [2] (b) The direction of the induced current reverses every half cycle as the coil rotates. [1]

Question 13 (a) $I = P/V = 2000 / 240 = 8.33\text{ A}$. [1] (b) Energy = $P \times t = 2\text{ kW} \times 0.5\text{ h} = 1\text{ kWh}$. [1] Cost = 1\text{ kWh} \times \0.30 = $0.30$. [1]

Question 14 (a) Two diverging lines of force pointing away from each charge, with a neutral region/gap in the middle. [2] (b) A charged object is brought near a conductor, causing charges to redistribute (polarize). [1] The conductor is then grounded or connected to another body to remove/add charge, leaving it with a net opposite charge. [1]

Question 15 (a) For ideal: $I_p/I_s = N_s/N_p \Rightarrow 5.0 / I_s = 1/10 \Rightarrow I_s = 50\text{ A}$. [2] (b) To reduce energy loss due to eddy currents. [1] Lamination breaks the paths of circulating currents, reducing heating. [1]