Secondary 4 Pure Physics Preliminary Examination Paper 1

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TuitionGoWhere Exam Practice (AI) - Pure Physics Secondary 4

PRELIM VERSION 1

Subject: Pure Physics
Level: Secondary 4
Paper: Electricity and Magnetism Section
Duration: 60 Minutes
Total Marks: 40 Marks

Name: __________________________ Class: __________ Date: __________

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Instructions to Candidates:

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

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Section A: Static and Current Electricity

Question 1 (a) A plastic rod is rubbed with a woollen cloth and becomes negatively charged. State the process by which the rod becomes charged. [1]

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(b) Sketch the electric field pattern around two identical negatively charged spheres placed 5 cm apart. [2]



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Question 2 A circuit contains a battery with an electromotive force (e.m.f.) of $12\text{ V}$ and a lamp that dissipates energy at a rate of $360\text{ mW}$. (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) State the function of the neutral wire in a three-pin plug. [1]

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(b) Explain one advantage of using a circuit breaker over a fuse in a household electrical system. [2]

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Question 4 Two resistors, $R_1 = 4\text{ }\Omega$ and $R_2 = 6\text{ }\Omega$, are connected in parallel. This combination is then connected in series with a $2\text{ }\Omega$ resistor and a $12\text{ V}$ battery. (a) Calculate the effective resistance of the entire circuit. [2]

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

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Section B: Magnetism and Electromagnetism

Question 5 (a) Describe the difference between a "hard" magnetic material and a "soft" magnetic material. [2]

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(b) A straight conductor carries a current of $5\text{ A}$. Sketch the magnetic field pattern around the conductor. [2]



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Question 6 A current-carrying wire is placed in a uniform magnetic field. (a) State the condition under which the force acting on the wire is maximum. [1]

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(b) If the current in the wire is reversed, describe the change in the direction of the force. [1]

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Question 7 A DC motor consists of a coil of wire rotating in a magnetic field. (a) Explain the role of the split-ring commutator in the operation of the motor. [2]

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(b) State one way to increase the speed of rotation of the motor. [1]

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Section C: Electromagnetic Induction and Transformers

Question 8 A coil of wire is connected to a galvanometer. A permanent magnet is moved quickly into the coil. (a) State the observation made on the galvanometer. [1]

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(b) Explain why the galvanometer needle deflects only momentarily. [2]

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Question 9 A transformer has a primary coil with 200 turns and a secondary coil with 50 turns. The primary voltage is $240\text{ V}$. (a) Calculate the secondary voltage, assuming the transformer is ideal. [2]

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



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Question 10 A real transformer has an efficiency of $80\%$. The secondary voltage is $12\text{ V}$ and the secondary current is $2.0\text{ A}$. The primary voltage is $240\text{ V}$. (a) Calculate the power output of the transformer. [1]

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(b) Calculate the current in the primary coil. [3]

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Answer Key - Pure Physics Secondary 4 (Prelim V1)

Question 1 (a) Charging by rubbing / Friction. [1] (b) Field lines pointing outwards from both spheres; a clear region of repulsion/gap in the center. [2]

Question 2 (a) $P = IV \Rightarrow I = P/V = 0.360 / 12 = 0.03\text{ A}$ (or $30\text{ mA}$). [1] (b) $R = V/I = 12 / 0.03 = 400\text{ }\Omega$. [1]

Question 3 (a) Provides a return path for the current to the supply / completes the circuit. [1] (b) Circuit breakers can be reset/reused without replacement, whereas fuses must be replaced after blowing. [2]

Question 4 (a) $1/R_p = 1/4 + 1/6 = 5/12 \Rightarrow R_p = 2.4\text{ }\Omega$. Total $R = 2.4 + 2 = 4.4\text{ }\Omega$. [2] (b) $I = V/R = 12 / 4.4 \approx 2.73\text{ A}$. [1]

Question 5 (a) Hard magnetic materials retain magnetism for a long time (permanent), while soft magnetic materials lose magnetism easily when the field is removed (temporary). [2] (b) Concentric circles around the wire with arrows indicating direction (Right-hand grip rule). [2]

Question 6 (a) When the wire is perpendicular to the magnetic field. [1] (b) The force will act in the opposite direction. [1]

Question 7 (a) It reverses the direction of the current in the coil every half-turn, ensuring the force on the sides of the coil always acts to rotate it in the same direction. [2] (b) Increase current / increase number of turns in the coil / use stronger magnets. [1]

Question 8 (a) The needle deflects momentarily. [1] (b) An EMF is induced only when there is a change in magnetic flux linkage (relative motion). Once the magnet stops moving, the flux is constant and the induced current drops to zero. [2]

Question 9 (a) $V_s/V_p = N_s/N_p \Rightarrow V_s = 240 \times (50/200) = 60\text{ V}$. [2] (b) A straight line passing through the origin. Gradient = $50/200 = 0.25$. [2]

Question 10 (a) $P_{out} = V_s I_s = 12 \times 2.0 = 24\text{ W}$. [1] (b) $\eta = P_{out} / P_{in} \Rightarrow 0.8 = 24 / (240 \times I_p)$ $I_p = 24 / (0.8 \times 240) = 24 / 192 = 0.125\text{ A}$. [3] (1 mark for formula/substitution, 1 mark for calculation, 1 mark for unit).