Secondary 4 Pure Physics Preliminary Examination Paper 3

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TuitionGoWhere Exam Practice (AI)

Subject: Pure Physics
Level: Secondary 4
Paper: Preliminary Examination (Version 3)
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 calculations, show all working clearly.
4. Use $g = 10\text{ m/s}^2$ where applicable.

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Section A: Structured Questions (60 Marks)

Question 1 A student uses a plastic rod rubbed with wool to charge a small metal sphere by induction. (a) Describe the process of charging the metal sphere negatively using this method. [3]


(b) The charged sphere is then brought near a thin stream of water. State and explain the observation. [2]


(c) Draw the electric field pattern between two identical positive point charges. [2]

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

(b) Determine the total current flowing from the battery. [2]

(c) Calculate the potential difference across $R_1$. [1]

(d) If $R_2$ is replaced by a resistor of $2\text{ }\Omega$, state how the total current in the circuit changes. Explain your answer. [2]

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Question 3 A lamp in a specialized industrial torch is connected to a 15V DC power source. The lamp dissipates energy at a rate of 450 mW. (a) Calculate the current flowing through the lamp. [2]

(b) Calculate the resistance of the lamp filament. [2]

(c) If the power source is replaced by a 9V source, calculate the new power dissipated by the lamp, assuming resistance remains constant. [2]

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Question 4 A household appliance is connected to the AC mains supply. (a) State the function of the earth wire in the appliance. [1] (b) The appliance is protected by a 5A fuse. Explain the purpose of this fuse. [2] (c) State one advantage of using a circuit breaker instead of a fuse for this appliance. [1] (d) The appliance has a power rating of 2.2 kW, 230 V. Calculate the operating current. [2]

(e) Determine if the 5A fuse is suitable for this appliance. Justify your answer. [2]

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Question 5 A transformer is used to step down the voltage from 240V to 12V. The transformer has an efficiency of 80%. The secondary current is 4.0 A. (a) Calculate the power output of the transformer. [2]

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

(c) If the transformer were ideal, calculate the primary current. [2]

(d) 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 6 A coil of wire is connected to a sensitive galvanometer. A strong permanent magnet is moved quickly into the coil. (a) State the observation on the galvanometer. [1] (b) Explain the observation in (a) using the concept of magnetic flux. [3] (c) The magnet is now held stationary inside the coil. State the reading on the galvanometer and explain why. [2]

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Question 7 A D.C. motor consists of a rectangular coil placed in a uniform magnetic field. (a) Describe how the motor is made to rotate continuously. [3]

(b) State two ways to increase the speed of rotation of the motor. [2]

(c) Explain the role of the split-ring commutator in the motor. [2]

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Question 8 A step-up transformer has a primary coil of 200 turns and a secondary coil of 1200 turns. (a) Calculate the secondary voltage if the primary voltage is 110V. [2]

(b) If the primary current is 2.0 A, calculate the secondary current for an ideal transformer. [2] (c) Explain why the core of the transformer is laminated. [2]

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Question 9 A student investigates the relationship between the length of a wire and its resistance. (a) State two variables that must be kept constant during this investigation. [2]

(b) Describe how the student should set up the circuit to measure the resistance of the wire. [3]

(c) If the length of the wire is doubled, state what happens to the resistance. Explain your answer. [2]

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Question 10 A metal sphere is charged positively. It is then placed inside a conducting metal box. (a) Describe the distribution of charges on the inner and outer surfaces of the metal box. [3]

(b) A galvanometer is connected to the box. If the positively charged sphere is removed quickly, state the observation on the galvanometer. [1] (c) Explain the observation in (b). [2]

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Answer Key - Pure Physics Preliminary (Version 3)

Question 1 (a)

• Bring the positively charged rod near the metal sphere; electrons are attracted to the side near the rod. [1]
• Earth the opposite side of the sphere; electrons flow from earth to the sphere. [1]
• Remove earth connection before removing the rod. [1] (b) The stream of water bends towards the sphere. [1] The negatively charged sphere attracts the polar water molecules (or induces opposite charge). [1] (c) Field lines pointing radially outwards from both charges, with a neutral point (zero field) exactly halfway between them. [2]

Question 2 (a) $1/R = 1/4 + 1/6 = 5/12 \Rightarrow R = 12/5 = 2.4\text{ }\Omega$ [2] (b) $I = V/R = 12 / 2.4 = 5.0\text{ A}$ [2] (c) $V = 12\text{ V}$ (Parallel circuits have same PD) [1] (d) Total current increases. [1] Replacing $6\text{ }\Omega$ with $2\text{ }\Omega$ decreases the effective resistance, thus increasing current for a constant voltage. [1]

Question 3 (a) $P = IV \Rightarrow I = P/V = 0.450 / 15 = 0.03\text{ A}$ [2] (b) $R = V/I = 15 / 0.03 = 500\text{ }\Omega$ [2] (c) $P = V^2/R = 9^2 / 500 = 81 / 500 = 0.162\text{ W}$ (or 162 mW) [2]

Question 4 (a) Provides a low-resistance path to earth to prevent the metal casing from becoming live. [1] (b) To prevent excessive current from flowing. [1] If current exceeds 5A, the fuse melts and breaks the circuit. [1] (c) It can be reset without needing replacement. [1] (d) $I = P/V = 2200 / 230 \approx 9.57\text{ A}$ [2] (e) Not suitable. [1] The operating current (9.57A) is higher than the fuse rating (5A), so the fuse would blow immediately. [1]

Question 5 (a) $P_{out} = V_s I_s = 12 \times 4.0 = 48\text{ W}$ [2] (b) $\eta = P_{out} / P_{in} \Rightarrow P_{in} = 48 / 0.8 = 60\text{ W}$. [1] $P_{in} = V_p I_p \Rightarrow I_p = 60 / 240 = 0.25\text{ A}$ [2] (c) $V_p I_p = V_s I_s \Rightarrow I_p = (12 \times 4) / 240 = 0.2\text{ A}$ [2] (d) Straight line through origin. [1] Gradient = $12/240 = 0.05$. [1]

Question 6 (a) The galvanometer needle deflects momentarily. [1] (b) Moving the magnet changes the magnetic flux linkage through the coil. [1] This induces an EMF (Faraday's Law). [1] The EMF drives a current, causing deflection. [1] (c) Zero reading. [1] There is no change in magnetic flux linkage when the magnet is stationary. [1]

Question 7 (a) Current flows through the coil, creating a magnetic field. [1] This field interacts with the external magnetic field to produce a force (Lorentz force). [1] The forces on opposite sides of the coil create a couple/torque. [1] (b) Increase current; Increase strength of permanent magnets; Increase number of turns in coil. (Any 2) [2] (c) Reverses the direction of current in the coil every half turn. [1] This ensures the torque remains in the same direction for continuous rotation. [1]

Question 8 (a) $V_s/V_p = N_s/N_p \Rightarrow V_s = 110 \times (1200/200) = 110 \times 6 = 660\text{ V}$ [2] (b) $I_s/I_p = N_p/N_s \Rightarrow I_s = 2.0 \times (200/1200) = 2.0 / 6 \approx 0.33\text{ A}$ [2] (c) To reduce energy loss due to eddy currents. [2]

Question 9 (a) Material of the wire; Cross-sectional area (thickness) of the wire; Temperature of the wire. (Any 2) [2] (b) Connect battery, ammeter, and wire in series. [1] Connect a voltmeter in parallel across the wire. [1] Use a meter ruler to measure length. [1] (c) Resistance doubles. [1] Resistance is directly proportional to length ($R \propto L$). [1]

Question 10 (a) Inner surface of box becomes negatively charged. [1] Outer surface of box becomes positively charged. [1] The charges are redistributed to cancel the field inside the conductor. [1] (b) The needle deflects momentarily. [1] (c) Removing the sphere changes the magnetic/electric flux (or redistributes charges) within the box. [1] This induces an EMF/current in the connected circuit. [1]