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Secondary 4 Pure Physics Electricity Magnetism Quiz

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Questions

Secondary 4 Pure Physics Quiz - Electricity Magnetism

Name: __________________________
Class: __________________________
Date: ___________________________
Score: _________ / 45

Duration: 45 minutes
Total Marks: 45

Instructions:

Answer all questions.
Write your answers in the spaces provided.
Show all working clearly. Marks are awarded for correct working steps.
Use $g = 10 \text{ m/s}^2$ where applicable (though not required for this topic).
Assume standard mains voltage is 230 V unless stated otherwise.

Section A: Static Electricity and Electric Fields (Questions 1–5)

1. A polythene rod is rubbed with a dry cloth and becomes negatively charged.
(a) Explain, in terms of electron transfer, how the rod becomes negatively charged. [2]

(b) The charged rod is brought near a small piece of neutral paper. The paper is attracted to the rod. Explain why this attraction occurs. [2]

2. Figure 1 shows two isolated point charges, $+Q$ and $-Q$ , separated by a distance.
(a) On Figure 1, sketch the pattern of the electric field lines between the two charges. Include at least four field lines. [2]
(Space for sketch)

(b) State the direction of the force experienced by a positive test charge placed at the midpoint between $+Q$ and $-Q$ . [1]

3. A student investigates the electric field around a positively charged metal sphere.
(a) Define electric field strength. [1]

(b) If the charge on the sphere is doubled, state and explain what happens to the electric field strength at a fixed distance $r$ from the center. [2]

4. Lightning is a natural discharge of static electricity.
(a) Describe how charge separation occurs in a thundercloud to create the potential difference necessary for lightning. [2]

(b) A lightning strike transfers a charge of 20 C in 0.002 s. Calculate the average current during the strike. [2]

5. Electrostatic precipitators are used in factory chimneys to remove ash particles.
Explain how the ash particles become charged and how they are removed from the smoke. [3]

Section B: Current Electricity and D.C. Circuits (Questions 6–12)

6. A wire of length $L$ and cross-sectional area $A$ has a resistance of $R$ .
(a) State the relationship between resistance and the length of the wire. [1]

(b) A second wire is made of the same material but has length $2L$ and cross-sectional area $A/2$ . Express the resistance of this second wire in terms of $R$ . [2]

7. Figure 2 shows the current-voltage (I-V) characteristic graph for a filament lamp.
(a) Describe the shape of the graph and explain why it is not a straight line through the origin. [3]

(b) Calculate the resistance of the lamp when the potential difference across it is 6.0 V and the current is 0.5 A. [1]

8. A battery has an electromotive force (e.m.f.) of 12 V.
(a) Define electromotive force. [2]

(b) Explain the difference between e.m.f. and potential difference (p.d.) in terms of energy conversion. [2]

9. In the circuit shown in Figure 3, three resistors are connected in series with a 12 V battery.
$R_1 = 2 \, \Omega$ , $R_2 = 4 \, \Omega$ , $R_3 = 6 \, \Omega$ .
(a) Calculate the total resistance of the circuit. [1]

(b) Calculate the current flowing through the circuit. [2]

10. Two resistors, $R_A = 10 \, \Omega$ and $R_B = 10 \, \Omega$ , are connected in parallel across a 12 V supply.
(a) Calculate the combined resistance of the parallel combination. [2]

(b) Calculate the total current supplied by the battery. [2]

11. A potential divider circuit consists of a 12 V battery connected in series with a fixed resistor $R_1 = 200 \, \Omega$ and a variable resistor $R_2$ . The output voltage is taken across $R_2$ .
(a) Explain the purpose of a potential divider circuit. [1]

(b) If the resistance of $R_2$ is adjusted to $400 \, \Omega$ , calculate the output voltage. [3]

12. A student measures the resistance of a component using an ammeter and a voltmeter.
(a) Draw a circuit diagram showing the correct connection of the ammeter and voltmeter to measure the resistance of a resistor. [2]
(Space for diagram)

(b) Why must the ammeter have very low resistance and the voltmeter have very high resistance? [2]

Section C: Practical Electricity and Magnetism (Questions 13–20)

13. An electric kettle is rated at 230 V, 2.5 kW.
(a) Calculate the current flowing through the kettle when it is operating normally. [2]

(b) Calculate the energy consumed by the kettle if it is used for 3 minutes. [2]

14. Explain the function of the earth wire in a three-pin plug for a metal-cased appliance. [3]

15. A fuse is rated at 5 A.
(a) State what happens to the fuse if a current of 8 A flows through it. [1]

(b) Why is it dangerous to replace a 5 A fuse with a 13 A fuse in an appliance designed for 5 A? [2]

16. Figure 4 shows a simple d.c. motor consisting of a rectangular coil placed between the poles of a magnet.
(a) State the effect that causes the coil to rotate. [1]

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

17. A straight wire carrying a current is placed in a uniform magnetic field.
(a) State the condition required for the wire to experience a maximum force. [1]

(b) If the current is reversed, what happens to the direction of the force? [1]

18. Describe an experiment to demonstrate electromagnetic induction using a bar magnet, a coil of wire, and a sensitive galvanometer. Include what is observed when:
(a) The magnet is pushed into the coil. [1]

(b) The magnet is held stationary inside the coil. [1]

19. A transformer has 500 turns on the primary coil and 100 turns on the secondary coil. The primary coil is connected to a 240 V a.c. supply.
(a) Calculate the secondary voltage. [2]

(b) State whether this is a step-up or step-down transformer. [1]

20. In a real transformer, the efficiency is less than 100%.
(a) Give one reason for energy loss in a transformer. [1]

(b) The primary current is 0.5 A and the primary voltage is 240 V. The secondary voltage is 48 V and the secondary current is 2.0 A. Calculate the efficiency of the transformer. [3]

Answers

Secondary 4 Pure Physics Quiz - Electricity Magnetism: Answer Key

1.
(a) Electrons are transferred from the dry cloth to the polythene rod. [1] The rod gains excess electrons, giving it a net negative charge. [1]
(b) The negative rod repels electrons in the paper to the far side, leaving the near side positively charged (induction). [1] The attractive force between the rod and the near positive side is stronger than the repulsive force from the far negative side (due to distance), resulting in net attraction. [1]

2.
(a) Sketch should show curved lines starting from $+Q$ and ending at $-Q$ . Lines should not cross. Arrows should point from $+$ to $-$ . At least 4 lines. [2]
(b) Towards the negative charge ( $-Q$ ). [1]

3.
(a) Electric field strength is the force experienced per unit positive charge placed at that point. [1]
(b) The electric field strength doubles. [1] Since $E \propto Q$ (for a fixed distance), doubling the source charge $Q$ doubles the field strength $E$ . [1]

4.
(a) Ice particles and water droplets collide within the cloud. [1] Electrons are transferred, causing the top of the cloud to become positively charged and the bottom negatively charged (or vice versa), creating a large potential difference. [1]
(b) $I = Q / t$ [1]
$I = 20 / 0.002 = 10,000 \text{ A}$ [1]

5.
Ash particles pass through a grid/corona discharge which gives them a negative charge. [1] They are then attracted to positively charged collector plates. [1] The plates are vibrated/shaken to dislodge the ash, which falls into a collector hopper. [1]

6.
(a) Resistance is directly proportional to length. [1]
(b) $R = \rho L / A$ . New wire: $L' = 2L$ , $A' = A/2$ .
$R' = \rho (2L) / (A/2) = 4 (\rho L / A) = 4R$ . [2]

7.
(a) The graph curves with decreasing gradient (current increases less rapidly as voltage increases). [1] As current increases, the temperature of the filament increases. [1] Higher temperature causes metal ions to vibrate more, increasing collisions with electrons, thus increasing resistance. [1]
(b) $R = V / I = 6.0 / 0.5 = 12 \, \Omega$ . [1]

8.
(a) E.m.f. is the energy converted from non-electrical forms (e.g., chemical) to electrical energy per unit charge passing through the source. [2]
(b) E.m.f. involves conversion of non-electrical energy to electrical energy (in the source). [1] Potential difference involves conversion of electrical energy to other forms (e.g., heat, light) in the circuit components. [1]

9.
(a) $R_{total} = R_1 + R_2 + R_3 = 2 + 4 + 6 = 12 \, \Omega$ . [1]
(b) $I = V / R_{total} = 12 / 12 = 1.0 \text{ A}$ . [2]
(c) $V_2 = I \times R_2 = 1.0 \times 4 = 4.0 \text{ V}$ . [2]

10.
(a) $1/R_{total} = 1/R_A + 1/R_B = 1/10 + 1/10 = 2/10$ .
$R_{total} = 10 / 2 = 5 \, \Omega$ . [2]
(b) $I_{total} = V / R_{total} = 12 / 5 = 2.4 \text{ A}$ . [2]

11.
(a) To provide a variable output voltage from a fixed input voltage. [1]
(b) $V_{out} = V_{in} \times [R_2 / (R_1 + R_2)]$ [1]
$V_{out} = 12 \times [400 / (200 + 400)]$ [1]
$V_{out} = 12 \times (400 / 600) = 12 \times (2/3) = 8.0 \text{ V}$ . [1]

12.
(a) Ammeter in series with the resistor. Voltmeter in parallel across the resistor. [2]
(b) Ammeter must have low resistance so it does not significantly reduce the current in the circuit. [1] Voltmeter must have high resistance so it draws negligible current, ensuring the voltage measured is accurate. [1]

13.
(a) $P = IV \Rightarrow I = P / V$ [1]
$I = 2500 / 230 = 10.87 \text{ A}$ (approx 10.9 A). [1]
(b) $E = Pt$ [1]
$t = 3 \times 60 = 180 \text{ s}$ .
$E = 2500 \times 180 = 450,000 \text{ J}$ (or 450 kJ). [1]

14.
The earth wire connects the metal casing to the ground. [1] If the live wire touches the casing, a large current flows to earth. [1] This blows the fuse/c trips the breaker, disconnecting the supply and preventing electric shock to the user. [1]

15.
(a) The fuse wire melts/blows, breaking the circuit. [1]
(b) The appliance wiring is designed to handle only up to 5 A safely. [1] A 13 A fuse will not blow until the current exceeds 13 A, allowing excessive current to heat the wiring, potentially causing a fire. [1]

16.
(a) The motor effect (force on a current-carrying conductor in a magnetic field). [1]
(b) It reverses the direction of the current in the coil every half rotation. [1] This ensures the force on the coil always acts in the same rotational direction, allowing continuous rotation. [1]
(c) Increase the current; Increase the magnetic field strength; Increase the number of turns on the coil. (Any 2). [2]

17.
(a) The wire must be perpendicular ( $90^\circ$ ) to the magnetic field lines. [1]
(b) The direction of the force reverses. [1]

18.
(a) The galvanometer needle deflects in one direction. [1]
(b) The galvanometer needle shows no deflection (returns to zero). [1]
(c) The galvanometer needle deflects in the opposite direction. [1]

19.
(a) $V_s / V_p = N_s / N_p$ [1]
$V_s / 240 = 100 / 500$
$V_s = 240 \times (1/5) = 48 \text{ V}$ . [1]
(b) Step-down transformer. [1]
(c) Transformers rely on a changing magnetic field to induce an e.m.f. in the secondary coil. [1] D.c. produces a constant magnetic field, so there is no change in flux linkage and no induced e.m.f. [1]

20.
(a) Heat loss in the coils due to resistance; Eddy currents in the core; Hysteresis loss; Flux leakage. (Any 1). [1]
(b) Input Power $P_{in} = V_p I_p = 240 \times 0.5 = 120 \text{ W}$ . [1]
Output Power $P_{out} = V_s I_s = 48 \times 2.0 = 96 \text{ W}$ . [1]
Efficiency $= (P_{out} / P_{in}) \times 100\% = (96 / 120) \times 100\% = 80\%$ . [1]