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Secondary 4 Pure Physics Preliminary Examination Paper 4

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TuitionGoWhere Practice Paper - Pure Physics Secondary 4

TuitionGoWhere Secondary School (AI)
PRELIMINARY EXAMINATION 2024
Version 4 of 5

Subject: Pure Physics
Level: Secondary 4
Paper: 2 (Structured Questions)
Duration: 1 hour 45 minutes
Total Marks: 60

Name: __________________________
Class: __________________________
Date: __________________________

INSTRUCTIONS TO CANDIDATES

Write your name, class, and date in the spaces provided.
Answer all questions.
Write your answers in the spaces provided on this question paper.
All working must be clearly shown.
The use of an approved scientific calculator is expected.
Where appropriate, take the acceleration of free fall $g = 10 \text{ m/s}^2$ .

Section A

Answer all questions in this section.

1. A student investigates the properties of electric charge using two suspended polystyrene balls, A and B. Ball A is positively charged.

(a) State the law of electrostatics regarding the interaction between like charges.
[1]

(b) Ball B is brought close to Ball A, and they repel each other. State the charge on Ball B.
[1]

2. Figure 2.1 shows a simple circuit containing a battery, a switch, and a lamp.

(a) Define electric current in terms of charge flow.
[1]

(b) If $12 \text{ C}$ of charge flows through the lamp in $4 \text{ s}$ , calculate the current in the circuit.
[2]

3. A resistor has a potential difference of $6.0 \text{ V}$ across it and a current of $0.5 \text{ A}$ flowing through it.

(a) Calculate the resistance of the resistor.
[2]

(b) The resistor is made of a wire of length $L$ and cross-sectional area $A$ . State how the resistance changes if:
(i) The length $L$ is doubled.
[1]

(ii) The cross-sectional area $A$ is doubled.
[1]

4. Figure 4.1 shows a circuit with two resistors, $R_1 = 4.0 \, \Omega$ and $R_2 = 6.0 \, \Omega$ , connected in series to a $12 \text{ V}$ battery.

(a) Calculate the total resistance of the circuit.
[1]

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

5. A household electric kettle is rated at $240 \text{ V}$ and $2.4 \text{ kW}$ .

(a) Calculate the current flowing through the kettle when it is operating normally.
[2]

(b) Calculate the electrical energy consumed by the kettle if it is used for $5 \text{ minutes}$ . Give your answer in Joules.
[2]

(c) State one safety feature found in a 3-pin plug that protects the user from electric shock if the live wire touches the metal casing of the kettle.
[1]

Section B

Answer all questions in this section.

6. Figure 6.1 shows a transformer used to step down the voltage from $240 \text{ V}$ to $12 \text{ V}$ for a laptop charger. The primary coil has $2000$ turns.

(a) Calculate the number of turns in the secondary coil.
[2]

(b) The transformer is $90\%$ efficient. The current in the secondary coil is $2.0 \text{ A}$ . Calculate the current in the primary coil.
[3]

7. Figure 7.1 shows a straight wire carrying a current placed between the poles of a U-shaped magnet. The wire experiences a force and moves upwards.

(a) State the rule used to determine the direction of the force on a current-carrying conductor in a magnetic field.
[1]

(b) State two ways to increase the magnitude of the force on the wire.
[2]

8. A student performs an experiment to investigate electromagnetic induction. She moves a bar magnet into a solenoid connected to a sensitive galvanometer.

(a) State what is observed on the galvanometer when the magnet is:
(i) Moved quickly into the solenoid.
[1]

(ii) Held stationary inside the solenoid.
[1]

(b) Explain why an electromotive force (e.m.f.) is induced in the solenoid when the magnet moves.
[2]

9. Figure 9.1 shows the input and output voltage waveforms of an a.c. generator.

(a) Define the frequency of an a.c. supply.
[1]

(b) If the period of the waveform is $0.02 \text{ s}$ , calculate the frequency.
[2]

10. Figure 10.1 shows a simple d.c. motor.

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

(b) State two factors that would increase the speed of rotation of the motor.
[2]

Section C

Answer all questions in this section.

11. A student sets up a potential divider circuit using a $12 \text{ V}$ battery and two resistors, $R_A$ and $R_B$ , connected in series. $R_A = 200 \, \Omega$ and $R_B = 400 \, \Omega$ . The output voltage is taken across $R_B$ .

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

(b) Calculate the output voltage across $R_B$ .
[2]

(c) A lamp is now connected in parallel with $R_B$ . State and explain what happens to the output voltage across the lamp compared to the value calculated in (b).
[3]

12. Figure 12.1 shows the wiring of a 3-pin plug.

(a) Identify the wires connected to terminals X, Y, and Z.
[3]
X: _________________________
Y: _________________________
Z: _________________________

(b) Explain why the fuse is always connected to the live wire and not the neutral wire.
[2]

(c) A hairdryer is rated at $1200 \text{ W}$ and $240 \text{ V}$ . Available fuses are $3 \text{ A}$ , $5 \text{ A}$ , and $13 \text{ A}$ .
(i) Calculate the normal operating current of the hairdryer.
[2]

(ii) Select the most suitable fuse for the hairdryer and explain your choice.
[2]

13. Figure 13.1 shows a circuit containing a thermistor and a fixed resistor connected in series to a $6.0 \text{ V}$ supply. The resistance of the thermistor decreases as temperature increases.

(a) Explain how the resistance of a thermistor changes with temperature in terms of charge carriers.
[2]

(b) As the temperature of the thermistor increases, state and explain what happens to the reading on the voltmeter connected across the fixed resistor.
[3]

14. A high-voltage transmission line carries a current of $500 \text{ A}$ at a voltage of $400 \text{ kV}$ . The total resistance of the transmission lines is $10 \, \Omega$ .

(a) Calculate the power loss in the transmission lines due to heating.
[2]

(b) Explain why high voltage is used for long-distance power transmission.
[2]

15. Figure 15.1 shows a coil of wire rotating in a magnetic field to generate electricity.

(a) State the name of this device.
[1]

(b) Explain why the induced e.m.f. is zero when the plane of the coil is perpendicular to the magnetic field lines.
[2]

(c) Sketch a graph of the induced e.m.f. against time for one complete rotation of the coil, starting from the position where the plane of the coil is parallel to the magnetic field.
[2]

16. A student wants to determine the resistance of a wire. She has a battery, an ammeter, a voltmeter, a variable resistor, and the wire.

(a) Draw a circuit diagram showing how the components should be connected to measure the resistance of the wire.
[3]

(b) Explain how the variable resistor helps in obtaining a reliable value for the resistance.
[2]

17. Figure 17.1 shows a magnetic relay used to switch on a high-current motor using a low-current switch.

(a) Explain how closing switch S causes the motor to start.
[3]

(b) State one advantage of using a relay in this circuit.
[1]

18. Two identical lamps are connected to a $12 \text{ V}$ battery. In Circuit A, they are in series. In Circuit B, they are in parallel.

(a) Compare the brightness of the lamps in Circuit A and Circuit B. Explain your answer.
[3]

(b) If one lamp breaks in Circuit A, what happens to the other lamp?
[1]

19. Figure 19.1 shows a cathode ray oscilloscope (CRO) trace. The Y-gain is set to $2 \text{ V/cm}$ and the time-base is set to $5 \text{ ms/cm}$ .

(a) The peak of the waveform is $3 \text{ cm}$ from the center line. Calculate the peak voltage.
[2]

(b) One complete cycle occupies $4 \text{ cm}$ horizontally. Calculate the frequency of the signal.
[3]

20. A student investigates the relationship between the length of a wire and its resistance.

(a) State the variable that must be kept constant to ensure a fair test.
[1]

(b) The student plots a graph of Resistance ( $R$ ) against Length ( $L$ ). Describe the expected shape of this graph.
[1]

END OF PAPER

Answers

TuitionGoWhere Practice Paper - Pure Physics Secondary 4

PRELIMINARY EXAMINATION 2024
Version 4 of 5
ANSWER KEY AND MARKING SCHEME

Subject: Pure Physics
Level: Secondary 4

Section A

1.
(a) Like charges repel. [1]
(b) Positive. [1]
(c) The charge flows to earth / The ball becomes neutral / Electrons flow from earth to neutralize the positive charge. [1]

2.
(a) Electric current is the rate of flow of electric charge. [1]
(b) $I = Q / t$ [1]
$I = 12 / 4 = 3.0 \text{ A}$ [1]

3.
(a) $R = V / I$ [1]
$R = 6.0 / 0.5 = 12 \, \Omega$ [1]
(b) (i) Resistance doubles / Increases by factor of 2. [1]
(ii) Resistance halves / Decreases by factor of 2. [1]

4.
(a) $R_{total} = R_1 + R_2 = 4.0 + 6.0 = 10.0 \, \Omega$ [1]
(b) $I = V / R_{total}$ [1]
$I = 12 / 10.0 = 1.2 \text{ A}$ [1]
(c) $V_2 = I \times R_2$ [1]
$V_2 = 1.2 \times 6.0 = 7.2 \text{ V}$ [1]

5.
(a) $P = IV \Rightarrow I = P / V$ [1]
$I = 2400 / 240 = 10 \text{ A}$ [1]
(b) $E = Pt$ [1]
$t = 5 \times 60 = 300 \text{ s}$
$E = 2400 \times 300 = 720,000 \text{ J}$ [1]
(c) Earth wire. [1]

Section B

6.
(a) $\frac{V_s}{V_p} = \frac{N_s}{N_p}$ [1]
$\frac{12}{240} = \frac{N_s}{2000}$
$N_s = \frac{12 \times 2000}{240} = 100 \text{ turns}$ [1]
(b) Efficiency = $\frac{P_{out}}{P_{in}} \times 100\%$
$P_{out} = V_s I_s = 12 \times 2.0 = 24 \text{ W}$ [1]
$0.90 = \frac{24}{P_{in}} \Rightarrow P_{in} = \frac{24}{0.90} = 26.67 \text{ W}$ [1]
$P_{in} = V_p I_p \Rightarrow 26.67 = 240 \times I_p$
$I_p = \frac{26.67}{240} = 0.11 \text{ A}$ (or $0.111 \text{ A}$ ) [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 magnetic flux linkage, and thus no induced e.m.f. [1]

7.
(a) Fleming’s Left-Hand Rule. [1]
(b) 1. Increase the current. [1]
2. Increase the strength of the magnetic field. [1]
(c) Downwards. [1]

8.
(a) (i) The needle deflects (momentarily). [1]
(ii) No deflection / Needle stays at zero. [1]
(b) There is a change in magnetic flux linkage through the solenoid. [1]
This change induces an e.m.f. (Faraday’s Law). [1]
(c) The direction of the induced current is such that it opposes the change producing it. [1]

9.
(a) The number of complete cycles per second. [1]
(b) $f = 1 / T$ [1]
$f = 1 / 0.02 = 50 \text{ Hz}$ [1]
(c) Voltage can be easily stepped up or down using transformers (reducing energy loss during transmission). [1]

10.
(a) It reverses the direction of current in the coil every half rotation. [1]
This ensures the torque acts in the same direction, allowing continuous rotation. [1]
(b) 1. Increase the current. [1]
2. Increase the strength of the magnetic field / Increase number of turns on coil. [1]

Section C

11.
(a) $R_{total} = 200 + 400 = 600 \, \Omega$
$I = V / R = 6.0 / 600 = 0.01 \text{ A}$ [2]
(b) $V_{out} = I \times R_B = 0.01 \times 400 = 4.0 \text{ V}$ [2]
(c) The output voltage decreases. [1]
Connecting the lamp in parallel with $R_B$ reduces the combined resistance of that section. [1]
This causes a larger proportion of the voltage to drop across $R_A$ , leaving less voltage across the parallel combination (lamp). [1]

12.
(a) X: Earth [1]
Y: Neutral [1]
Z: Live [1]
(b) If the fuse is on the neutral wire and it blows, the appliance is still connected to the high voltage live wire. [1]
This poses a shock hazard if someone touches the internal parts. Connecting to live ensures the circuit is disconnected from high voltage when the fuse blows. [1]
(c) (i) $I = P / V = 1200 / 240 = 5.0 \text{ A}$ [2]
(ii) $5 \text{ A}$ fuse. [1]
The operating current is $5 \text{ A}$ . A $3 \text{ A}$ fuse would blow immediately. A $13 \text{ A}$ fuse would allow excessive current to flow without blowing, potentially causing damage. The $5 \text{ A}$ fuse is the closest standard value above or equal to the operating current (Note: In practice, a slightly higher fuse like 5A or next standard up if 5A is borderline, but 5A is the calculated rating). Accept 5A or 13A with valid reasoning, but 5A is ideal for exact rating. [1]

13.
(a) As temperature increases, more charge carriers (electrons) are released / become free to move. [1]
This decreases the resistance. [1]
(b) The resistance of the thermistor decreases. [1]
The total resistance of the circuit decreases, so the current increases. [1]
Since $V = IR$ for the fixed resistor, and $I$ increases while $R$ is constant, the voltage across the fixed resistor increases. [1]

14.
(a) $P_{loss} = I^2 R$ [1]
$P_{loss} = 500^2 \times 10 = 250,000 \times 10 = 2,500,000 \text{ W}$ ( $2.5 \text{ MW}$ ) [1]
(b) High voltage allows for lower current for the same power transmitted ( $P=IV$ ). [1]
Lower current reduces power loss due to heating in the cables ( $P_{loss} = I^2 R$ ). [1]

15.
(a) A.C. Generator / Alternator. [1]
(b) At this position, the sides of the coil are moving parallel to the magnetic field lines. [1]
Therefore, they do not cut any magnetic field lines, so no e.m.f. is induced. [1]
(c) Sine wave starting at max positive (or negative), crossing zero at 1/4 period, max negative at 1/2 period, etc. [2] (1 for shape, 1 for starting point/phase).

16.
(a) Battery in series with ammeter, variable resistor, and wire. Voltmeter in parallel with the wire. [3] (1 for series loop, 1 for ammeter placement, 1 for voltmeter in parallel).
(b) It allows the current/voltage to be varied. [1]
Multiple readings can be taken to plot a graph or calculate an average, reducing random error. [1]

17.
(a) Closing S allows current to flow through the electromagnet coil. [1]
The electromagnet becomes magnetized and attracts the soft iron armature. [1]
The armature pivots and closes the contacts in the high-current motor circuit, allowing the motor to run. [1]
(b) It allows a low-current switch to control a high-current device safely. [1]

18.
(a) Lamps in Circuit B (parallel) are brighter. [1]
In parallel, each lamp receives the full $12 \text{ V}$ . In series, the voltage is shared ( $6 \text{ V}$ each). [1]
Since $P = V^2/R$ , higher voltage means higher power and brightness. [1]
(b) The other lamp goes off. [1]
(c) The other lamp stays on (with same brightness). [1]

19.
(a) Peak Voltage = Height $\times$ Y-gain [1]
$V_{peak} = 3 \text{ cm} \times 2 \text{ V/cm} = 6 \text{ V}$ [1]
(b) Period $T = \text{Width} \times \text{Time-base}$ [1]
$T = 4 \text{ cm} \times 5 \text{ ms/cm} = 20 \text{ ms} = 0.02 \text{ s}$ [1]
$f = 1 / T = 1 / 0.02 = 50 \text{ Hz}$ [1]

20.
(a) Cross-sectional area / Thickness / Material / Temperature. [1] (Any one)
(b) A straight line passing through the origin. [1]
(c) The wire gets hot due to the heating effect of current ( $I^2 R$ ). [1]
As temperature increases, the resistance of the metal wire increases, causing the graph to curve upwards / results to be inaccurate if not controlled. [1]

END OF MARKING SCHEME