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Secondary 4 Pure Physics Practice Paper 2

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TuitionGoWhere Practice Paper - Pure Physics Secondary 4

TuitionGoWhere Practice Paper (AI) Version: 2 of 5

Subject: Pure Physics Level: Secondary 4 Paper: Practice Paper (Electricity & Magnetism Focus) Duration: 1 hour 30 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 in this question paper.
The number of marks is given in brackets [ ] at the end of each question or part question.
You may use a scientific calculator.
Take the acceleration of free fall, $g = 10 \text{ m/s}^2$ .
Assume the density of water is $1000 \text{ kg/m}^3$ and specific heat capacity of water is $4200 \text{ J/(kg}^\circ\text{C)}$ where applicable.

Section A: Structured Questions

Answer all questions in this section.

1. Fig 1.1 shows a simple circuit containing a battery, a switch, a lamp, and an ammeter.

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

(b) The ammeter reads $0.5 \text{ A}$ . Calculate the total charge that flows through the lamp in 2 minutes. [2] Charge = __________________________ C

(c) State the direction of conventional current flow in the circuit relative to the electron flow. [1]

2. A student investigates the resistance of a wire. She measures the potential difference (p.d.) across the wire and the current through it for different lengths of the wire.

(a) State Ohm’s Law. [1]

(b) The student plots a graph of Current ( $I$ ) against Potential Difference ( $V$ ). The graph is a straight line passing through the origin. What does this indicate about the resistance of the wire? [1]

(c) If the length of the wire is doubled while keeping the cross-sectional area and material constant, state and explain what happens to its resistance. [2]

3. Fig 3.1 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] Total Resistance = __________________________ $\Omega$

(b) Calculate the current flowing through the circuit. [2] Current = __________________________ A

(c) Calculate the potential difference across resistor $R_2$ . [2] p.d. across $R_2$ = __________________________ V

4. Fig 4.1 shows a circuit with two resistors, $R_A = 10 \, \Omega$ and $R_B = 10 \, \Omega$ , connected in parallel to a $6 \text{ V}$ battery.

(a) Calculate the combined resistance of the two resistors. [2] Combined Resistance = __________________________ $\Omega$

(b) Calculate the total current supplied by the battery. [2] Total Current = __________________________ A

(c) State one advantage of connecting household appliances in parallel rather than in series. [1]

5. An electric kettle is rated at $240 \text{ V}$ , $2000 \text{ W}$ .

(a) Calculate the current flowing through the kettle when it is operating at normal brightness. [2] Current = __________________________ A

(b) Calculate the electrical energy consumed by the kettle if it is switched on for 5 minutes. [2] Energy = __________________________ J

(c) The kettle is connected to a mains supply via a plug containing a fuse. Explain the purpose of the fuse in this circuit. [2]

6. Fig 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 on the secondary coil. [2] Number of turns = __________________________

(b) The transformer is assumed to be 100% efficient. If the current in the secondary coil is $2.0 \text{ A}$ , calculate the current in the primary coil. [2] Primary Current = __________________________ A

(c) In reality, transformers are not 100% efficient. State one reason for energy loss in a transformer. [1]

7. Fig 7.1 shows a bar magnet suspended freely. A second bar magnet is brought close to the North pole of the suspended magnet.

(a) State the law of magnetism regarding like poles. [1]

(b) Describe the pattern of magnetic field lines around a single bar magnet. Include the direction of the lines. [2]

(c) Explain why steel is used to make permanent magnets while soft iron is used for electromagnets. [2]

8. Fig 8.1 shows a wire carrying a current placed between the poles of a U-shaped magnet. The wire experiences a force and moves upwards.

(a) Name 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]

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

9. Fig 9.1 shows a simple d.c. motor.

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

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

10. Fig 10.1 shows a coil of wire connected to a sensitive galvanometer. A bar magnet is moved towards the coil.

(a) State what is observed on the galvanometer when the magnet is moved towards the coil. [1]

(b) State what is observed on the galvanometer when the magnet is held stationary inside the coil. [1]

(c) Explain, in terms of magnetic field lines, why an e.m.f. is induced in the coil when the magnet moves. [2]

Section B: Free-Response Questions

Answer all questions in this section.

11. A student sets up a circuit to determine the resistance of an unknown resistor $X$ . The circuit includes a power supply, an ammeter, a voltmeter, a variable resistor, and resistor $X$ .

(a) Draw the circuit diagram for this experiment. Ensure the voltmeter and ammeter are connected correctly. [3]

(b) The student obtains the following readings:

Voltmeter reading: $4.5 \text{ V}$
Ammeter reading: $0.3 \text{ A}$

Calculate the resistance of resistor $X$ . [2] Resistance = __________________________ $\Omega$

(c) The student repeats the experiment with different settings of the variable resistor and plots a graph of $V$ against $I$ . Explain how the resistance can be determined from this graph. [2]

12. Fig 12.1 shows the wiring of a 3-pin plug for an electric iron.

(a) Identify the wires labelled A, B, and C. [3] A: __________________________ B: __________________________ C: __________________________

(b) The fuse in the plug is rated at $13 \text{ A}$ . The iron is rated at $240 \text{ V}$ , $2400 \text{ W}$ . (i) Calculate the normal operating current of the iron. [2] Current = __________________________ A

(ii) Explain why a $13 \text{ A}$ fuse is appropriate for this appliance. [2]

(c) Explain the safety role of the earth wire (Wire C) in the event that the live wire accidentally touches the metal casing of the iron. [3]

13. Fig 13.1 shows an a.c. generator.

(a) Describe how an alternating current is produced in the external circuit as the coil rotates. [3]

(b) State two factors that affect the magnitude of the induced e.m.f. in the generator. [2]

(c) Sketch a graph of the induced e.m.f. against time for one complete rotation of the coil, starting from the vertical position. [2]

14. A transformer is used to transmit electrical power over long distances.

(a) Explain why electrical power is transmitted at high voltage. [3]

(b) A power station generates electricity at $11 \text{ kV}$ and $200 \text{ A}$ . This is stepped up to $132 \text{ kV}$ for transmission. Assuming the transformer is ideal, calculate the current in the transmission lines. [3] Current = __________________________ A

(c) Why is alternating current (a.c.) used for transformers instead of direct current (d.c.)? [2]

15. Fig 15.1 shows a cathode ray oscilloscope (CRO) trace.

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

(b) The time-base setting of the CRO is $5 \text{ ms/cm}$ . One complete wave occupies $4 \text{ cm}$ horizontally. Calculate the frequency of the a.c. supply. [3] Frequency = __________________________ Hz

(c) If the peak voltage of the supply is $10 \text{ V}$ and the y-gain setting is $2 \text{ V/cm}$ , calculate the vertical height of the trace from the center line to the peak. [2] Height = __________________________ cm

Section C: Application and Analysis

Answer all questions in this section.

16. Fig 16.1 shows a circuit containing a thermistor and a fixed resistor connected in series to a $12 \text{ V}$ supply. A voltmeter is connected across the fixed resistor.

(a) Describe how the resistance of a thermistor changes as the temperature increases. [1]

(b) Explain what happens to the reading on the voltmeter as the temperature of the thermistor increases. [3]

(c) Suggest a practical application for this circuit. [1]

17. A light-dependent resistor (LDR) is used in a street light control circuit.

(a) Describe how the resistance of an LDR changes as light intensity increases. [1]

(b) In the dark, the resistance of the LDR is high. Explain how this property can be used to switch on a street light automatically. You may refer to a potential divider circuit in your answer. [3]

18. Fig 18.1 shows a relay switch used to control a high-voltage motor using a low-voltage switch.

(a) Explain how closing the low-voltage switch causes the high-voltage motor to start. [3]

(b) Why is a relay used instead of connecting the low-voltage switch directly to the motor? [2]

19. A student investigates the magnetic field pattern around a straight current-carrying wire.

(a) Describe the shape of the magnetic field lines around the wire. [1]

(b) State the rule used to determine the direction of these field lines. [1]

(c) If the current in the wire is increased, what happens to the strength of the magnetic field? [1]

(d) Describe an experiment to demonstrate the magnetic field pattern around the wire. Include the materials used and the observation. [3]

20. Fig 20.1 shows a simple circuit with a diode, a resistor, and an a.c. supply.

(a) State the property of a diode that allows it to be used in this circuit. [1]

(b) Sketch the output voltage waveform across the resistor if the input is a sinusoidal a.c. voltage. [2]

(c) Name this process. [1]

(d) Suggest how the output can be smoothed to produce a steadier d.c. voltage. [2]

End of Paper

Answers

TuitionGoWhere Practice Paper - Pure Physics Secondary 4

Answer Key and Marking Scheme

Version: 2 of 5

Section A: Structured Questions

1. (a) Electric current is the rate of flow of electric charge. [1] (b)

Time $t = 2 \text{ minutes} = 120 \text{ s}$ [1]
Charge $Q = I \times t = 0.5 \times 120 = 60 \text{ C}$ [1] (c) Conventional current flows in the opposite direction to electron flow. [1]

2. (a) Ohm’s Law states that the current flowing through a metallic conductor is directly proportional to the potential difference across it, provided physical conditions (such as temperature) remain constant. [1] (b) The resistance is constant. [1] (c)

Resistance increases. [1]
Resistance is directly proportional to length ( $R \propto L$ ). Doubling the length doubles the resistance. [1]

3. (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]

4. (a)

$\frac{1}{R_{total}} = \frac{1}{R_A} + \frac{1}{R_B} = \frac{1}{10} + \frac{1}{10} = \frac{2}{10}$ [1]
$R_{total} = \frac{10}{2} = 5.0 \, \Omega$ [1] (b)
$I_{total} = V / R_{total}$ [1]
$I_{total} = 6 / 5.0 = 1.2 \text{ A}$ [1] (c) If one appliance fails, the others continue to work. / Each appliance receives the full mains voltage. [1]

5. (a)

$P = IV \Rightarrow I = P / V$ [1]
$I = 2000 / 240 = 8.33 \text{ A}$ [1] (b)
Time $t = 5 \text{ minutes} = 300 \text{ s}$ [1]
$E = P \times t = 2000 \times 300 = 600,000 \text{ J}$ (or $600 \text{ kJ}$ ) [1] (c) The fuse melts/breaks the circuit if the current exceeds the rated value, preventing overheating and fire. [2]

6. (a)

$\frac{V_s}{V_p} = \frac{N_s}{N_p}$ [1]
$\frac{12}{240} = \frac{N_s}{2000} \Rightarrow N_s = \frac{12 \times 2000}{240} = 100 \text{ turns}$ [1] (b)
For ideal transformer: $V_p I_p = V_s I_s$ [1]
$240 \times I_p = 12 \times 2.0 \Rightarrow I_p = \frac{24}{240} = 0.1 \text{ A}$ [1] (c) Heating of coils due to resistance / Eddy currents in the core / Hysteresis loss / Flux leakage. [1]

7. (a) Like poles repel each other. [1] (b) Lines emerge from the North pole and enter the South pole. [1] The lines are closer together near the poles where the field is stronger. [1] (c) Steel is a hard magnetic material that retains magnetism (permanent). [1] Soft iron is a soft magnetic material that loses magnetism easily when current is switched off (temporary). [1]

8. (a) Fleming’s Left-Hand Rule. [1] (b)

Increase the current. [1]
Increase the magnetic field strength (use stronger magnets). [1] (c) The direction of the force reverses (moves downwards). [1]

9. (a) It reverses the direction of current in the coil every half rotation. [1] This ensures the torque/force acts in the same direction, allowing continuous rotation. [1] (b)

Increase the current. [1]
Increase the magnetic field strength / Increase the number of turns on the coil. [1]

10. (a) The needle deflects (in one direction). [1] (b) No deflection (needle stays at zero). [1] (c) The moving magnet causes the magnetic field lines to cut through the coil. [1] This change in magnetic flux linkage induces an e.m.f. (Faraday’s Law). [1]

Section B: Free-Response Questions

11. (a)

Power supply, switch, variable resistor, ammeter, and resistor $X$ in series. [1]
Voltmeter connected in parallel across resistor $X$ only. [1]
Correct symbols used. [1] (b)
$R = V / I$ [1]
$R = 4.5 / 0.3 = 15 \, \Omega$ [1] (c) The resistance is equal to the gradient (slope) of the $V-I$ graph. [2]

12. (a)

A: Live [1]
B: Neutral [1]
C: Earth [1] (b) (i) $I = P / V = 2400 / 240 = 10 \text{ A}$ [2] (ii) The operating current ( $10 \text{ A}$ ) is less than the fuse rating ( $13 \text{ A}$ ), so the fuse does not blow during normal operation. [1] However, it is close enough to blow if a significant fault occurs, providing protection. [1] (c) If the live wire touches the casing, a large current flows through the earth wire to the ground. [1] This large current melts the fuse/blows the circuit breaker. [1] This disconnects the live supply, making the casing safe to touch. [1]

13. (a) As the coil rotates, it cuts magnetic field lines. [1] This induces an e.m.f. in the coil. [1] As the coil passes the vertical position, the sides of the coil swap positions relative to the poles, reversing the direction of the induced current. [1] (b)

Speed of rotation. [1]
Strength of the magnetic field / Number of turns on the coil. [1] (c) Sine wave starting at zero, reaching a positive peak, crossing zero, reaching a negative peak, and returning to zero. [2]

14. (a) High voltage reduces the current for the same power ( $P=IV$ ). [1] Lower current reduces energy loss due to heating in the transmission cables ( $P_{loss} = I^2 R$ ). [1] This makes transmission more efficient. [1] (b)

$V_p I_p = V_s I_s$ (Ideal) [1]
$11,000 \times 200 = 132,000 \times I_s$ [1]
$I_s = \frac{2,200,000}{132,000} = 16.67 \text{ A}$ [1] (c) Transformers work on the principle of electromagnetic induction, which requires a changing magnetic field. [1] A.c. provides a continuously changing current/field, whereas d.c. produces a constant field which does not induce e.m.f. in the secondary coil. [1]

15. (a) Frequency is the number of complete waves (or cycles) produced per second. [1] (b)

Period $T = 4 \text{ cm} \times 5 \text{ ms/cm} = 20 \text{ ms} = 0.02 \text{ s}$ [1]
$f = 1 / T$ [1]
$f = 1 / 0.02 = 50 \text{ Hz}$ [1] (c)
Height = Peak Voltage / y-gain [1]
Height = $10 / 2 = 5 \text{ cm}$ [1]

Section C: Application and Analysis

16. (a) Resistance decreases as temperature increases. [1] (b)

As temperature increases, resistance of thermistor decreases. [1]
Total resistance of the circuit decreases, so total current increases. [1]
Since $V = IR$ for the fixed resistor, and $I$ increases, the p.d. across the fixed resistor (voltmeter reading) increases. [1] (c) Fire alarm / Temperature sensor. [1]

17. (a) Resistance decreases as light intensity increases. [1] (b)

The LDR and a fixed resistor form a potential divider. [1]
In the dark, LDR resistance is high, so the voltage across the LDR is high. [1]
This high voltage can be used to trigger a transistor/switch to turn on the light. (Or: When light increases, LDR resistance drops, voltage across it drops, turning off the light). [1]

18. (a)

Closing the low-voltage switch allows current to flow through the electromagnet coil. [1]
The electromagnet becomes magnetized and attracts the iron armature. [1]
The armature pivots and closes the high-voltage contacts, completing the motor circuit. [1] (b)
To isolate the user from the high-voltage circuit (safety). [1]
To allow a low-power switch to control a high-power device. [1]

19. (a) Concentric circles centered on the wire. [1] (b) Right-Hand Grip Rule. [1] (c) The magnetic field strength increases. [1] (d)

Place a straight wire vertically through a horizontal card. [1]
Sprinkle iron filings on the card and tap gently. [1]
Observation: The filings arrange themselves in concentric circles around the wire. [1]

20. (a) It allows current to flow in only one direction (unidirectional). [1] (b) Graph showing only the positive half-cycles of the sine wave (zero during negative half-cycles). [2] (c) Half-wave rectification. [1] (d) Connect a capacitor in parallel with the resistor (load). [1] The capacitor charges during the peak and discharges during the gap, smoothing the output. [1]