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Secondary 4 Pure Physics Practice Paper 4

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TuitionGoWhere Practice Paper - Pure Physics Secondary 4

TuitionGoWhere Practice Paper (AI)
Version: 4 of 5
Subject: Pure Physics
Level: Secondary 4
Paper: Electricity & Magnetism Practice
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 on the 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 due to gravity, $g = 10 \text{ m/s}^2$ .
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 relevant.

Section A: Multiple Choice & Short Structured Questions (20 Marks)

1. Which of the following correctly describes the direction of conventional current and electron flow in a metallic conductor?
[1]
A. Conventional current flows from negative to positive; electrons flow from positive to negative.
B. Conventional current flows from positive to negative; electrons flow from negative to positive.
C. Both conventional current and electrons flow from positive to negative.
D. Both conventional current and electrons flow from negative to positive.

2. A positively charged rod is brought near a neutral metal sphere without touching it. The sphere is then earthed momentarily while the rod is still near, and then the earth connection is removed, followed by the removal of the rod. What is the final charge on the sphere?
[1]
A. Positive
B. Negative
C. Neutral
D. It depends on the material of the sphere.

3. Two resistors, $R_1 = 4 \, \Omega$ and $R_2 = 6 \, \Omega$ , are connected in series to a 12 V battery. What is the potential difference across $R_2$ ?
[1]
A. 4.8 V
B. 7.2 V
C. 12.0 V
D. 20.0 V

4. Which graph best represents the current-voltage (I-V) characteristic of a filament lamp?
[1]
A. A straight line through the origin.
B. A curve with increasing gradient as V increases.
C. A curve with decreasing gradient as V increases.
D. A horizontal line.

5. In a household electrical circuit, why is the fuse always connected to the live wire?
[1]
A. To ensure the appliance is disconnected from the high potential when the fuse blows.
B. To allow the neutral wire to carry the return current safely.
C. Because the live wire has lower resistance than the neutral wire.
D. To protect the neutral wire from overheating.

6. A transformer has 500 turns on the primary coil and 100 turns on the secondary coil. If the primary voltage is 240 V, what is the secondary voltage?
[1]
A. 12 V
B. 48 V
C. 1200 V
D. 2400 V

7. State the function of the earth wire in a three-pin plug.
[1]

8. Define the electromotive force (e.m.f.) of a battery.
[1]

9. A current of 2.0 A flows through a resistor for 30 seconds. Calculate the total charge that passes through the resistor.
[2]

Answer space

10. Explain why soft iron is preferred over steel for the core of an electromagnet used in a scrapyard crane.
[2]

Section B: Structured Questions (25 Marks)

11. Figure 11.1 shows a simple circuit containing a battery, a switch, a fixed resistor $R$ , and a thermistor $T$ .

(Imagine Figure 11.1: Battery in series with Switch, Resistor R, and Thermistor T. A voltmeter is connected in parallel across the Thermistor T.)

(a) State how the resistance of the 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) The battery has an e.m.f. of 12 V and negligible internal resistance. At a certain temperature, the resistance of the thermistor is $200 \, \Omega$ and the fixed resistor $R$ is $400 \, \Omega$ . Calculate the current flowing in the circuit.
[2]

Answer space

12. A student investigates the relationship between the length of a wire and its resistance. She uses a wire of uniform cross-sectional area and measures the resistance for different lengths.

(a) State the relationship between the resistance of a wire and its length.
[1]

(b) The student finds that a 50 cm length of the wire has a resistance of $2.5 \, \Omega$ . Calculate the resistance of a 1.2 m length of the same wire.
[2]

Answer space

13. Figure 13.1 shows a rectangular coil ABCD placed between the poles of a permanent magnet. The coil is connected to a DC supply via a split-ring commutator.

(Imagine Figure 13.1: A DC motor setup. Magnet N on left, S on right. Coil vertical. Current flows A to B to C to D.)

(a) State the direction of the force on side AB of the coil.
[1]

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

Section C: Free Response & Application Questions (15 Marks)

14. A transformer is used to step down the voltage from 240 V to 12 V to power a lamp rated at "12 V, 24 W". The transformer is assumed to be 100% efficient.

(a) Calculate the current flowing through the lamp when it is operating at normal brightness.
[2]

Answer space

(b) Calculate the current in the primary coil of the transformer.
[2]

Answer space

(c) In reality, transformers are not 100% efficient. State one reason for energy loss in a transformer and suggest a design feature that minimizes this loss.
[2]
Reason: __________________________________________________________________
Design Feature: __________________________________________________________

15. Figure 15.1 shows the wiring of a 3-pin plug for an electric kettle.

(Imagine Figure 15.1: Diagram of a 3-pin plug with terminals L, N, E. Wires colored Brown, Blue, Green/Yellow.)

(a) Identify the correct color of the insulation for the wire connected to terminal E (Earth).
[1]

(b) The kettle is rated at 240 V, 2.0 kW.
(i) Calculate the normal operating current of the kettle.
[2]

Answer space

(ii) Available fuses are 3 A, 5 A, and 13 A. Which fuse should be used in the plug? Explain your choice.
[2]
Fuse: ____________________________________________________________________
Explanation: ______________________________________________________________

(c) Explain why it is dangerous to use a 13 A fuse for an appliance that normally draws 2 A, if the appliance develops a fault that causes a current of 10 A to flow.
[2]

16. A charged particle enters a uniform magnetic field moving perpendicular to the field lines.

(a) Describe the path taken by the particle while it is inside the magnetic field.
[1]

(b) State the factor that determines whether the force on the particle is directed into the page or out of the page.
[1]

End of Paper

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Answers

TuitionGoWhere Practice Paper - Pure Physics Secondary 4 (Answer Key)

Version: 4 of 5
Subject: Pure Physics
Topic: Electricity & Magnetism

Section A: Multiple Choice & Short Structured Questions

1. B
Conventional current is defined as flowing from positive to negative. Electrons, being negatively charged, flow from negative to positive. [1]

2. B
Induction causes negative charges to accumulate on the side near the rod. Earthing allows positive charges to leave (or electrons to enter from earth). When earth is removed, the sphere retains the negative charge. [1]

3. B
Total Resistance $R_T = 4 + 6 = 10 \, \Omega$ . Current $I = V/R = 12/10 = 1.2 \text{ A}$ . Voltage across $R_2 = I \times R_2 = 1.2 \times 6 = 7.2 \text{ V}$ . [1]

4. C
As voltage increases, the filament heats up, resistance increases, so the current increases at a slower rate. The gradient (I/V) decreases. [1]

5. A
If the fuse is on the live wire, blowing it disconnects the appliance from the high voltage source, preventing electric shock if the case is touched. [1]

6. B
Using $\frac{V_s}{V_p} = \frac{N_s}{N_p}$ : $V_s = 240 \times \frac{100}{500} = 240 \times 0.2 = 48 \text{ V}$ . [1]

7. To provide a low-resistance path to the ground for any leakage current, preventing the metal casing of the appliance from becoming live and causing electric shock. [1]

8. The energy supplied by the source per unit charge passing through it. (Or: Work done by the source in driving a unit charge around a complete circuit). [1]

9. Formula: $Q = I \times t$ [1]
Calculation: $Q = 2.0 \times 30 = 60 \text{ C}$ [1]

10. Soft iron is easily magnetized and demagnetized (soft magnetic material). [1]
Steel retains magnetism (hard magnetic material), which would cause the scrap to stick to the crane even when switched off. [1]

Section B: Structured Questions

11. (a) Resistance decreases. [1]

(b) As temperature increases, resistance of thermistor decreases. [1]
The total resistance of the circuit decreases, so the current in the circuit increases. [1]
However, the voltmeter is across the thermistor. Since $V_T = I \times R_T$ , and $R_T$ decreases significantly while $I$ increases slightly, the potential difference across the thermistor decreases. (Alternatively: The fixed resistor takes a larger share of the voltage as its resistance becomes a larger proportion of the total). [1]

(c) Total Resistance $R_{total} = 200 + 400 = 600 \, \Omega$ . [1]
Current $I = \frac{V}{R} = \frac{12}{600} = 0.02 \text{ A}$ . [1]

12. (a) Resistance is directly proportional to length. [1]

(b) Ratio of lengths: $\frac{1.2 \text{ m}}{0.5 \text{ m}} = 2.4$ . [1]
New Resistance $R = 2.5 \, \Omega \times 2.4 = 6.0 \, \Omega$ . [1]

13. (a) Upwards (or towards the top of the page). [1]
(Using Fleming's Left Hand Rule: Field N to S (Left to Right), Current A to B (assume into page or out depending on diagram orientation, but standard vertical coil usually implies force is vertical). If AB is the side where current flows 'into' the page relative to N-S field, force is Up. If 'out', force is Down. Assuming standard diagram where AB is left side and current goes up/down, force is perpendicular to field. Let's assume standard: Field Left->Right. Current in AB is 'into page' -> Force Up. Current in CD is 'out of page' -> Force Down.)
Note: Accept "Upwards" or "Downwards" depending on specific current direction defined in student's mental model of the diagram, provided reasoning is consistent. Standard answer for 'side AB' in many textbooks with current flowing away from viewer is Up.

(b) To reverse the direction of the current in the coil every half rotation. [1]
This ensures that the force on the sides of the coil always acts in the same rotational direction, allowing continuous rotation. [1]

(c) Any two of:
1. Increase the current. [1]
2. Use a stronger magnet (increase magnetic field strength). [1]
3. Increase the number of turns on the coil.
4. Increase the area of the coil.

Section C: Free Response & Application Questions

14. (a) Formula: $P = IV \Rightarrow I = P/V$ [1]
Calculation: $I = \frac{24}{12} = 2.0 \text{ A}$ . [1]

(b) For 100% efficiency, Power Input = Power Output = 24 W. [1]
Primary Current $I_p = \frac{P}{V_p} = \frac{24}{240} = 0.1 \text{ A}$ . [1]

(c) Reason: Heating of the coils due to resistance (Copper loss) OR Eddy currents in the core OR Hysteresis loss. [1]
Design Feature: Use thick copper wires (low resistance) OR Laminated soft iron core. [1]

15. (a) Green and Yellow. [1]

(b) (i) Formula: $P = IV \Rightarrow I = P/V$ [1]
Calculation: $I = \frac{2000}{240} = 8.33 \text{ A}$ . [1]

(ii) Fuse: 13 A. [1]
Explanation: The normal operating current is 8.33 A. A 3 A or 5 A fuse would blow immediately. The 13 A fuse is the next standard value above the operating current, allowing normal operation while protecting against excessive currents. [1]

(c) A 10 A current is significantly higher than the normal 2 A, indicating a fault that could cause overheating and fire. [1]
However, 10 A is less than the 13 A fuse rating, so the fuse will NOT blow. The wire inside the appliance may overheat and cause a fire before the fuse reacts. [1]

16. (a) Circular path (or arc of a circle). [1]

(b) The sign of the charge (positive or negative) OR the direction of motion of the particle. [1]
(Accept: The direction of the velocity vector relative to the field).

(c) Opposite to the direction of motion of the electron. [1]
(Conventional current is defined as the flow of positive charge, opposite to electron flow).

End of Marking Scheme