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O Level Physics Practice Paper 5

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TuitionGoWhere Practice Paper - Physics O-Level

TuitionGoWhere Practice Paper (AI)
Version: 5 of 5
Subject: Physics (6091)
Level: O-Level
Topic: Electricity and Magnetism (Topics 13–19)
Paper: Structured Questions
Duration: 1 hour
Total Marks: 40

Name: __________________________
Class: __________________________
Date: __________________________

Instructions to Candidates:

Write your name, class, and date in the spaces above.
Answer all questions.
Write your answers in the spaces provided in this booklet.
All working must be clearly shown.
You may use a calculator.
Take the acceleration of free fall, $g = 10 \text{ m/s}^2$ .

Section A: Structured Questions

Answer all questions in this section.

1. A student rubs a polythene rod with a dry cloth. The rod 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. Fig. 2.1 shows a simple circuit containing a battery, a switch, a fixed resistor $R$ , and a thermistor $T$ .

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

The resistance of the thermistor decreases as the temperature increases.

(a) State what happens to the reading on the voltmeter when the temperature of the thermistor increases.

_________________________________________________________________________ [1]

(b) Explain your answer to (a) by referring to the potential divider principle or current changes.

_________________________________________________________________________ [2]

3. A filament lamp is rated at $6.0 \text{ V}$ , $0.5 \text{ A}$ .

(a) Calculate the resistance of the lamp when it is operating at its rated voltage.

_________________________________________________________________________ [2]

(b) The current-voltage (I-V) characteristic graph for this lamp is a curve, not a straight line. Explain why the resistance of the filament changes as the voltage increases.

_________________________________________________________________________ [2]

4. Fig. 4.1 shows two resistors, $R_1 = 4.0 \, \Omega$ and $R_2 = 6.0 \, \Omega$ , connected in parallel across a $12 \text{ V}$ battery.

(a) Calculate the combined resistance of the two resistors.

_________________________________________________________________________ [2]

(b) Calculate the total current supplied by the battery.

_________________________________________________________________________ [2]

5. A student investigates the resistance of a wire. She measures the length $L$ and the resistance $R$ for different lengths of the same wire.

(a) State the relationship between the resistance $R$ and the length $L$ of the wire.

_________________________________________________________________________ [1]

(b) The student cuts the wire in half and twists the two halves together to form a thicker wire of half the original length. State and explain how the new resistance compares to the original resistance.

_________________________________________________________________________ [2]

6. Fig. 6.1 shows the wiring of a three-pin plug for an electric heater.

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

_________________________________________________________________________ [1]

(b) Explain the purpose of the fuse in the plug.

_________________________________________________________________________ [2]

7. A bar magnet is suspended freely so it can rotate in a horizontal plane.

(a) State the direction in which the magnet will come to rest.

_________________________________________________________________________ [1]

(b) Describe how you would use a plotting compass to map the magnetic field lines around a bar magnet.

_________________________________________________________________________ [2]

8. Fig. 8.1 shows a straight wire carrying a current flowing vertically upwards. The wire passes through a horizontal card.

(a) Sketch the pattern of the magnetic field lines on the card. Include arrows to show the direction of the field.

Graph space

[2]

(b) State the rule used to determine the direction of this magnetic field.

_________________________________________________________________________ [1]

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

(a) State the function of the split-ring commutator in the motor.

_________________________________________________________________________ [2]

(b) Suggest two changes that would increase the speed of rotation of the motor coil.

_____________________________________________________________________ [2]

10. A transformer has 500 turns on the primary coil and 100 turns on the secondary coil. The primary coil is connected to a $240 \text{ V}$ a.c. supply.

(a) Calculate the voltage across the secondary coil.

_________________________________________________________________________ [2]

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

_________________________________________________________________________ [1]

_________________________________________________________________________ [2]

Section B: Free Response Questions

Answer all questions in this section.

11. Fig. 11.1 shows a circuit used to measure the resistance of a component $X$ . The circuit includes a battery, a switch, an ammeter, component $X$ , and a variable resistor. A voltmeter is connected across component $X$ .

(a) Draw the circuit diagram for Fig. 11.1 using standard symbols.

[3]

(b) The variable resistor is adjusted. The following readings are obtained:

Voltage (V)	Current (A)
2.0	0.4
4.0	0.8
6.0	1.2

(i) Plot a graph of Current (y-axis) against Voltage (x-axis) on the grid below.

(Grid space implied)

[3]

(ii) Determine the resistance of component $X$ from your graph. Show your working.

_________________________________________________________________________ [2]

(iii) State whether component $X$ is an ohmic conductor. Give a reason for your answer.

_________________________________________________________________________ [1]

12. A student sets up an experiment to investigate electromagnetic induction. She moves a bar magnet into a coil of wire connected to a sensitive galvanometer (center-zero ammeter).

(a) Describe what is observed on the galvanometer when:

(i) The magnet is pushed quickly into the coil.

_________________________________________________________________________ [1]

(ii) The magnet is held stationary inside the coil.

_________________________________________________________________________ [1]

(iii) The magnet is pulled quickly out of the coil.

_________________________________________________________________________ [1]

(b) Explain, in terms of magnetic field lines, why an e.m.f. is induced when the magnet moves.

_________________________________________________________________________ [2]

_____________________________________________________________________ [3]

13. High-voltage transmission lines are used to transmit electrical power over long distances.

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

_________________________________________________________________________ [3]

(b) A power station generates $100 \text{ MW}$ of power at $25 \text{ kV}$ . This is stepped up to $400 \text{ kV}$ for transmission.

(i) Calculate the current in the transmission lines at $400 \text{ kV}$ .

_________________________________________________________________________ [2]

(ii) The total resistance of the transmission lines is $5.0 \, \Omega$ . Calculate the power loss in the lines.

_________________________________________________________________________ [2]

14. Fig. 14.1 shows a relay switch used to control a high-voltage motor using a low-voltage circuit.

(a) Explain how closing switch S in the low-voltage circuit causes the high-voltage motor to start.

_________________________________________________________________________ [4]

(b) Why is soft iron used for the core of the electromagnet in the relay?

_________________________________________________________________________ [1]

15. A cathode ray oscilloscope (CRO) displays a waveform from an a.c. generator.

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

_________________________________________________________________________ [1]

(b) The time-base setting on the CRO is $5 \text{ ms/cm}$ . One complete cycle of the wave occupies $4 \text{ cm}$ horizontally.

(i) Calculate the period of the wave.

_________________________________________________________________________ [2]

(ii) Calculate the frequency of the wave.

_________________________________________________________________________ [2]

16. Fig. 16.1 shows a beam of electrons entering a magnetic field directed into the plane of the paper.

(a) State the direction of the force acting on the electrons.

_________________________________________________________________________ [1]

(b) Explain why the electrons move in a circular path while in the magnetic field.

_________________________________________________________________________ [2]

_________________________________________________________________________ [1]

17. A household uses an electric kettle rated at $2.4 \text{ kW}$ . The kettle is used for 15 minutes each day.

(a) Calculate the energy consumed by the kettle in one day in kilowatt-hours (kWh).

_________________________________________________________________________ [2]

(b) If the cost of electricity is $\$ 0.25$ per kWh, calculate the cost of using the kettle for 30 days.

_________________________________________________________________________ [2]

18. Two identical lamps are connected in series to a battery. Another two identical lamps are connected in parallel to an identical battery.

(a) Compare the brightness of the lamps in the series circuit with the lamps in the parallel circuit.

_________________________________________________________________________ [1]

(b) Explain your answer in terms of voltage and current.

_________________________________________________________________________ [3]

19. A logic circuit is required to sound an alarm when it is dark AND when sound is detected.

(a) Identify the input sensors required for this circuit.

_____________________________________________________________________ [2]

(b) Name the logic gate required to process these inputs.

_________________________________________________________________________ [1]

[1]

20. A student wants to determine the resistivity of a metal wire.

(a) List the measurements the student needs to take.

_________________________________________________________________________ [3]

(b) Describe how the student can ensure the measurements are accurate.

_________________________________________________________________________ [2]

End of Paper

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Answers

TuitionGoWhere Practice Paper - Physics O-Level (Answers)

Version: 5 of 5
Topic: Electricity and Magnetism

Section A: Structured Questions

1. (a) Electrons are transferred from the 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). [1]

2. (a) The voltmeter reading decreases. [1]

(b) As temperature increases, the resistance of the thermistor decreases. [1]
In a series circuit, the voltage is shared proportional to resistance. Since the thermistor's resistance decreases relative to the fixed resistor, it takes a smaller share of the supply voltage. [1]
(Alternatively: Total resistance decreases, current increases, voltage across fixed resistor $V=IR$ increases, so voltage across thermistor decreases). [1]

3. (a) $R = V / I$ [1]
$R = 6.0 / 0.5 = 12 \, \Omega$ [1]

(b) As voltage increases, current increases, causing the filament to heat up. [1]
Higher temperature causes the metal lattice ions to vibrate more vigorously. [1]
This increases the frequency of collisions with electrons, increasing resistance. [1]
(Max 2 marks)

4. (a) $\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2}$ [1]
$\frac{1}{R_{total}} = \frac{1}{4} + \frac{1}{6} = \frac{3}{12} + \frac{2}{12} = \frac{5}{12}$
$R_{total} = \frac{12}{5} = 2.4 \, \Omega$ [1]

(b) $I = V / R$ [1]
$I = 12 / 2.4 = 5.0 \text{ A}$ [1]

5. (a) Resistance is directly proportional to length ( $R \propto L$ ). [1]

(b) The new resistance is one-quarter of the original. [1]
Halving the length halves the resistance. [1]
Doubling the cross-sectional area (by twisting two halves) halves the resistance again. [1]
Total factor: $1/2 \times 1/2 = 1/4$ .

6. (a) Green and yellow. [1]

(b) The fuse contains a thin wire that melts if the current exceeds a safe value. [1]
This breaks the circuit, preventing overheating and fire. [1]

7. (a) North-South direction. [1]

(b) Place a plotting compass near the magnet. [1]
Mark the position of the needle ends. [1]
Move the compass so one end is at the previous mark, and mark the new position. [1]
Repeat to form a line, then join the dots. [1]
(Max 2 marks)

8. (a) Concentric circles centered on the wire. [1]
Arrows showing counter-clockwise direction (using Right-Hand Grip Rule). [1]

(b) Right-Hand Grip Rule. [1]

9. (a) To reverse the direction of current in the coil every half rotation. [1]
This ensures the force on the coil always acts in the same rotational direction, maintaining continuous rotation. [1]

(b) 1. Increase the current. [1]
2. Use stronger magnets (increase magnetic field strength). [1]
(Or: Increase number of turns on the coil).

10. (a) $\frac{V_s}{V_p} = \frac{N_s}{N_p}$ [1]
$V_s = 240 \times (\frac{100}{500}) = 240 \times 0.2 = 48 \text{ V}$ [1]

(b) Step-down transformer. [1]

(c) A d.c. supply produces a constant current, which creates a constant magnetic field. [1]
There is no change in magnetic flux linkage through the secondary coil, so no e.m.f. is induced. [1]

Section B: Free Response Questions

11. (a) Correct symbols for battery, switch, ammeter (in series), voltmeter (in parallel across X), variable resistor (in series). [3]
(1 mark for series loop correct, 1 mark for voltmeter parallel, 1 mark for correct symbols)

(b)(i) Points plotted correctly. [1]
Straight line through origin. [1]
Axes labeled with units. [1]

(b)(ii) Gradient calculation or $R = V/I$ from a point. [1]
$R = 2.0 / 0.4 = 5.0 \, \Omega$ (or $4.0/0.8$ , etc.). [1]

(b)(iii) Yes, it is an ohmic conductor. [1]
Because the I-V graph is a straight line through the origin (current is proportional to voltage). [1]

12. (a)(i) The needle deflects (kicks) in one direction. [1]

(a)(ii) The needle returns to zero (no deflection). [1]

(a)(iii) The needle deflects in the opposite direction. [1]

(b) When the magnet moves, the magnetic field lines cut across the coil (or magnetic flux through the coil changes). [1]
This change in flux induces an e.m.f. (Faraday's Law). [1]

13. (a) Power loss $P = I^2 R$ . [1]
Transmitting at high voltage reduces the current for the same power ( $P=VI$ ). [1]
Lower current significantly reduces power loss due to heating in the cables. [1]

(b)(i) $P = VI \Rightarrow I = P / V$ [1]
$I = 100 \times 10^6 / (400 \times 10^3) = 100,000 / 400 = 250 \text{ A}$ [1]

(b)(ii) $P_{loss} = I^2 R$ [1]
$P_{loss} = 250^2 \times 5.0 = 62,500 \times 5 = 312,500 \text{ W}$ (or $312.5 \text{ kW}$ ) [1]

14. (a) Closing switch S allows current to flow through the electromagnet coil. [1]
The core becomes magnetized. [1]
It attracts the soft iron armature. [1]
The armature pivots and closes the contacts in the high-voltage circuit. [1]
This allows current to flow to the motor. [1]
(Max 4 marks)

(b) Soft iron is easily magnetized and demagnetized. [1]
This allows the relay to switch off quickly when the current is cut.

15. (a) The number of complete waves (or cycles) passing a point per second. [1]

(b)(i) Period $T = \text{horizontal distance} \times \text{time-base}$ [1]
$T = 4 \text{ cm} \times 5 \text{ ms/cm} = 20 \text{ ms}$ (or $0.02 \text{ s}$ ) [1]

(b)(ii) $f = 1 / T$ [1]
$f = 1 / 0.02 = 50 \text{ Hz}$ [1]

16. (a) Downwards (towards the bottom of the page). [1]
(Note: Electrons are negative, so use Left Hand Rule but reverse direction, or Right Hand Rule for positive current opposite to electron flow).

(b) The magnetic force is always perpendicular to the velocity of the electrons. [1]
This force acts as a centripetal force, changing the direction but not the speed, resulting in circular motion. [1]

17. (a) Time in hours $= 15 / 60 = 0.25 \text{ h}$ . [1]
Energy $= P \times t = 2.4 \text{ kW} \times 0.25 \text{ h} = 0.6 \text{ kWh}$ . [1]

(b) Total energy for 30 days $= 0.6 \times 30 = 18 \text{ kWh}$ . [1]
Cost $= 18 \times \$ 0.25 = $4.50$. [1]

18. (a) The lamps in the parallel circuit are brighter. [1]

(b) In parallel, each lamp receives the full battery voltage. [1]
In series, the voltage is shared between the two lamps (each gets half). [1]
Since $P = V^2/R$ , higher voltage means higher power and greater brightness. [1]
(Alternatively: Current in parallel branch is higher than in series loop).

19. (a) 1. Light Dependent Resistor (LDR). [1]
2. Microphone (or sound sensor). [1]

(b) AND gate. [1]

20. (a) 1. Length of the wire ( $L$ ). [1]
2. Diameter of the wire ( $d$ ) (to calculate cross-sectional area). [1]
3. Resistance of the wire ( $R$ ) (using voltmeter and ammeter). [1]

(b) Measure diameter at several points along the wire and take an average to account for irregularities. [1]
Ensure the wire is straight when measuring length, or use a meter rule with the wire taut. [1]
(Other valid answers: Keep current low to prevent heating changing resistance; use zero-error corrected instruments).