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O Level Physics Practice Paper 3

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TuitionGoWhere Practice Paper - Physics O-Level

TuitionGoWhere Practice Paper (AI)

Subject: Physics
Level: O-Level (Syllabus 6091)
Paper: Practice Paper - Version 3 of 5
Topic: Electricity and Magnetism (Topics 13–19)
Duration: 1 hour 15 minutes
Total Marks: 50

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 booklet.
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 $1000 \text{ kg/m}^3$ .

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 circuit containing a battery, a fixed resistor $R$ , and a thermistor $T$ .

(Diagram Description: A series circuit with a 12 V battery, a fixed resistor R, and a 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 Ohm’s Law. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ [2]

3. A filament lamp is rated at $12 \text{ V}$ , $24 \text{ W}$ . (a) Calculate the current flowing through the lamp when it is operating at normal brightness.

Current = ____________________ A [2]

(b) Calculate the resistance of the filament at normal brightness.

Resistance = ____________________ $\Omega$ [2]

(c) The 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 three resistors connected in a combination of series and parallel.

Resistor $A = 4.0 \, \Omega$
Resistor $B = 4.0 \, \Omega$
Resistor $C = 4.0 \, \Omega$

Resistors $A$ and $B$ are connected in parallel. This combination is connected in series with resistor $C$ .

(a) Calculate the combined resistance of resistors $A$ and $B$ .

Combined Resistance = ____________________ $\Omega$ [2]

(b) Calculate the total resistance of the entire circuit.

Total Resistance = ____________________ $\Omega$ [1]

5. A homeowner uses an electric kettle rated at $2.4 \text{ kW}$ , $240 \text{ V}$ . (a) Calculate the suitable fuse rating for the plug of this kettle from the following options: $3 \text{ A}$ , $5 \text{ A}$ , $13 \text{ A}$ . Show your working.

Working: ........................................................................................................................................ ........................................................................................................................................

Fuse Rating = ____________________ A [2]

(b) The kettle is used for 15 minutes. Calculate the electrical energy consumed in kilowatt-hours (kWh).

Energy = ____________________ kWh [2]

Cost = ____________________ cents [1]

6. Fig. 6.1 shows a simple d.c. motor.

(Diagram Description: A rectangular coil ABCD placed between the poles of a magnet (N on left, S on right). The coil is connected to a split-ring commutator and carbon brushes.)

(a) State the rule used to determine the direction of the force on the sides of the coil. ........................................................................................................................................ [1]

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

....................................................................................................................................
.................................................................................................................................... [2]

7. A transformer is 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.

Number of turns = ____________________ [2]

(b) The laptop draws a current of $2.0 \text{ A}$ from the secondary coil. Assuming the transformer is $100\%$ efficient, calculate the current in the primary coil.

Current = ____________________ A [2]

(c) Explain why transformers only work with alternating current (a.c.) and not direct current (d.c.). ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ [2]

8. Fig. 8.1 shows a bar magnet being pushed into a solenoid connected to a sensitive galvanometer.

(Diagram Description: A bar magnet with North pole facing a solenoid. The magnet is moving towards the solenoid. The solenoid is connected to a galvanometer which shows a deflection.)

(a) State what is observed on the galvanometer when the magnet is: (i) pushed into the solenoid: .................................................................................... (ii) held stationary inside the solenoid: .................................................................... (iii) pulled out of the solenoid: ................................................................................ [3]

(b) State Lenz’s Law. ........................................................................................................................................ ........................................................................................................................................ [2]

9. High-voltage transmission lines are used to transmit electrical power over long distances. (a) Explain why electrical power is transmitted at high voltage. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ [2]

(b) A power station generates $500 \text{ MW}$ of power. This is transmitted at $400 \text{ kV}$ . Calculate the current in the transmission lines.

Current = ____________________ A [2]

10. Fig. 10.1 shows the magnetic field pattern around a straight current-carrying wire.

(Diagram Description: Concentric circles around a wire. Current flows upwards.)

(a) Describe how you would experimentally determine the shape and direction of this magnetic field. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ [2]

(b) State two factors that affect the strength of the magnetic field around the wire.

....................................................................................................................................
.................................................................................................................................... [2]

Section B: Free Response Questions

Answer all questions in this section.

11. A student investigates the relationship between the length of a resistance wire and its resistance. (a) Draw a circuit diagram that the student could use for this investigation. Include a power source, an ammeter, a voltmeter, and the resistance wire with a sliding contact (jockey) or crocodile clips to vary the length.

[3]

(b) The student obtains the following results:

Length (cm)	Resistance ( $\Omega$ )
20	1.0
40	2.0
60	3.0
80	4.0
100	5.0

(i) Plot a graph of Resistance ( $y$ -axis) against Length ( $x$ -axis) on the grid below.

[3]

(ii) Describe the relationship between resistance and length based on the graph. ........................................................................................................................................ ........................................................................................................................................ [1]

(iii) Determine the resistance per centimeter of the wire from your graph.

Resistance per cm = ____________________ $\Omega/\text{cm}$ [1]

12. Fig. 12.1 shows a circuit used to control a heater. The circuit includes a thermistor, a variable resistor, a transistor switch, and a relay that controls the heater.

(Diagram Description: A potential divider with a thermistor and variable resistor feeds into the base of an NPN transistor. The collector circuit contains a relay coil. The relay switch controls a separate high-voltage heater circuit.)

(a) Explain how the circuit works to turn on the heater when the temperature drops below a certain level. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ [4]

(b) State the purpose of the diode connected in parallel with the relay coil. ........................................................................................................................................ ........................................................................................................................................ [1]

(c) Suggest one modification to the circuit so that the heater turns on when the light intensity decreases (using an LDR instead of a thermistor). ........................................................................................................................................ ........................................................................................................................................ [1]

13. An electric motor lifts a load of mass $50 \text{ kg}$ vertically through a height of $4.0 \text{ m}$ in $8.0 \text{ s}$ . The motor is connected to a $120 \text{ V}$ supply and draws a current of $25 \text{ A}$ . (a) Calculate the useful power output of the motor (power used to lift the load).

Useful Power = ____________________ W [3]

(b) Calculate the total electrical power input to the motor.

Input Power = ____________________ W [2]

Efficiency = ____________________ % [2]

(d) Explain why the efficiency of the motor is less than $100\%$ . ........................................................................................................................................ ........................................................................................................................................ [1]

14. Fig. 14.1 shows an a.c. generator.

(Diagram Description: A coil rotating in a magnetic field, connected to slip rings and brushes, outputting to an oscilloscope.)

(a) Explain why an alternating voltage is induced in the coil. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ [2]

(b) The oscilloscope trace shows a peak voltage of $10 \text{ V}$ and a time period of $0.04 \text{ s}$ . (i) Calculate the frequency of the a.c. supply.

  Frequency = ____________________ Hz [2]
  
  (ii) State what happens to the trace on the oscilloscope if the coil is rotated at twice the speed.
  1. Peak voltage: ....................................................................................................
  2. Time period: ..................................................................................................... [2]

15. Safety in domestic electrical circuits is crucial. (a) Explain the difference between double insulation and earth wiring. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ [3]

(b) Why is it dangerous to touch a live wire with wet hands? ........................................................................................................................................ ........................................................................................................................................ [1]

(c) A fuse is rated at $5 \text{ A}$ . Explain what happens if a current of $8 \text{ A}$ flows through the circuit. ........................................................................................................................................ ........................................................................................................................................ ........................................................................................................................................ [2]

Answers

TuitionGoWhere Practice Paper - Physics O-Level

Answer Key and Marking Scheme Version 3 of 5

Section A: Structured Questions

1. (a) Electrons are transferred from the cloth to the 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 [1], 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 [1]. (Note: 2 marks max. Accept: "Opposite charges attract" if induction is implied).

2. (a) The voltmeter reading decreases [1]. (b) As temperature increases, resistance of thermistor decreases [1]. In a series circuit, voltage is shared proportional to resistance ( $V=IR$ or potential divider) [1]. Since the thermistor's resistance decreases relative to the fixed resistor, it takes a smaller share of the supply voltage [1].

3. (a) $P = VI \Rightarrow I = P/V$ [1]. $I = 24 / 12 = 2.0 \text{ A}$ [1]. (b) $V = IR \Rightarrow R = V/I$ [1]. $R = 12 / 2.0 = 6.0 \, \Omega$ [1]. (c) As voltage increases, current increases, causing the filament to heat up [1]. Higher temperature causes metal ions to vibrate more, increasing collisions with electrons, thus increasing resistance [1].

4. (a) For parallel resistors: $1/R_p = 1/R_A + 1/R_B$ [1]. $1/R_p = 1/4 + 1/4 = 2/4 = 1/2$ . $R_p = 2.0 \, \Omega$ [1]. (b) Total Resistance $R_T = R_p + R_C$ [1]. $R_T = 2.0 + 4.0 = 6.0 \, \Omega$ [1]. (Note: 1 mark for correct addition).

5. (a) $P = VI \Rightarrow I = P/V$ [1]. $I = 2400 \text{ W} / 240 \text{ V} = 10 \text{ A}$ [1]. The fuse must be slightly higher than the operating current. $13 \text{ A}$ is the suitable choice [1]. (Note: If calculation is wrong but logic for selecting next highest fuse is correct, award 1 mark). (b) Time $= 15/60 = 0.25 \text{ h}$ [1]. Energy $= P \times t = 2.4 \text{ kW} \times 0.25 \text{ h} = 0.6 \text{ kWh}$ [1]. (c) Cost $= 0.6 \times 25 = 15$ cents [1].

6. (a) Fleming’s Left-Hand Rule [1]. (b) It reverses 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, allowing continuous rotation [1]. (c) Any two from: Increase current [1], Increase magnetic field strength [1], Increase number of turns on coil [1].

7. (a) $V_p / V_s = N_p / N_s$ [1]. $240 / 12 = 2000 / N_s$ . $20 = 2000 / N_s \Rightarrow N_s = 2000 / 20 = 100$ turns [1]. (b) $V_p I_p = V_s I_s$ (for 100% efficiency) [1]. $240 \times I_p = 12 \times 2.0$ . $240 I_p = 24 \Rightarrow I_p = 24 / 240 = 0.1 \text{ A}$ [1]. (c) Transformers rely on a changing magnetic field to induce voltage in the secondary coil [1]. D.C. produces a constant magnetic field, so no EMF is induced [1].

8. (a) (i) Deflection in one direction [1]. (ii) No deflection (zero reading) [1]. (iii) Deflection in the opposite direction [1]. (b) The direction of the induced current (or EMF) is such that it opposes the change producing it [2]. (1 mark for "opposes change", 1 mark for context).

9. (a) High voltage reduces the current for the same power ( $P=VI$ ) [1]. Lower current reduces energy loss due to heating in the cables ( $P_{loss} = I^2 R$ ) [1]. (b) $P = VI \Rightarrow I = P/V$ [1]. $I = 500 \times 10^6 \text{ W} / 400 \times 10^3 \text{ V} = 500,000,000 / 400,000 = 1250 \text{ A}$ [1].

10. (a) Place plotting compasses around the wire [1]. Mark the direction the North pole points. Move compass to follow the field line [1]. OR Sprinkle iron filings on a card around the wire and tap gently [1]. (b) Any two from: Magnitude of current [1], Distance from the wire [1].

Section B: Free Response Questions

11. (a) Circuit Diagram:

Power source (battery/cell) [1].
Ammeter in series with the resistance wire [1].
Voltmeter in parallel across the length of the wire being tested [1].
Variable length mechanism (jockey/crocodile clips) indicated [1]. (Max 3 marks).

(b) (i) Graph:

Axes labeled correctly with units (Length/cm, Resistance/ $\Omega$ ) [1].
Points plotted correctly [1].
Straight line of best fit passing through origin [1]. (ii) Resistance is directly proportional to length [1]. (iii) Gradient calculation: $R/L$ . E.g., $5.0 / 100 = 0.05 \, \Omega/\text{cm}$ [1].

12. (a) When temperature drops, resistance of thermistor increases [1]. This causes the voltage across the thermistor (and thus the base of the transistor) to increase (potential divider action) [1]. When base voltage reaches a threshold, the transistor switches on [1]. Current flows through the relay coil, magnetizing it and closing the switch to turn on the heater [1]. (b) To protect the transistor from the high back-EMF induced in the relay coil when it switches off [1]. (c) Replace the thermistor with an LDR [1]. (Note: Depending on circuit configuration, may need to swap positions of sensor and variable resistor, but "Replace thermistor with LDR" is the primary modification).

13. (a) Work Done (GPE gain) $= mgh = 50 \times 10 \times 4.0 = 2000 \text{ J}$ [1]. Power Output $= \text{Work} / \text{time} = 2000 / 8.0 = 250 \text{ W}$ [2]. (1 mark for work, 1 for power). (b) Power Input $= VI = 120 \times 25 = 3000 \text{ W}$ [2]. (c) Efficiency $= (\text{Useful Output} / \text{Total Input}) \times 100\%$ [1]. Efficiency $= (250 / 3000) \times 100\% = 8.33\%$ [1]. (Accept 8.3%). (d) Energy is lost as heat due to resistance in the motor coils and friction in the bearings [1].

14. (a) The coil cuts through magnetic field lines as it rotates [1]. This changes the magnetic flux linkage through the coil, inducing an EMF (Faraday's Law) [1]. The direction of cutting reverses every half turn, causing alternating voltage [1]. (2 marks max). (b) (i) $f = 1/T = 1 / 0.04 = 25 \text{ Hz}$ [2]. (ii) 1. Peak voltage doubles (increases) [1]. 2. Time period halves (decreases) [1].

15. (a) Double Insulation: The appliance has a plastic casing and no exposed metal parts, so no earth wire is needed [1]. Earth Wiring: Metal-cased appliances are connected to earth so that if the live wire touches the case, the current flows to earth, blowing the fuse and preventing shock [1]. (b) Water (especially with impurities) is a good conductor of electricity [1]. Wet hands lower the skin's resistance, allowing a larger current to flow through the body, increasing the risk of severe shock [1]. (c) The fuse wire heats up due to the excessive current ( $I^2 R$ heating) [1]. It melts/blows, breaking the circuit and stopping the flow of current [1].