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O Level Physics Practice Paper 2

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TuitionGoWhere Practice Paper - Physics O-Level

TuitionGoWhere Secondary School (AI)


Subject:	Physics (6091)
Level:	O-Level
Paper:	PRACTICE – Electricity & Magnetism
Version:	2 of 5
Duration:	1 hour 15 minutes
Total Marks:	60

Name: _________________________

Class: _________________________

Date: _________________________

Instructions to Candidates

This paper consists of three sections: Section A, Section B, and Section C.
Answer all questions.
Write your answers in the spaces provided.
Show all working clearly; marks are awarded for method.
Include units in all final numerical answers.
The number of marks is given in brackets [ ] at the end of each question or part question.
You may use a scientific calculator.
Take g = 10 m/s² where required.

Section A: Short Answer and Data Interpretation (20 marks)

Answer all questions in this section.

1. A student rubs a polythene rod with a woollen cloth. The rod becomes negatively charged.

(a) Explain, in terms of electron transfer, how the polythene rod becomes negatively charged. [2]

(b) The charged rod is brought near a small piece of uncharged aluminium foil resting on a table. The foil is attracted to the rod. Explain why this happens. [2]

2. The diagram below shows two charged parallel metal plates. The top plate is positively charged and the bottom plate is negatively charged.

Image pending generation for this question.

    + + + + + + + + + + + +
    
    – – – – – – – – – – – –

(a) On the diagram above, sketch the electric field lines between the plates. Use arrows to show the direction of the field. [2]

(b) A small positive test charge is placed at the midpoint between the plates. State the direction of the force experienced by the test charge. [1]

3. A circuit contains a 12 V battery connected in series with an ammeter, a switch, and two resistors of 4.0 Ω and 8.0 Ω.

(a) Draw the circuit diagram using standard symbols. [2]

(b) When the switch is closed, the ammeter reads 1.0 A. Calculate the total resistance of the circuit. [2]

(c) The 4.0 Ω resistor is replaced with a 12 Ω resistor. Predict and explain what happens to the ammeter reading. [2]

4. A student investigates the I-V characteristic of a filament lamp. The results are shown in the table below.

Voltage / V	0	1.0	2.0	3.0	4.0	5.0	6.0
Current / A	0	0.12	0.18	0.22	0.25	0.27	0.29

(a) On the grid below, plot a graph of current (y-axis) against voltage (x-axis). Draw a smooth curve through the points. [3]

Current / A
0.30 |
     |
0.25 |
     |
0.20 |
     |
0.15 |
     |
0.10 |
     |
0.05 |
     |
   0 |____|____|____|____|____|____|____
     0   1.0  2.0  3.0  4.0  5.0  6.0
                Voltage / V

(b) Using your graph, determine the resistance of the filament lamp when the voltage is 3.0 V. [2]

(c) Explain why the resistance of the filament lamp changes as the voltage increases. [2]

Section B: Structured Questions (24 marks)

Answer all questions in this section.

5. A bar magnet is pushed into a coil of wire connected to a sensitive centre-zero galvanometer, as shown below.

    N [====] S  →→→  | coil |
                      |______|
                          |
                     +   G   –

(a) As the magnet enters the coil, the galvanometer needle deflects to the right. Explain why an EMF is induced in the coil. [2]

(b) State two ways in which the size of the induced EMF could be increased. [2]

(c) The magnet is now pulled out of the coil at the same speed. State and explain what is observed on the galvanometer. [2]

6. An electric kettle is rated at 240 V, 2200 W.

(a) Calculate the current flowing through the kettle when it is operating at its rated voltage. [2]

(b) The kettle is used to heat 1.5 kg of water from 25 °C to 100 °C. The specific heat capacity of water is 4200 J/(kg °C).

(i) Calculate the energy required to heat the water. [2]

(ii) The kettle is 80% efficient. Calculate the time taken to heat the water. [3]

(c) Explain why the heating element of the kettle is made of a material with high resistivity. [2]

7. The diagram below shows a transformer. The primary coil has 1200 turns and is connected to a 240 V a.c. supply. The secondary coil has 300 turns.

Image pending generation for this question.

    240 V a.c.  ────────  ││  ────────  output
        input     1200   ││   300
                  turns  ││  turns
                         ││
                    iron core

(a) Calculate the output voltage of the transformer. [2]

(b) The secondary coil is connected to a 12 Ω resistor. Calculate the current in the secondary circuit. [2]

(c) Assuming the transformer is 100% efficient, calculate the current in the primary coil. [2]

(d) Explain why the core of the transformer is made of soft iron rather than steel. [2]

8. A student sets up the circuit shown below to investigate the potential divider principle. The circuit contains a 6.0 V battery, a fixed resistor R of 10 Ω, and a variable resistor (rheostat) connected in series.

    6.0 V
     │
     ├───[R = 10 Ω]───┬───[rheostat]───┐
     │                │                │
     └────────────────┴────────────────┘

(a) The rheostat is adjusted so that its resistance is 20 Ω. Calculate the potential difference across the 10 Ω resistor. [3]

(b) The rheostat is now adjusted so that the potential difference across the 10 Ω resistor is 4.5 V. Calculate the resistance of the rheostat in this condition. [3]

Section C: Data-Based and Application Questions (16 marks)

Answer all questions in this section.

9. A factory uses an electrostatic precipitator to remove dust particles from waste gases before they are released into the atmosphere. The diagram below shows the basic structure.

Image pending generation for this question.

    waste gases ──→  │  +  │  +  │  +  │  → clean gases
    with dust        │grid │grid │grid │
                     │  –  │  –  │  –  │
                     └─────┴─────┴─────┘
                        collecting plates (earthed)

(a) Explain how the dust particles become charged as they pass through the precipitator. [2]

(b) Explain why the charged dust particles are attracted to the collecting plates. [2]

(c) State one advantage of using an electrostatic precipitator compared to a mechanical filter. [1]

(d) Suggest why the collecting plates need to be cleaned regularly. [1]

10. A student designs a temperature-sensing circuit using a thermistor and a fixed resistor to automatically switch on a fan when the temperature exceeds 30 °C. The circuit is shown below.

    9.0 V
     │
     ├───[thermistor]───┬───[relay coil]───┐
     │                  │                  │
     │                  ├───[R = 500 Ω]───┤
     │                  │                  │
     └──────────────────┴──────────────────┘

The relay switches on when the potential difference across the relay coil reaches 3.0 V. The resistance of the thermistor at various temperatures is given in the table.

Temperature / °C	10	20	30	40	50
Thermistor resistance / Ω	2000	1200	700	400	250

(a) Calculate the potential difference across the relay coil when the temperature is 20 °C. [3]

(b) Determine whether the fan will be switched on at 20 °C. Explain your answer. [2]

(c) The student wants the fan to switch on at a lower temperature of 25 °C. The thermistor resistance at 25 °C is 900 Ω. Suggest one change that could be made to the circuit to achieve this, without changing the battery voltage. Explain your reasoning. [3]

11. An electric motor is used to lift a load of mass 50 kg through a vertical height of 8.0 m in 4.0 s. The motor operates from a 240 V supply and draws a current of 5.0 A.

(a) Calculate the useful work done by the motor in lifting the load. [2]

(b) Calculate the electrical energy supplied to the motor during the 4.0 s. [2]

(c) Calculate the efficiency of the motor. [2]

(d) Explain what happens to the energy that is not converted to useful work. [2]

END OF PAPER

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Answers

TuitionGoWhere Practice Paper - Physics O-Level

ANSWER KEY AND MARKING SCHEME

Paper: PRACTICE – Electricity & Magnetism
Version: 2 of 5
Total Marks: 60

Section A: Short Answer and Data Interpretation (20 marks)

1. (a) Explain how the polythene rod becomes negatively charged. [2]

Answer: When the polythene rod is rubbed with the woollen cloth, electrons are transferred from the wool to the polythene rod [1]. The polythene rod gains electrons, giving it an overall negative charge [1].

Marking notes:

Award [1] for stating electrons transfer from wool to polythene.
Award [1] for stating rod gains electrons / becomes negatively charged.
Accept: "Polythene has a greater affinity for electrons than wool."

1. (b) Explain why the uncharged aluminium foil is attracted to the charged rod. [2]

Answer: The negative charges on the polythene rod repel the free electrons in the aluminium foil to the far side of the foil [1]. This leaves the side of the foil nearest the rod with a net positive charge (induced positive charge). The attraction between the negative rod and the induced positive charge on the foil is stronger than the repulsion between the rod and the negative charges on the far side (because the positive charges are closer), resulting in a net attractive force [1].

Marking notes:

Award [1] for describing charge separation / induction in the foil.
Award [1] for explaining net attraction due to distance/proximity.
Accept: "Electrostatic induction causes attraction."

2. (a) Sketch electric field lines between parallel plates. [2]

Answer: Field lines should be drawn as straight, parallel, equally spaced lines from the positive (top) plate to the negative (bottom) plate [1]. Arrows should point downwards from positive to negative [1].

Marking notes:

Award [1] for straight, parallel, equally spaced lines.
Award [1] for correct direction (downwards).
Deduct [1] if lines are curved or non-uniform.

2. (b) Direction of force on a positive test charge at midpoint. [1]

Answer: Downwards / towards the negative plate / from positive to negative plate. [1]

3. (a) Draw circuit diagram. [2]

Answer: Circuit should show: battery (two cells, long line positive, short line negative), ammeter (circle with A), switch (open or closed), and two resistors (rectangles or zigzag lines) all connected in series with connecting wires. [2]

Marking notes:

Award [1] for correct symbols.
Award [1] for correct series arrangement.
Deduct [1] if any component missing.

3. (b) Calculate total resistance. [2]

Answer: R_total = V / I = 12 V / 1.0 A = 12 Ω [1] for formula/substitution, [1] for correct answer with unit.

3. (c) Predict and explain ammeter reading change when 4.0 Ω replaced with 12 Ω. [2]

Answer: The ammeter reading will decrease [1]. The total resistance increases (from 12 Ω to 20 Ω), so by Ohm's law (I = V/R), the current decreases since the voltage remains constant [1].

Marking notes:

Award [1] for stating current decreases.
Award [1] for correct explanation using Ohm's law / increased resistance.

4. (a) Plot I-V graph. [3]

Answer: Points plotted correctly: (0,0), (1.0, 0.12), (2.0, 0.18), (3.0, 0.22), (4.0, 0.25), (5.0, 0.27), (6.0, 0.29) [1]. Smooth curve drawn through points [1]. Axes labelled with quantities and units [1].

Marking notes:

Award [1] for all points plotted accurately (± half small square).
Award [1] for smooth curve (not dot-to-dot straight lines).
Award [1] for labelled axes (Current/A and Voltage/V).
Curve should show decreasing gradient (resistance increasing).

4. (b) Determine resistance at 3.0 V from graph. [2]

Answer: From graph, at V = 3.0 V, I = 0.22 A [1]. R = V/I = 3.0/0.22 = 13.6 Ω (accept 13–14 Ω) [1].

Marking notes:

Award [1] for correct reading from graph.
Award [1] for correct calculation with unit.

4. (c) Explain why resistance changes with voltage. [2]

Answer: As voltage (and current) increases, the filament gets hotter [1]. The increased temperature causes increased vibration of the metal atoms/ions in the filament, which increases the resistance to electron flow / increases collisions between electrons and atoms [1].

Marking notes:

Award [1] for linking to temperature increase.
Award [1] for explaining increased atomic vibrations / increased collisions / increased resistance.

Section B: Structured Questions (24 marks)

5. (a) Explain why EMF is induced as magnet enters coil. [2]

Answer: As the magnet moves into the coil, the magnetic field lines passing through the coil change / the magnetic flux linking the coil changes [1]. By Faraday's law of electromagnetic induction, a changing magnetic flux induces an EMF in the coil [1].

Marking notes:

Award [1] for stating magnetic field/flux through coil changes.
Award [1] for linking to electromagnetic induction / Faraday's law.

5. (b) State two ways to increase induced EMF. [2]

Answer (any two, 1 mark each):

Move the magnet faster / increase speed of magnet.
Use a stronger magnet.
Increase the number of turns in the coil.
Use a coil with a larger cross-sectional area.

5. (c) Observation when magnet pulled out at same speed. [2]

Answer: The galvanometer needle deflects to the left / in the opposite direction [1]. This is because the magnetic flux through the coil is now decreasing (instead of increasing), so the induced EMF is in the opposite direction (Lenz's law) [1].

Marking notes:

Award [1] for stating opposite deflection.
Award [1] for explaining flux decreasing / Lenz's law / opposite direction of induced EMF.

6. (a) Calculate current through kettle. [2]

Answer: P = VI → I = P/V = 2200 W / 240 V [1] = 9.17 A (accept 9.2 A) [1].

6. (b)(i) Calculate energy required to heat water. [2]

Answer: E = mcΔθ = 1.5 × 4200 × (100 – 25) [1] = 1.5 × 4200 × 75 = 472,500 J (or 473 kJ) [1].

6. (b)(ii) Calculate time taken (80% efficiency). [3]

Answer: Efficiency = Useful energy output / Total energy input [1]. 0.80 = 472,500 / E_input → E_input = 472,500 / 0.80 = 590,625 J [1]. P = E/t → t = E/P = 590,625 / 2200 = 268.5 s ≈ 270 s (or 4 min 30 s) [1].

Marking notes:

Award [1] for correct efficiency formula/substitution.
Award [1] for calculating total input energy.
Award [1] for correct time with unit.

6. (c) Explain why heating element has high resistivity. [2]

Answer: A material with high resistivity produces more heat for a given current (P = I²R) [1]. This ensures that most of the electrical energy is converted to thermal energy in the heating element, making the kettle efficient at heating water [1].

Marking notes:

Award [1] for linking high resistivity to greater heating effect.
Award [1] for explaining efficiency / practical purpose.

7. (a) Calculate output voltage of transformer. [2]

Answer: V_s / V_p = N_s / N_p → V_s / 240 = 300 / 1200 [1]. V_s = 240 × (300/1200) = 60 V [1].

7. (b) Calculate current in secondary circuit. [2]

Answer: I_s = V_s / R = 60 V / 12 Ω [1] = 5.0 A [1].

7. (c) Calculate current in primary coil (100% efficiency). [2]

Answer: V_p × I_p = V_s × I_s → 240 × I_p = 60 × 5.0 [1]. I_p = 300 / 240 = 1.25 A [1].

7. (d) Explain why core is soft iron, not steel. [2]

Answer: Soft iron is easily magnetised and demagnetised [1]. In a transformer, the core needs to be repeatedly magnetised and demagnetised as the alternating current changes direction. Steel would retain magnetism (is harder to demagnetise), which would cause energy losses and reduce efficiency [1].

Marking notes:

Award [1] for stating soft iron is easily magnetised/demagnetised.
Award [1] for explaining why this is needed (AC operation) or why steel is unsuitable (retains magnetism / hysteresis losses).

8. (a) Calculate PD across 10 Ω resistor (rheostat = 20 Ω). [3]

Answer: Total resistance = 10 + 20 = 30 Ω [1]. Circuit current I = V/R_total = 6.0/30 = 0.20 A [1]. V_10Ω = I × R = 0.20 × 10 = 2.0 V [1].

8. (b) Calculate rheostat resistance when V_10Ω = 4.5 V. [3]

Answer: Current through 10 Ω resistor: I = V/R = 4.5/10 = 0.45 A [1]. Total resistance: R_total = V_supply/I = 6.0/0.45 = 13.33 Ω [1]. Rheostat resistance = R_total – 10 = 13.33 – 10 = 3.33 Ω (accept 3.3 Ω) [1].

Alternative method: V_rheostat = 6.0 – 4.5 = 1.5 V. I = 4.5/10 = 0.45 A. R_rheostat = 1.5/0.45 = 3.33 Ω.

Section C: Data-Based and Application Questions (16 marks)

9. (a) Explain how dust particles become charged. [2]

Answer: The high voltage grids ionise the air molecules / create ions [1]. The dust particles collide with these ions and gain charge (become charged) [1].

Marking notes:

Award [1] for mentioning ionisation of air.
Award [1] for dust gaining charge through contact/collision with ions.

9. (b) Explain why charged dust attracted to collecting plates. [2]

Answer: The charged dust particles experience an electric force in the electric field between the grids and the earthed plates [1]. The dust particles (charged by the grids) are attracted towards the oppositely charged / earthed collecting plates [1].

Marking notes:

Award [1] for mentioning electric field/force.
Award [1] for attraction to oppositely charged/earthed plates.

9. (c) State one advantage over mechanical filter. [1]

Answer (any one):

Can remove very fine/small particles.
Does not clog as easily / lower maintenance.
Does not impede gas flow significantly.
More effective at removing particulate matter.

9. (d) Suggest why plates need regular cleaning. [1]

Answer: Dust accumulates on the plates, reducing the effectiveness of the precipitator / reducing the electric field strength / the plates become insulated by the dust layer.

10. (a) Calculate PD across relay coil at 20 °C. [3]

Answer: At 20 °C, thermistor resistance = 1200 Ω [1]. Total resistance = 1200 + 500 = 1700 Ω [1]. Circuit current I = 9.0/1700 = 0.00529 A. V_relay = I × R = 0.00529 × 500 = 2.65 V (accept 2.6–2.7 V) [1].

Alternative (potential divider formula): V_relay = [500/(1200+500)] × 9.0 = (500/1700) × 9.0 = 2.65 V.

10. (b) Will fan switch on at 20 °C? Explain. [2]

Answer: No, the fan will not switch on [1]. The PD across the relay coil (2.65 V) is less than the required 3.0 V to activate the relay [1].

10. (c) Suggest change to switch fan on at 25 °C (thermistor = 900 Ω). [3]

Answer: Increase the value of the fixed resistor R [1]. At 25 °C, with thermistor = 900 Ω, a larger fixed resistor would increase the fraction of the supply voltage across the relay coil [1]. For V_relay = 3.0 V: 3.0 = [R/(900+R)] × 9.0 → R = 450 Ω (but current R = 500 Ω, so R should be increased to achieve 3.0 V at 900 Ω thermistor). Wait — check: 3.0/9.0 = R/(900+R) → 1/3 = R/(900+R) → 900+R = 3R → 2R = 900 → R = 450 Ω. So the fixed resistor should be decreased to 450 Ω to switch on at 25 °C [1 for correct reasoning].

Marking notes:

Award [1] for identifying need to change fixed resistor value.
Award [1] for correct direction (decrease to 450 Ω, or increase if reasoning about threshold is sound).
Award [1] for clear explanation using potential divider principle.
Accept: "Decrease the fixed resistor to 450 Ω so that V_relay reaches 3.0 V when thermistor is 900 Ω."

11. (a) Calculate useful work done. [2]

Answer: Work done = mgh = 50 × 10 × 8.0 [1] = 4000 J [1].

11. (b) Calculate electrical energy supplied. [2]

Answer: E = VIt = 240 × 5.0 × 4.0 [1] = 4800 J [1].

11. (c) Calculate efficiency. [2]

Answer: Efficiency = (Useful work output / Energy input) × 100% = (4000/4800) × 100% [1] = 83.3% (accept 83%) [1].

11. (d) Explain what happens to wasted energy. [2]

Answer: The energy that is not converted to useful work is mostly dissipated as thermal energy (heat) [1] in the motor windings due to the resistance of the coils (I²R losses), and also as sound energy and friction in the moving parts [1].

Marking notes:

Award [1] for identifying heat/thermal energy as main waste.
Award [1] for explaining source (resistance in coils, friction, sound).

END OF ANSWER KEY

Marking scheme based on O-Level Physics (6091) assessment standards. Marks awarded for correct method, appropriate working, and correct units.