O Level Combined Science Physical Sciences Quiz

O-Level Combined Science Quiz - Physical Sciences: Answer Key

Total Marks: 50

---

Section A: Short Answer and Structured Questions (20 marks)

1. State the difference between a scalar quantity and a vector quantity. Give one example of each. [2 marks]

Answer:

• A scalar quantity has magnitude only, while a vector quantity has both magnitude and direction. [1 mark]
• Scalar example: distance, speed, mass, time, energy, temperature (any one correct). [0.5 marks]
• Vector example: displacement, velocity, acceleration, force, weight, momentum (any one correct). [0.5 marks]

Marking notes: Award 1 mark for correct distinction. Award 0.5 marks each for valid examples. Do not accept "scalar has no direction" without mentioning magnitude.

---

2. A student measures the length of a table as 1.52 m using a metre rule. (a) State the precision of the metre rule. [1 mark] (b) Express this length in centimetres and in millimetres. [2 marks]

Answer: (a) 0.1 cm or 1 mm or 0.001 m [1 mark] (b) 1.52 m = 152 cm [1 mark]; 1.52 m = 1520 mm [1 mark]

Marking notes: Accept any valid expression of precision. For (b), both conversions must be correct with units for full marks.

---

3. An object has a mass of 250 g and a volume of 50 cm³. (a) Calculate the density of the object in g/cm³. [2 marks] (b) The density of water is 1.0 g/cm³. State and explain whether the object will float or sink in water. [2 marks]

Answer: (a) Density = mass / volume = 250 / 50 = 5.0 g/cm³ [2 marks: 1 for formula/substitution, 1 for correct answer with unit] (b) The object will sink. [1 mark] The density of the object (5.0 g/cm³) is greater than the density of water (1.0 g/cm³). [1 mark]

Marking notes: For (a), award 1 mark for correct working even if final answer has minor error. For (b), must state "sink" and give comparative density explanation.

---

4. State the Principle of Conservation of Energy. [2 marks]

Answer: Energy cannot be created or destroyed. [1 mark] It can only be converted/transformed/transferred from one form to another. [1 mark] OR: The total energy in an isolated/closed system remains constant.

Marking notes: Both parts required for full marks. Accept "total energy is conserved" as equivalent to second part. Do not accept "energy is always conserved" alone.

---

5. A force of 15 N is applied perpendicularly to a spanner at a distance of 0.25 m from the centre of a bolt. Calculate the moment of the force about the bolt. [2 marks]

Answer: Moment = Force × perpendicular distance [1 mark] Moment = 15 × 0.25 = 3.75 N m [1 mark]

Marking notes: Award 1 mark for correct formula or substitution. Award 1 mark for correct answer with unit (accept N m or Nm).

---

6. Explain, using the particle model, how thermal energy is transferred through a metal rod when one end is heated. [3 marks]

Answer:

• Particles at the heated end gain kinetic energy and vibrate more vigorously. [1 mark]
• These vibrations are passed to neighbouring particles through collisions, transferring energy along the rod. [1 mark]
• In metals, free electrons also move and transfer kinetic energy rapidly through the metal, making metals good conductors. [1 mark]

Marking notes: Award marks for: (1) increased vibration at hot end, (2) transfer via particle collisions/vibrations, (3) role of free electrons in metals. Accept any valid particle-level explanation.

---

7. State two differences between boiling and evaporation. [2 marks]

Answer: Any two of the following (1 mark each):

• Boiling occurs at a specific temperature (boiling point); evaporation occurs at any temperature.
• Boiling occurs throughout the liquid; evaporation occurs only at the surface.
• Boiling produces bubbles; evaporation does not.
• Boiling is a rapid process; evaporation is a slow process.

Marking notes: Must state clear contrasting differences. Award 1 mark per valid difference.

---

8. A student of weight 500 N runs up a flight of 20 steps. Each step has a height of 15 cm. She takes 8.0 s to reach the top. (a) Calculate the total vertical height climbed. [1 mark] (b) Calculate the work done by the student. [1 mark] (c) Calculate the average power developed by the student. [2 marks]

Answer: (a) Total height = 20 × 0.15 = 3.0 m [1 mark] (b) Work done = Force × distance = 500 × 3.0 = 1500 J [1 mark] (c) Power = Work / time = 1500 / 8.0 = 187.5 W ≈ 188 W [2 marks: 1 for formula/substitution, 1 for correct answer with unit]

Marking notes: For (a), must convert cm to m. For (c), accept 187.5 W or 188 W.

---

Section B: Diagram and Data Interpretation (15 marks)

9. Figure 1 shows a ray of light travelling from air into a glass block. (a) State what happens to the speed of light as it enters the glass block. [1 mark] (b) State what happens to the wavelength of light as it enters the glass block. [1 mark] (c) The angle of refraction in the glass is 25°. Calculate the refractive index of the glass. [2 marks]

Answer: (a) The speed of light decreases. [1 mark] (b) The wavelength decreases. [1 mark] (c) n = sin i / sin r = sin 40° / sin 25° [1 mark] n = 0.6428 / 0.4226 = 1.52 [1 mark]

Marking notes: For (a) and (b), accept "reduces" or "becomes smaller". For (c), award 1 mark for correct formula/substitution, 1 mark for correct answer (accept 1.5 to 1.52).

---

10. Figure 2 shows a simple pendulum swinging between positions A and C. (a) At which position (A, B, or C) does the sphere have maximum kinetic energy? Explain. [2 marks] (b) At which position (A, B, or C) does the sphere have maximum gravitational potential energy? Explain. [2 marks] (c) State the energy conversion that occurs as the sphere moves from position A to position B. [1 mark]

Answer: (a) Position B. [1 mark] At the lowest point, the sphere has maximum speed, so kinetic energy is maximum. [1 mark] (b) Positions A and C. [1 mark] At the highest points, the sphere has maximum height, so gravitational potential energy is maximum. [1 mark] (c) Gravitational potential energy is converted to kinetic energy. [1 mark]

Marking notes: For (b), accept either A or C. Must link position to energy reasoning.

---

11. Figure 3 shows a U-tube manometer connected to a gas supply. (a) On which side of the U-tube is the pressure greater? Explain. [2 marks] (b) Calculate the pressure difference between the gas supply and the atmosphere. [2 marks]

Answer: (a) The gas supply side has greater pressure. [1 mark] The gas pressure pushes the water level down on the gas side and up on the atmosphere side. [1 mark] (b) Pressure difference = ρgh = 1000 × 10 × 0.080 [1 mark] = 800 Pa [1 mark]

Marking notes: For (b), must convert 8.0 cm to 0.080 m. Award 1 mark for correct substitution, 1 mark for correct answer with unit.

---

12. Figure 4 shows a circuit with two resistors in parallel across a 6.0 V battery. (a) Calculate the total resistance of the circuit. [2 marks] (b) Calculate the total current flowing from the battery. [2 marks]

Answer: (a) 1/R_total = 1/3.0 + 1/6.0 = 2/6.0 + 1/6.0 = 3/6.0 [1 mark] R_total = 6.0/3 = 2.0 Ω [1 mark] (b) I = V/R = 6.0/2.0 = 3.0 A [2 marks: 1 for formula/substitution, 1 for correct answer with unit]

Marking notes: For (a), accept alternative method. For (b), award full marks if correct current calculated using individual branch currents (I₁ = 2.0 A, I₂ = 1.0 A, total = 3.0 A).

---

Section C: Application and Analysis (15 marks)

13. A crane lifts a concrete block of mass 200 kg through a vertical height of 15 m in 30 s. (a) Calculate the weight of the concrete block. [1 mark] (b) Calculate the work done by the crane. [2 marks] (c) Calculate the power output of the crane. [2 marks] (d) Calculate the efficiency of the crane. [2 marks]

Answer: (a) Weight = mg = 200 × 10 = 2000 N [1 mark] (b) Work done = Force × distance = 2000 × 15 = 30,000 J [2 marks: 1 for formula, 1 for answer] (c) Power output = Work/time = 30,000/30 = 1000 W [2 marks: 1 for formula, 1 for answer] (d) Efficiency = (Power output / Power input) × 100% [1 mark] = (1000/1500) × 100% = 66.7% [1 mark]

Marking notes: Accept 67% for efficiency. Award method marks even if earlier parts have errors (error carried forward).

---

14. A student investigates pressure at different depths in a liquid. (a) Plot a graph of pressure against depth. [3 marks] (b) Describe the relationship. [1 mark] (c) Predict pressure at 25 cm depth. [1 mark] (d) State one variable kept constant. [1 mark]

Answer: (a)

Image pending generation for this question.

[3 marks: 1 for correct points, 1 for straight line, 1 for labelled axes] (b) Pressure is directly proportional to depth. [1 mark] (c) 3.0 kPa [1 mark] (d) Density of liquid / temperature / same liquid used (any one). [1 mark]

Marking notes: For (a), deduct 1 mark for each error. For (c), accept 2.9-3.1 kPa.

---

15. A metal block is heated using a 50 W heater for 5.0 minutes. (a) Calculate total electrical energy supplied. [2 marks] (b) Calculate thermal energy absorbed by the block. [2 marks] (c) Suggest one reason for energy difference. [1 mark]

Answer: (a) Time = 5.0 × 60 = 300 s [0.5 marks] Energy = Power × time = 50 × 300 = 15,000 J [1.5 marks] (b) Energy = mcΔθ = 0.50 × 400 × (65 - 25) [1 mark] = 0.50 × 400 × 40 = 8000 J [1 mark] (c) Energy is lost to the surroundings / heat lost to air / heater also heats the container (any valid reason). [1 mark]

Marking notes: For (a), award full marks for 15,000 J or 15 kJ. For (b), must show temperature change calculation.

---

16. Explain the observation of paper moving on water when detergent is added. [2 marks]

Answer:

• Detergent reduces the surface tension of water. [1 mark]
• The surface tension is higher in the surrounding water than where the detergent was added. The paper is pulled towards the region of higher surface tension (away from the detergent). [1 mark]

Marking notes: Must mention surface tension and the difference in surface tension causing movement.

---

17. A car of mass 1200 kg travelling at 20 m/s comes to rest in 5.0 s. (a) Calculate initial kinetic energy. [2 marks] (b) Calculate average braking force. [3 marks]

Answer: (a) KE = ½mv² = ½ × 1200 × (20)² [1 mark] = ½ × 1200 × 400 = 240,000 J [1 mark] (b) Work done by brakes = change in KE = 240,000 J [1 mark] Distance travelled: average speed = (20 + 0)/2 = 10 m/s; distance = 10 × 5.0 = 50 m [1 mark] Force = Work/distance = 240,000/50 = 4800 N [1 mark] OR: F = ma; a = (0 - 20)/5.0 = -4.0 m/s²; F = 1200 × 4.0 = 4800 N

Marking notes: Accept alternative methods. Award marks for correct physics reasoning even if final answer has arithmetic error.

---

18. Explain why a person feels colder on a windy day than on a calm day at the same temperature. [2 marks]

Answer:

• Wind increases the rate of evaporation of moisture/sweat from the skin. [1 mark]
• Evaporation requires thermal energy (latent heat), which is taken from the skin, causing cooling. [1 mark] OR: Wind removes the layer of warm air near the skin (convection), increasing heat loss from the body.

Marking notes: Must link wind to increased rate of heat loss. Accept either evaporation or convection explanation.

---

19. A student investigates convection using potassium permanganate in water. (a) Describe and explain the movement of coloured water. [2 marks] (b) State why potassium permanganate is used. [1 mark]

Answer: (a) The coloured water rises above the heated region, moves across the top, sinks at the far side, and moves along the bottom back to the heated region (convection current). [1 mark] Heating causes the water to expand, become less dense, and rise. Cooler, denser water sinks to replace it. [1 mark] (b) Potassium permanganate is used to make the movement of water visible / as a dye/tracer. [1 mark]

Marking notes: For (a), must describe the circulation pattern and explain in terms of density changes.

---

20. A loudspeaker produces sound of frequency 500 Hz. Speed of sound = 340 m/s. (a) Calculate the wavelength. [2 marks] (b) State what happens to wavelength if frequency increases. [1 mark]

Answer: (a) v = fλ → λ = v/f = 340/500 [1 mark] = 0.68 m [1 mark] (b) The wavelength decreases. [1 mark]

Marking notes: For (a), award 1 mark for correct formula/substitution, 1 mark for correct answer with unit. For (b), accept "becomes shorter" or "reduces".

---

END OF ANSWER KEY