Secondary 3 Physics Energy Power Quiz

Secondary 3 Physics Quiz - Energy Power - ANSWER KEY

Total Marks: 40

Section A: Multiple Choice (5 × 1 mark = 5 marks)

1. C. Wind [1 mark for correct answer]

2. C. 400 J Working: W = F × d = 50 × 8 = 400 J [1 mark for correct answer]

3. C. 30 J Working: GPE = mgh = 2.0 × 10 × 1.5 = 30 J [1 mark for correct answer]

4. C. Energy can be transferred from one store to another, but the total energy remains constant. [1 mark for correct answer]

5. C. 80% Working: Efficiency = (240/300) × 100% = 80% [1 mark for correct answer]

Section B: Short Answer (5 × 2 marks = 10 marks)

6. Energy cannot be created or destroyed; it can only be transferred from one store to another or transformed from one form to another. The total energy of an isolated system remains constant. [2 marks: 1 for "cannot be created or destroyed", 1 for "transferred/transformed" or "total energy constant"]

7. Kinetic energy is the energy possessed by an object due to its motion (depends on mass and speed). Gravitational potential energy is the energy possessed by an object due to its position in a gravitational field (depends on mass, height, and gravitational field strength). [2 marks: 1 for correct description of KE, 1 for correct description of GPE]

8. KE = ½mv² = ½ × 1200 × (15)² = ½ × 1200 × 225 = 135,000 J (or 135 kJ) [2 marks: 1 for correct formula and substitution, 1 for correct answer with units]

9. In any real machine, some energy is always dissipated as heat due to friction between moving parts, air resistance, or other resistive forces. This energy is transferred to the thermal energy store of the surroundings and cannot be used for useful work, so the useful output is always less than the total input. [2 marks: 1 for identifying energy dissipation/friction, 1 for explaining that useful output < total input]

10. The student is partially correct. When the ball rises, kinetic energy is indeed converted to gravitational potential energy (assuming negligible air resistance). When it falls, gravitational potential energy is converted back to kinetic energy. However, in reality, some energy is dissipated as heat due to air resistance, so the ball will not return to its original height or speed. The principle of conservation of energy still holds because the total energy (including thermal energy) remains constant. [2 marks: 1 for agreeing and explaining energy conversion, 1 for mentioning energy dissipation/air resistance]

Section C: Structured Questions (15 marks)

11. (a) Work done = Force × distance = 800 × 12 = 9600 J (or 9.6 kJ) [2 marks: 1 for correct formula, 1 for correct answer with units]

(b) Power = Work done / time = 9600 / 40 = 240 W [2 marks: 1 for correct formula, 1 for correct answer with units]

(c) Efficiency = (Useful work output / Work input) × 100% 80% = (9600 / Work input) × 100% Work input = 9600 / 0.80 = 12,000 J (or 12 kJ) [2 marks: 1 for correct rearrangement, 1 for correct answer with units]

12. (a) GPE = mgh = 500 × 10 × 30 = 150,000 J (or 150 kJ) [2 marks: 1 for correct formula and substitution, 1 for correct answer with units]

(b) By conservation of energy (assuming no energy loss): KE at B = GPE at A = 150,000 J (or 150 kJ) [2 marks: 1 for stating conservation of energy, 1 for correct answer]

(c) KE = ½mv² 150,000 = ½ × 500 × v² v² = 150,000 / 250 = 600 v = √600 ≈ 24.5 m/s [2 marks: 1 for correct substitution and rearrangement, 1 for correct answer with units]

(d) The measured speed is lower because, in reality, energy is lost due to friction between the car and the track, and air resistance. This energy is dissipated as heat and sound, so not all the initial GPE is converted to KE. [2 marks: 1 for identifying friction/air resistance, 1 for explaining energy dissipation]

13. (a) GPE = mgh = 0.50 × 10 × 2.0 = 10 J [2 marks: 1 for correct formula and substitution, 1 for correct answer with units]

(b) Power = Energy / time = 10 / 4.0 = 2.5 W [2 marks: 1 for correct formula, 1 for correct answer with units]

(c) Efficiency = (Useful power output / Power input) × 100% = (2.5 / 5.0) × 100% = 50% [2 marks: 1 for correct formula and substitution, 1 for correct answer]

(d) Any one of:

• Energy is lost as heat due to friction in the motor.
• Energy is lost as sound.
• Energy is lost due to resistance in the electrical wires. [1 mark for any valid reason]

14. (a) P = ½ρAv³ = ½ × 1.2 × 5000 × (8.0)³ = 0.5 × 1.2 × 5000 × 512 = 1,536,000 W (or 1.536 MW or 1536 kW) [2 marks: 1 for correct substitution, 1 for correct answer with units]

(b) Electrical power = 0.40 × 1,536,000 = 614,400 W (or 614.4 kW) [1 mark for correct answer with units]

(c) Advantage (any one):

• Renewable energy source (wind is freely available).
• Does not produce greenhouse gases during operation.
• Low operating costs once installed.

Disadvantage (any one):

• Intermittent/unreliable (depends on wind conditions).
• Can be noisy and visually intrusive.
• May pose a threat to birds.
• High initial installation costs. [2 marks: 1 for a valid advantage, 1 for a valid disadvantage]

15. (a) Useful mechanical work = 0.25 × 800 kJ = 200 kJ (or 200,000 J) [1 mark for correct answer with units]

(b) Energy transferred as heat = 800 kJ - 200 kJ = 600 kJ (or 600,000 J) [1 mark for correct answer with units]

(c) Time = Energy / Power = 200,000 J / 200 W = 1000 s (or 16 min 40 s) [2 marks: 1 for correct formula and substitution, 1 for correct answer with units]

(d) The student's body temperature increases because the energy transferred as heat (from metabolic processes) is not all dissipated to the surroundings quickly enough, causing a rise in internal thermal energy. [1 mark for linking heat production to temperature increase]

Section D: Data-Based and Application Questions (10 marks)

16. (a) GPE = mgh = 0.20 × 10 × 2.0 = 4.0 J [1 mark for correct answer with units]

(b) GPE at top of bounce = 0.20 × 10 × 1.6 = 3.2 J [1 mark for correct answer with units]

(c) Percentage retained = (3.2 / 4.0) × 100% = 80% [2 marks: 1 for correct formula, 1 for correct answer]

(d) The energy not retained (0.8 J) was dissipated as heat and sound during the impact with the ground, and transferred to the thermal energy store of the ball, ground, and surroundings. [1 mark for identifying energy dissipation as heat/sound]

17. (a) As wind speed increases, power output increases. The increase is non-linear; power output rises more rapidly at higher wind speeds. [1 mark for describing the positive, non-linear relationship]

(b) The formula shows that power is proportional to the cube of wind speed (v³). This means that if wind speed doubles, power output increases by a factor of 2³ = 8, explaining the rapid increase. [2 marks: 1 for identifying the v³ relationship, 1 for explaining the effect of doubling]

(c) At very high wind speeds, the forces on the turbine blades could cause structural damage or failure. Shutting down protects the turbine. [1 mark for identifying safety/protection from damage]

(d) Theoretical power at 8.0 m/s: P = ½ × 1.2 × 5000 × (8.0)³ = 1,536,000 W = 1536 kW Actual power output = 80 kW Efficiency = (80 / 1536) × 100% ≈ 5.2% [2 marks: 1 for correct theoretical power calculation, 1 for correct efficiency]

18. (a) The system stores energy by pumping water to a higher elevation, converting electrical energy into gravitational potential energy of the water. [1 mark for describing energy conversion to GPE]

(b) GPE = mgh = 5000 × 10 × 200 = 10,000,000 J (or 10 MJ) [1 mark for correct answer with units]

(c) Power = Energy / time = (mgh) / t 2.0 × 10⁶ W = (m × 10 × 200) / 1 s m = (2.0 × 10⁶) / 2000 = 1000 kg [2 marks: 1 for correct formula and substitution, 1 for correct answer with units]

(d) With 80% efficiency, more energy (and therefore more water mass) is needed to produce the same electrical output because some energy is lost to friction, turbulence, and generator inefficiencies. [1 mark for explaining energy losses require greater mass flow]

19. (a) Total incident power = Intensity × Area = 800 W/m² × 2.0 m² = 1600 W [1 mark for correct answer with units]

(b) Electrical power output = 0.18 × 1600 W = 288 W [1 mark for correct answer with units]

(c) Power = Voltage × Current 288 W = 12 V × I I = 288 / 12 = 24 A [2 marks: 1 for correct formula and substitution, 1 for correct answer with units]

(d) Advantage (any one):

• Renewable energy source (sunlight is freely available).
• Does not produce greenhouse gases during operation.
• Low operating costs.

Limitation (any one):

• Intermittent (depends on sunlight availability, e.g., night/cloudy days).
• Requires large areas for significant power generation.
• High initial installation costs. [2 marks: 1 for a valid advantage, 1 for a valid limitation]

20. (a) Energy supplied = Power × time = 50 W × (5 × 60 s) = 15,000 J (or 15 kJ) [1 mark for correct answer with units]

(b) Thermal energy gained = mcΔθ = 0.50 × 4200 × (45 - 25) = 0.50 × 4200 × 20 = 42,000 J (or 42 kJ) [2 marks: 1 for correct formula and substitution, 1 for correct answer with units]

(c) The energy supplied by the heater (15,000 J) is much less than the thermal energy gained by the water (42,000 J). This discrepancy suggests an error in the experimental data or calculations. In reality, the energy supplied should be greater than the energy gained by the water due to heat losses to the surroundings. The values provided in the question are inconsistent; a 50 W heater running for 5 minutes cannot provide enough energy to raise the temperature of 0.50 kg of water by 20°C. Students should identify this inconsistency and suggest that either the power, time, mass, or temperature change was measured incorrectly, or that significant energy was gained from the surroundings (which is unlikely). [2 marks: 1 for comparing values and noting the discrepancy, 1 for suggesting experimental error or heat gain from surroundings]

(d) Any one of:

• Use a lid or cover for the container to reduce heat loss to the air.
• Use a well-insulated container (e.g., a calorimeter) to minimize heat exchange with the surroundings.
• Stir the water continuously to ensure uniform temperature distribution. [1 mark for any valid improvement]

END OF ANSWER KEY