Electrical power test answers and mark scheme for grade 8 science (free PDF download)

Question 1 Meaning of power (2 marks)

1. The rate at which energy is transferred (energy per second).
2. Watt, W (1 joule per second).

Marking: 1 mark for each part.

Question 2 Cool box (2 marks)

1. P = V × I.
2. P = 12 × 3.5 = 42 W.

Marking: 1 mark for the formula, 1 mark for 42 W with unit.

Question 3 Hair dryer (3 marks)

1. 10 min = 1/6 h. E = 1.8 × 1/6 = 0.30 kWh.
2. E = 1800 W × 600 s = 1 080 000 J (check: 0.30 × 3 600 000).

Marking: Part a: 1 mark for the time in hours, 1 mark for 0.30 kWh. Part b: 1 mark.

Question 4 Monthly bill (3 marks)

1. 3018 − 2906 = 112 kWh.
2. Cost = 112 × 0.22 = 24.64 dollars.

Marking: Part a: 1 mark. Part b: 1 mark for the method, 1 mark for the answer.

Question 5 Blender fuse (3 marks)

1. I = P ÷ V = 1380 ÷ 230 = 6.0 A.
2. 13 A, the smallest one above the working current of 6.0 A.

Marking: Part a: 1 mark for the rearrangement, 1 mark for 6.0 A. Part b: 1 mark.

Question 6 Why kilowatt-hours? (3 marks)

1. The heater has a higher power, so it transfers more joules every second and uses more energy in the same hour, which costs more.
2. The kilowatt-hour is a much bigger unit than the joule, so bills show small convenient numbers instead of millions.

Marking: Part a: 1 mark for power as energy per second, 1 mark for linking more energy to more cost. Part b: 1 mark.

Question 7 Television or oven (3 marks)

1. Television: 0.120 × 4 = 0.48 kWh. Oven: 2.4 × 25/60 = 1.0 kWh.
2. The oven uses more, by 1.0 − 0.48 = 0.52 kWh.

Marking: Part a: 1 mark each. Part b: 1 mark.

Question 8 Correct the statement (1 mark)

Marking: 1 mark for stating that the time is needed.