Formative Practice 3.4 – Physics Form 5 Chapter 3 (Electricity)

Question 1:
The output power of a battery is 80 W. Determine the electrical energy that is supplied by
the battery in:
(a) 10 seconds
(b) 2 hours

Answer:
(a)
Power, P = 80 W
Time, t = 10 s

Electrical energy, E = Pt
= 80(10)
= 800 J


(b)
Power, P = 80 W

Time, t = 2 h
= 2 × 60 × 60
= 7 200 s

Electrical energy, E = Pt
= 80(7 200)
= 576 kJ

Question 2:
If the cost of electricity is 30 sen per unit, calculate the cost of using:
(a) 600 W LED television for 8 hours
(b) 1 kW vacuum cleaner for half an hour

Answer:
(a)
Power, P = 600 W
Time, t = 8 h

Energy used = 0.6 kW × 8 h
= 4.8 kWh

Unit cost = RM0.30
Cost of energy used = 4.8 × 0.30
= RM1.44


(b)
Power, P = 1 kW
Time, t = 0.5 h

Energy used = 1 kW × 0.5 h
= 0.5 kWh

Unit cost = RM0.30
Cost of energy used = 0.5 × 0.30
= RM0.15