Practice Questions

1.An electric heater uses 12.9kJ of energy whilst switched on and has a power rating of 600W. Calculate how long the heater was switched on for.

Energy = Power x time

Time = Energy/Power

Energy = 12.9kJ, 12900J

Time = 12900/600 = 21.5 seconds

2.A 3kW kettle draw a current of 12A from the mains supply. Calculate the potential difference of the mains supply

Power = potential difference x current

Potential difference = Power/current

Power = 3kW, 3000W

Potential difference = 3000/12 = 250V

3.A dishwasher has a power rating of 1.2kW  and a resistance of 40Ω. Calculate the current flowing through the dishwasher.

Power = Current² x Resistance

P = I² x R

I² = (P/R)

I = √(P/R)

I = √(1200/40)

I = √30

I = 5.5A