WAEC Physics 2017 Paper

Concept:Electrical energy consumed is the product of power and time.The cost is found by multiplying the energy in kilowatt-hours by the cost per kilowatt-hour.Explanation:The lamp has a power rating of $P = 100\text{ W}$ and is used for $t = 5$ hours.Convert the time correctly because power is given in watts and energy is required in kilowatt-hours.Total energy in watt-hours is $E = P \times t = 100 \times 5 = 500\text{ Wh}$.Convert watt-hours to kilowatt-hours using $1\text{ kWh} = 1000\text{ Wh}$:$E = \frac{500}{1000} = 0.5\text{ kWh}$.The cost of $1\text{ kWh}$ is $\text{N}5$.Therefore, total cost $= 0.5 \times 5 = \text{N}2.50$.The voltage value $240\text{ V}$ is not needed for this calculation because the power rating already includes it.Answer:Cost of lighting the lamp for 5 hours is $\text{N}2.50$.Correct option: A.