WAEC Physics 2024 Paper

Concept:When water vapor condenses, it first releases latent heat at constant temperature, then the resulting hot water releases additional heat while cooling to the final temperature.Explanation:Given: mass $m = 0.02\ \mathrm{kg}$, specific heat capacity $c = 4200\ \mathrm{J\,kg^{-1}K^{-1}}$, specific latent heat of vaporization $L_v = 2.26 \times 10^6\ \mathrm{J\,kg^{-1}}$.Water vapor condenses at $100^\circ\mathrm{C}$, and the condensed water finally cools to $97^\circ\mathrm{C}$.Heat released during condensation:$$Q_1 = mL_v = 0.02 \times 2.26 \times 10^6 = 45{,}200\ \mathrm{J}.$$Heat released when water cools from $100^\circ\mathrm{C}$ to $97^\circ\mathrm{C}$:$$Q_2 = mc\Delta T = 0.02 \times 4200 \times (100 - 97) = 252\ \mathrm{J}.$$Total heat extracted:$$Q = Q_1 + Q_2 = 45{,}200 + 252 = 45{,}452\ \mathrm{J}.$$Answer:$45{,}452\ \mathrm{J}$Option A.