WAEC Physics 2014 Paper

Concept:Heat lost by hot water equals heat gained by cold water.Explanation:Let $m_c$ be the mass of cold water and $m_h$ be the mass of hot water.Specific heat capacity $c$ is the same for both.Temperature rise of cold water: $\Delta T_c = 40^\circ C - 20^\circ C = 20^\circ C$.Temperature fall of hot water: $\Delta T_h = 70^\circ C - 40^\circ C = 30^\circ C$.Heat gained by cold water = heat lost by hot water:$m_c c \times 20 = m_h c \times 30$Cancel $c$: $20 m_c = 30 m_h$Thus, $\frac{m_c}{m_h} = \frac{30}{20} = \frac{3}{2}$.Therefore, the ratio of cold water to hot water is $3:2$.Answer:B. $3:2$