WAEC Physics 2010 Paper

Concept:A constant-volume gas thermometer shows a linear relation between pressure and temperature.Explanation:At ice point, $P_0 = 440$ mmHg at $0^\circ$C.At steam point, $P_{100} = 680$ mmHg at $100^\circ$C.Let $T$ be the temperature when the pressure is $P = 380$ mmHg.Using linear proportionality:$$T = \left(\frac{P - P_0}{P_{100} - P_0}\right) \times 100$$Substitute the values:$$T = \frac{380 - 440}{680 - 440} \times 100 = \frac{-60}{240} \times 100 = -25^\circ\text{C}$$Since the result is negative, it confirms that $380$ mmHg lies below the ice-point pressure.Answer:A. $-25^\circ$C