WAEC Physics 2022 Paper

Concept:A galvanometer is converted into a voltmeter by connecting a high resistance in series.At full-scale deflection, the total voltage equals the current times the total series resistance.Explanation:Let $r$ be the internal resistance of the galvanometer.Full-scale current is $I_g = 20\ \mathrm{mA} = 0.02\ \mathrm{A}$.The voltmeter reads $8\ \mathrm{V}$ at full scale.Series multiplier resistance is $R = 395\ \Omega$.Using Ohm's law for the whole circuit:$$V = I_g(r + R)$$Substitute the values:$$8 = 0.02(r + 395)$$$$r + 395 = \frac{8}{0.02} = 400$$$$r = 400 - 395 = 5\ \Omega$$Answer:The internal resistance of the galvanometer is $5.0\ \Omega$.Correct option: B. $5.0\ \Omega$