JAMB Physics 2020 Paper

Concept:Closing the key adds another parallel resistor, which lowers the total resistance and therefore increases the current.Explanation:When key K is open, only one $8\,\Omega$ resistor is in series with the $2\,\Omega$ resistor.Total resistance: $R = 8 + 2 = 10\,\Omega$.Current: $I = \dfrac{V}{R} = \dfrac{6}{10} = 0.6\,\text{A}$.When key K is closed, the two $8\,\Omega$ resistors are in parallel.Their effective resistance is given by $\dfrac{1}{R_p} = \dfrac{1}{8} + \dfrac{1}{8} = \dfrac{2}{8}$.So $R_p = 4\,\Omega$.This $4\,\Omega$ resistor is in series with the $2\,\Omega$ resistor, giving a total resistance of $4 + 2 = 6\,\Omega$.New current: $I = \dfrac{6}{6} = 1\,\text{A}$.Increase in current: $1 - 0.6 = 0.4\,\text{A}$.Answer:A. increase the current by 0.4A