(b) By Faraday’s law, $m \propto it$ $\therefore\frac{m_{1}}{m_{2}} = \frac{i_{1}t_{1}}{i_{2}t_{2}} \Rightarrow \frac{m}{m_{2}} = \frac{4 \times 120}{6 \times 40} \Rightarrow m_{2} = \frac{m}{2}$
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Ph.D. in Physics with 19+ years of teaching experience.