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The resistance of a wire is 'R' ohm. If it is melted and stretched to 'n' times its original length, its new resistance will be
Explanation
Volume constant, so $R \propto l^2$.
Detailed Solution
At constant volume, $R \propto l^2$: $\frac{R_2}{R_1} = \frac{l_2^2}{l_1^2} = \frac{n^2l_1^2}{l_1^2}$
$\frac{R_2}{R_1} = n^2$
$R_2 = n^2R_1$