A wire of radius $r$ has resistance $R$. If it is stretched to a radius of $\frac{{3r}}{4}$, its resistance becomes
Medium
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$\frac{{{R_1}}}{{{R_2}}} = {\left( {\frac{{{r_2}}}{{{r_1}}}} \right)^4}$ $ \Rightarrow $ $\frac{R}{{{R_2}}} = {\left( {\frac{{3r/4}}{r}} \right)^4} = \frac{{81}}{{256}} = {R_2} = \frac{{256}}{{81}}R$
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