$A$ constant voltage is applied between the two ends of a uniform metallic wire. Some heat is developed in it. The heat developed is doubled if
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Heat developed is
$H=\frac{V^{2}}{R} t$
Heat developed will be doubled when $R$ is halved. Further,
$R=\rho l /\left(\pi r^{2}\right)$ and $H=V^{2} \pi r^{2} t / \rho l .$ So heat produced when both the length and radius of the wire are doubled.
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