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A current $/$ is passing through a wire having two sections $P$ and $Q$ of uniform diameters $d$ and $d / 2$ respectively. If the mean drift velocity of electrons in sections $P$ and $Q$ is denoted by $v_P$ and $v_Q$ respectively, then
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$v_p={ }^{\frac{1}{4}} v_Q$
Drift velocity $v_d=\frac{i}{n e A} \Rightarrow v_d \propto \frac{1}{A}$ or $v_d \propto \frac{1}{d^2}$
$\Rightarrow \frac{v_P}{v_Q}=\left(\frac{d_Q}{d_p}\right)^2=\left(\frac{d / 2}{d}\right)^2=\frac{1}{4} \Rightarrow v_P=\frac{1}{4} v_Q$.
$\Rightarrow \frac{v_P}{v_Q}=\left(\frac{d_Q}{d_p}\right)^2=\left(\frac{d / 2}{d}\right)^2=\frac{1}{4} \Rightarrow v_P=\frac{1}{4} v_Q$.
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