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A potential divider circuit is connected with a dc source of $20 \mathrm{~V}$, a light emitting diode of glow in voltage $1.8 \mathrm{~V}$ and a zener diode of breakdown voltage of $3.2 \mathrm{~V}$. The length (PR) of the resistive wire is $20 \mathrm{~cm}$. The minimum length of PQ to just glow the LED is _____$\mathrm{cm}$
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The correct answer is:
5
$\begin{aligned} & \mathrm{PR}=20 \mathrm{~cm} \\ & \mathrm{~V}_{\mathrm{PQ}}=\frac{1}{4} \times \mathrm{R}_{\mathrm{PR}} \\ & \ell_{\min }(\mathrm{PQ})=\frac{1}{4} \times 20 \\ & =5 \mathrm{~cm}\end{aligned}$
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