A potentiometer wire of length 300 cm is connected in series with a… | JEE Main 2022 (26 Jul, Shift 2)

Q50JEE Main 2022 · 26 JUL · SHIFT 2Medium

Potentiometer

A potentiometer wire of length $300\text{ cm}$ is connected in series with a resistance 780 $\Omega$ and a standard cell of emf $4\text{ V}$ . A constant current flows through potentiometer wire. The length of the null point for cell of emf $20\text{ mV}$ is found to be $60\text{ cm}$ . The resistance of the potentiometer wire is ____________ $\Omega$ .

Numerical answer type. Work it out, then open the answer.

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Answer: 20

P.d. across 300 cm $= 20\ \text{mV} \times \frac{300}{60} = 0.1\ \text{V}$ .
$\frac{4r}{780 + r} = 0.1 \Rightarrow 3.9r = 78$
$r = 20\ \Omega$ .

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