In the experimental set up of metre bridge shown in the figure, the… | JEE Main 2019 (11 Jan, Shift 2)

Q70JEE Main 2019 · 11 JAN · SHIFT 2Medium

Wheatstone Bridge & Metre Bridge

In the experimental set up of metre bridge shown in the figure, the null point is obtained at a distance of 40 cm from A. If a 10 $\Omega$ resistor is connected in series with R1, the null point shifts by 10 cm. The resistance that should be connected in parallel with (R1 + 10) $\Omega$ such that the null point shifts back to its initial position is:

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Answer: (d)

$\dfrac{R_1}{R_2} = \dfrac{40}{60} \Rightarrow R_1 = \dfrac{2}{3}R_2$ .
Adding $10\ \Omega$ to $R_1$ moves the null point to 50 cm: $R_1 + 10 = R_2$ .
$\dfrac{2}{3}R_2 + 10 = R_2 \Rightarrow R_2 = 30\ \Omega,\ R_1 = 20\ \Omega$ .
Need $30 \parallel R = 20$ : $\dfrac{30R}{30+R} = 20 \Rightarrow R = 60\ \Omega$ .
So the answer is (d) $60\ \Omega$ .

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Wheatstone Bridge & Metre Bridge question from JEE Main 2019 (11 Jan, Shift 2)

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