As shown in the figure, a potentiometer wire of resistance 20 Ω and length 300 cm is connected with resistance box (R.B.) and a standard cell of emf 4 V. For a resistance 'R' of resistance box introduced into the circuit, the null point for a cell of 20 mV is found to be 60 cm. The value of 'R ' is Ω.

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answer is 780.

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Detailed Solution

$\begin{array}{l} R_{W} = 20 Ω \\ ε_{0} = 4 V \\ (\frac{4 \times 20}{20 + R}) \times \frac{60}{300} = 20 \times 10^{- 3} \\ \Rightarrow \frac{4}{20 + R} = 5 \times 10^{- 3} \\ \Rightarrow 20 + R = 800 \\ \Rightarrow R = 780 Ω \end{array}$

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