As shown in the figure, a potentiometer wire of resistance \(20\,\Omega\) 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 \(.....\Omega\)

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Answer & Solution

Correct Answer

(A) \(780\)

Step-by-step Solution

Detailed explanation

\(E =\frac{ AC }{ AB }\left( V _{ A }- V _{ B }\right)\)

\(\therefore 20 \times 10^{-3}=\frac{60}{300} \times \frac{4 \times 20}{ R +20}\)

\(\therefore R =780\,\Lambda\)

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