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  4. 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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Q. 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 ____ $\Omega$.Physics Question Image

JEE MainJEE Main 2022Current Electricity

Solution:

$ 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$