Question 431: Current Electricity

Question

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

Accepted Answer

$$l = 3m$$, $${R_w} = 20\,\Omega $$
$${\varepsilon _0} = 4V$$
$${{4 	imes 20} \over {20 + R}} 	imes {{60} \over {300}} = 20 	imes {10^{ - 3}}$$
$${4 \over {20 + R}} = 5 	imes {10^{ - 3}}$$
$$20 + R = 800$$
$$R = 780\,\Omega $$

Current Electricity

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