A potentiometer circuit has been set up for finding the internal… | NEET 2014 (AIPMT)

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Potentiometer

A potentiometer circuit has been set up for finding the internal resistance of a given cell. The main battery, used across the potentiometer wire, has an emf of 2.0 V and a negligible internal resistance. The potentiometer wire itself is 4 m long. When the resistance R, connected across the given cell, has values of (i) infinity, (ii) 9.5 $\Omega$

the balancing lengths on the potentiometer wire are found to be 3 m and 2.85 m, respectively. The value of internal resistance of the cell is

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Answer: (c)

$R = \infty$ gives the open-circuit length $l_0 = 3$ m.
$r = R\dfrac{l_0-l}{l} = 9.5\times\dfrac{3-2.85}{2.85}$
$r = \dfrac{9.5\times0.15}{2.85} = 0.5\ \Omega$
So the answer is (c) $0.5\ \Omega$ .

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