Cell of emf X is connected across a resistor R. The potential difference across the wire is measured as Y. The internal resistance of'the cell should be

X - Y/R

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(X - Y)R/Y

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(X - Y)Y/R

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(X - Y)R

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Solution

The correct option is A (X - Y)R/Y
E.m.f = X, Resistance = R, Potential difference = Y
$E = I (R + r)$ putting $I = V / R$
Internal resistance, $r = (\frac{E}{V} - 1) R$
$= (\frac{X}{Y} - 1) R = (\frac{X - Y}{Y}) R$

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