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The efficiency of a cell when connected to a resistance R is 60%. What will be its efficiency if the external resistance is increased to six times.

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10%

60%

90%

80%

answer is A.

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Detailed Solution

For I^st case

efficiency =

$\frac{TPD}{EMF} \times 100 = \frac{ER / R + r}{E} = \frac{R}{R + r}$

$0 .6 = \frac{R}{R + r}$

Hence $r = \frac{2}{3} R$

$In 2^{nd} case$

$x = \frac{6 R}{6 R + r}$

Put the value of r and get the answer.

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