First slide

Cell

Question

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.

Difficult

A

90%
B

60%
C

80%
D

10%

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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