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Standard XII

Physics

Series Combination of Cells

Five cells ea...

Question

Five cells each of emf E and internal resistance r send the same amount of current through an external resistance R whether the cells are connected in parallel or in series. Then, the ratio $(\frac{R}{r})$ is

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\frac{1}{2}

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\frac{1}{5}

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Solution

The correct option is D 1
When 5 cell are joined in parallel as shown,
$i = \frac{n e t e m f}{t o t a l r e s i s t a n c e}$
$i = \frac{5 E}{5 R + r}$ .... (i)
When 5 cell are joined in series as shown,
$i = \frac{5 E}{5 r + R}$ .... (ii)
So, according to the questions
$\frac{5 E}{5 R + r} = \frac{5 E}{5 r + R}$
or $4 R = 4 r \Rightarrow \frac{R}{r} = \frac{4}{4}$
$\frac{R}{r} = 1$

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Five cells each of emf E and internal resistance r send the same amount of current through an external resistance R whether the cells are connected in parallel or in series. Then, the ratio (Rr) is

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