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  4. Four cells, each of emf E and internal resistance r, are connected in series across an external resistance R. By mistake one of the cells is connected in reverse. Then the current in the external circuit is

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Q. Four cells, each of emf $E$ and internal resistance $r$, are connected in series across an external resistance $R$. By mistake one of the cells is connected in reverse. Then the current in the external circuit is

WBJEEWBJEE 2014Current Electricity

Solution:

Total emf of the cell $=3 E-E=2 E$
image
Total internal resistance $=4 r$
$\therefore $ Total resistance of the circuit $=4 r+R$
So, the current in the external circuit $\left(\because i=\frac{V}{R}\right)$
$\therefore i=\frac{2 E}{4 r+R}$