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  4. A cell of emf E is connected with an external resistance R, then potential difference across cell is V. The internal resistance of a cell will be

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Q. A cell of emf $E$ is connected with an external resistance $R$, then potential difference across cell is $V$. The internal resistance of a cell will be

Current Electricity

Solution:

Let the current in the circuit be $I = \frac{V}{R}$
Across the cell, $V=E-$ Ir
$r=\frac{E-V}{I} \Rightarrow \frac{E-V}{V / R}$
$r=\left(\frac{E-V}{V}\right) R$