Two cells with same emf $E$ but different internal resistances,$r_1$ and $r_2$,are connected in series to an external resistance $R$. If the potential difference across the first cell is zero,then the value of $R$ is

Four identical cells of emf $E$ and internal resistance $r$ are to be connected in series. If one of the cells is connected wrongly,the equivalent emf and effective internal resistance of the combination are

To draw maximum current from a combination of cells,how should the cells be grouped?

In the network shown,the potential difference between $A$ and $B$ is ................. $V$ $(R = r_1 = r_2 = r_3 = 1 \Omega, E_1 = 3 \, V, E_2 = 2 \, V, E_3 = 1 \, V)$.

The potential difference in open circuit for a cell is $2.2\, V$. When a $4\, \Omega$ resistor is connected between its two electrodes,the potential difference becomes $2\, V$. The internal resistance of the cell will be .............. $\Omega$.

When an external resistance of $5\text{ }\Omega$ is connected across the terminals of a cell, a current of $0.25\text{ A}$ flows through it. When the $5\text{ }\Omega$ resistor is replaced by a $2\text{ }\Omega$ resistor, a current of $0.5\text{ A}$ flows through it. The internal resistance of the cell is . . . . . . $\Omega$.