When a current of $2\, A$ flows in a battery from negative to positive terminal, the potential difference across it is $12\, V$. If a current of $3\, A$ flowing in the opposite direction produces a potential difference of $15\, V$, the $emf$ of the battery is .............. $V$.

The potential difference between the terminals of a cell is $20 \ V$ when a current of $2 \ A$ flows through the circuit. When the direction of current in the circuit is reversed,the potential difference between the terminals of the cell is $30 \ V$. The internal resistance of the cell is (in $\Omega$)

$A$ circuit of resistance $R$ is connected to $n$ similar cells. If the current in the circuit is the same when the cells are connected in series or in parallel,and the internal resistance of each cell is $r$,then:

The internal resistance of a cell is the resistance of

Consider a parallel combination of the cells shown in the figure. The potential difference between $B_1$ and $B_2$ is

Two cells $E_1$ and $E_2$ having equal $EMF$ $E$ and internal resistances $r_1$ and $r_2$ $(r_1 > r_2)$ respectively are connected in series. This combination is connected to an external resistance $R$. It is observed that the potential difference across the cell $E_1$ becomes zero. The value of $R$ will be