The $e.m.f.$ of a cell is $6 \ V$. When a current of $2 \ A$ flows through it,the potential difference across its terminals is $3 \ V$. The internal resistance of the cell is ............. $\Omega$.

$n$ identical cells are joined in series,with two cells $A$ and $B$ in the loop having reversed polarities. The $EMF$ of each cell is $E$ and the internal resistance is $r$. The potential difference across cell $A$ or $B$ is (where $n > 4$):

Two sources of equal $emf$ are connected to an external resistance $R$. The internal resistances of the two sources are $R_1$ and $R_2$ $(R_2 > R_1)$. If the potential difference across the source having internal resistance $R_2$ is zero,then:

Two cells,having the same $e.m.f.$ $E$,are connected in series through an external resistance $R$. The cells have internal resistances $r_1$ and $r_2$ $(r_1 > r_2)$ respectively. When the circuit is closed,the potential difference across the first cell is zero. The value of $R$ is

Two batteries of different $e.m.f.$ values and internal resistances are connected in series with each other and with an external load resistor. The current is $3.0 \,A$. When the polarity of one battery is reversed,the current becomes $1.0 \,A$. The ratio of the $e.m.f.$ values of the two batteries is ............

$n$ identical cells,each of emf $E$ and internal resistance $r$,are joined in series to form a row. $m$ such rows are joined in parallel across a load resistance $R$. The current in each cell is: