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 ............. $\Omega$.

Four identical cells of $EMF$ $E$ and internal resistance $r$ are connected as shown in the figure. Find the terminal voltage across any one cell.

Two sources of equal emfs $E$ are connected in series. This combination is connected to an external resistance $R$. The internal resistances of the two sources are $r_{1}$ and $r_{2}$ $(r_{1} > r_{2})$. If the potential difference across the source of internal resistance $r_{1}$ is zero,then the value of $R$ will be:

Given below are two statements $:$
Statement-$I$ : The equivalent emf of two non-ideal batteries connected in parallel is smaller than either of the two emfs.
Statement-$II$ : The equivalent internal resistance of two non-ideal batteries connected in parallel is smaller than the internal resistance of either of the two batteries.
In the light of the above statements,choose the correct answer from the options given below.

$A$ battery of $24$ cells,each of emf $1.5\, V$ and internal resistance $2\, \Omega$,is to be connected to send the maximum current through a $12\, \Omega$ resistor. The correct arrangement of cells will be:

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