Electromotive force is the force,which is able to maintain a constant

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 .............. $V$.

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.

$12$ cells,each having the same $emf$ $E$ and internal resistance $r$,are connected in series,but some cells are wrongly connected. This arrangement is connected in series with an ammeter and two additional cells (each of $emf$ $E$ and internal resistance $r$). The current is $3 \, A$ when the cells and the battery aid each other,and it is $2 \, A$ when they oppose each other. The number of cells wrongly connected is:

$A$ galvanometer is connected in series with an unknown resistor $R$ and two identical batteries of $1.5\, V$ each. When the batteries are connected in series,the galvanometer shows a current of $1\, A$. When the batteries are connected in parallel,it shows a current of $0.6\, A$. The internal resistance of each battery is:

When a current of $2 \ A$ flows through a resistor of $2 \ \Omega$,the terminal voltage across the cell is $E/2$. The internal resistance of the cell is ........ $\Omega$.