If the $EMF$ of each cell is $E = 1.5 \text{ V}$ and internal resistance is $r$, as shown in the figure, what is the current $i$ flowing through the circuit (in $\text{ A}$)?

Twelve cells,each having emf $E$ volts,are connected in series and are kept in a closed box. Some of these cells are wrongly connected with positive and negative terminals reversed. This $12$-cell battery is connected in series with an ammeter,an external resistance $R$ ohms,and a two-cell battery (two cells of the same type used earlier,connected perfectly in series). The current in the circuit when the $12$-cell battery and $2$-cell battery aid each other is $3 \text{ A}$,and it is $2 \text{ A}$ when they oppose each other. Then,the number of cells in the $12$-cell battery that are connected wrongly is:

What is called the internal resistance of a cell?

Two batteries of different emfs and different internal resistances are connected as shown. The voltage across $AB$ in volts is

Write the equation for the equivalent electromotive force (emf) of a series connection of cells.

To get maximum current through a resistance of $2.5\,\Omega$,one can use $m$ rows of cells,each row having $n$ cells. The internal resistance of each cell is $0.5\,\Omega$. What are the values of $n$ and $m$ if the total number of cells is $45$?