The combination of two identical cells,whether connected in series or parallel combination,provides the same current through an external resistance of $2 \,\Omega$. The value of internal resistance of each cell is ............ $\Omega$.

$n$ identical cells,each of $e.m.f.$ $E$ and internal resistance $r$,are connected in series. This combination is connected in series with an external resistor $R$. What is the current flowing through $R$?

The potential difference in open circuit for a cell is $2.2\, V$. When a $4\, \Omega$ resistor is connected between its two electrodes,the potential difference becomes $2\, V$. The internal resistance of the cell will be .............. $\Omega$.

An electric eel generates an electric current using biological cells called electroplaques. The eel has a total of $5000$ electroplaques arranged in $100$ rows. This arrangement is shown in the figure. Each electroplaque has an $emf$ of $0.15\ V$ and an internal resistance of $0.25\ \Omega$. The water surrounding the eel completes the circuit between its head and tail. If the resistance of the water is $500\ \Omega$,the electric current generated by the eel in the water is .......... $A$.

If there are $3$ parallelly connected cells of emf $\varepsilon_1 = 1.2 \ V, \varepsilon_2 = 1.4 \ V$ and $\varepsilon_3 = 1.5 \ V$ and of internal resistances $r_1 = 0.1 \ \Omega, r_2 = 0.2 \ \Omega$ and $r_3 = 0.3 \ \Omega$,then find $\frac{\varepsilon_{eq}}{r_{eq}} = $ . . . . . . $V \Omega^{-1}$.

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: