$32$ cells,each of $emf$ $3V$,are connected in series and kept in a box. Externally,the combination shows an $emf$ of $84V$. The number of cells reversed in the combination is

Two cells of equal $e.m.f.$ $E$ and internal resistances $r_1$ and $r_2$ $(r_1 > r_2)$ are connected in series. On connecting this combination to an external resistance $R$,it is observed that the potential difference across the first cell becomes zero. The value of $R$ will be

In the circuit shown,the potential difference between $A$ and $B$ is ............. $V$.

Two batteries of emf $\varepsilon_1$ and $\varepsilon_2$ (where $\varepsilon_2 > \varepsilon_1$) and internal resistances $r_1$ and $r_2$ respectively are connected in parallel as shown.

Four identical cells, each of $2 \, V$ e.m.f., are connected in parallel. They supply current to an external circuit consisting of two $15 \, \Omega$ resistors connected in parallel. The terminal voltage of the equivalent cell, as measured by an ideal voltmeter, is $1.6 \, V$. Calculate the internal resistance of each cell in $\Omega$.

For two cells having the same $EMF$ $E$ and internal resistance $r$,the current passing through an external resistor of $6 \ \Omega$ is the same when both cells are connected either in parallel or in series. The value of internal resistance $r$ is . . . . . . $ \ \Omega$.