Thousand cells of same emf $E$ and same internal resistance $r$ are connected in series in the same order without an external resistance. The potential drop across $399$ cells is found to be ......... $E$.

In the given diagram,an ideal voltmeter $V$ reads $1.45\,V$. Then the relation between $r_1$ and $r_2$ is:

$12$ cells,each having the same emf $E$ and internal resistance $r$,are connected in series and kept in a closed box. Some of the cells are wrongly connected. This battery is connected in series with an ammeter and two cells,each identical to the previous cells. The current is $3 \, A$ when the external cells aid the battery and $2 \, A$ when the external cells oppose the battery. How many cells in the battery are wrongly connected?

$A$ cell $E_{1}$ of $emf 6 V$ and internal resistance $2 \Omega$ is connected with another cell $E_{2}$ of $emf 4 V$ and internal resistance $8 \Omega$ as shown in the figure. The potential difference across points $X$ and $Y$ is............ $V$.

Two batteries of different $e.m.f.$ values and internal resistances are connected in series with each other and with an external load resistor. The current is $3.0 \,A$. When the polarity of one battery is reversed,the current becomes $1.0 \,A$. The ratio of the $e.m.f.$ values of the two batteries is ............

Two cells of emf $E_{1}$ and $E_{2}$ are joined in opposition (such that $E_{1} > E_{2}$). If $r_{1}$ and $r_{2}$ are the internal resistances and $R$ is the external resistance,then the terminal potential difference across the external resistance $R$ is: