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

$n$ 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 $R$?

$(a)$ Six lead-acid type of secondary cells each of $emf$ $2.0 \;V$ and internal resistance $0.015 \;\Omega$ are joined in series to provide a supply to a resistance of $8.5\; \Omega$. What are the current drawn from the supply and its terminal voltage?
$(b)$ $A$ secondary cell after long use has an $emf$ of $1.9 \;V$ and a large internal resistance of $380\; \Omega$. What maximum current can be drawn from the cell? Could the cell drive the starting motor of a car?

The $e.m.f.$ of a primary cell is $2 \ V$. When it is short-circuited,it gives a current of $4 \ A$. The internal resistance of the primary cell is ............. $\Omega$.

A circuit consists of three batteries of emf $E_{1} = 1 \text{ V}$, $E_{2} = 2 \text{ V}$, and $E_{3} = 3 \text{ V}$ and internal resistances $1 \Omega$, $2 \Omega$, and $1 \Omega$ respectively, which are connected in parallel as shown in the figure. The potential difference between points $P$ and $Q$ is: (in $\text{ V}$)

Two identical cells give the same current through an external resistance of $2 \Omega$ regardless of whether the cells are connected in series or in parallel. The internal resistance of each cell is: (in $\Omega$)