$A$ circuit of resistance $R$ is connected to $n$ similar cells. If the current in the circuit is the same when the cells are connected in series or in parallel,and the internal resistance of each cell is $r$,then:

Two identical cells each of emf $1.5\,V$ are connected in series across a $10\,\Omega$ resistance. An ideal voltmeter connected across $10\,\Omega$ resistance reads $1.5\,V$. The internal resistance of each cell is $......\Omega$.

$A$ cell of $emf\;4\,V$ and internal resistance $0.5\,\Omega$ is connected to a $7.5\,\Omega$ external resistance. The terminal potential difference of the cell is $.....\,V$.

$A$ cell can supply currents of $1 \ A$ and $0.5 \ A$ via resistances of $2.5 \ \Omega$ and $10 \ \Omega$ respectively. The internal resistance of the cell is (in $\Omega$)

In how many ways can a combination of cells be done? Explain.

Two cells are connected in opposition as shown. Cell $E_1$ has an electromotive force (emf) of $8 \ V$ and an internal resistance of $2 \ \Omega$; cell $E_2$ has an emf of $2 \ V$ and an internal resistance of $4 \ \Omega$. The terminal potential difference of cell $E_2$ is: (in $V$)