Two cells,each of $e.m.f. \ E$ and internal resistance $r$,are connected in parallel across an external resistor $R$. If the power delivered to the resistor $R$ is maximum,then:

If six identical cells each having an $e.m.f.$ of $6\,V$ are connected in parallel,the $e.m.f.$ of the combination is ................ $V$.

Two cells with same emf $E$ but different internal resistances,$r_1$ and $r_2$,are connected in series to an external resistance $R$. If the potential difference across the first cell is zero,then the value of $R$ is

Infinite number of cells having $emf$ and internal resistance $(E, r)$,$(\frac{E}{n}, \frac{r}{n})$,$(\frac{E}{n^2}, \frac{r}{n^2})$,$(\frac{E}{n^3}, \frac{r}{n^3})$... are connected in series across an external resistance of $\frac{nr}{n+1}$. The current flowing through the external resistor is:

$A$ cell of internal resistance $r$ is connected to an external resistance $R$. The current will be maximum in $R$,if

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 $V$)