When does the current drawn from a cell become maximum?

Two identical cells each of emf $E$ and internal resistance $r$ are connected in parallel with an external resistance $R$. To get maximum power developed across $R$,the value of $R$ is

In an electric circuit,a cell of certain $emf$ provides a potential difference of $1.25\, V$ across a load resistance of $5\, \Omega$. However,it provides a potential difference of $1\, V$ across a load resistance of $2\, \Omega$. The $emf$ of the cell is given by $\frac{x}{10}\, V$. Then the value of $x$ is ..... .

Two cells of internal resistance $r_{1}$ and $r_{2}$ and of the same emf are connected in series across a resistor of resistance $R$. If the terminal potential difference across the cell of internal resistance $r_{1}$ is zero,then the value of $R$ is

When a current of $2 \ A$ flows through a resistor of $2 \ \Omega$,the terminal voltage across the cell is $E/2$. The internal resistance of the cell is ........ $\Omega$.

The circuit in the figure shows two cells connected in opposition to each other. Cell $E_1$ has an $emf$ of $6 \ V$ and an internal resistance of $2 \ \Omega$. Cell $E_2$ has an $emf$ of $4 \ V$ and an internal resistance of $8 \ \Omega$. Find the potential difference between the points $A$ and $B$.