The $e.m.f.$ of a cell is generally........

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

$10$ identical cells,each of potential $E$ and internal resistance $r$,are connected in series to form a closed circuit. What is the potential difference across any $3$ cells (in $E$)?

The variation of terminal potential difference $(V)$ with current $(I)$ flowing through a cell is as shown in the graph. The $EMF$ $(E)$ and internal resistance $(r)$ of the cell are:

Ten identical cells each emf $2 \, V$ and internal resistance $1 \, \Omega$ are connected in series with two cells wrongly connected. $A$ resistor of $10 \, \Omega$ is connected to the combination. What is the current through the resistor (in $ \, A$)?

Eels are able to generate current with biological cells called electroplaques. The electroplaques in an eel are arranged in $100$ rows,each row stretching horizontally along the body of the fish containing $5000$ electroplaques. The arrangement is shown below. Each electroplaque has an emf of $0.15\, V$ and internal resistance of $0.25\,\Omega$. The water surrounding the eel completes a circuit between the head and its tail. If the water surrounding it has a resistance of $500\,\Omega$,the current an eel can produce in water is about .............. $A$.