Write the equation for the equivalent electromotive force (emf) of a series connection of cells.

$A$ car battery of $e.m.f.$ $12\,V$ and internal resistance $5 \times 10^{-2}\,\Omega$,receives a current of $60\,A$ from an external source. What is the terminal voltage of the battery (in $,V$)?

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 ........ $\Omega$.

$A$ resistor of resistance $R$ is connected between the terminals of a cell of $\operatorname{emf} E$ and internal resistance $r$. If $I$ is the current through the circuit,the terminal potential difference of the cell is given by:

$A$ galvanometer together with an unknown resistance in series is connected to two identical batteries each of $1.5\, V$. When the batteries are connected in series,the galvanometer records a current of $1\, A$,and when the batteries are in parallel,the current is $0.6\, A$. What is the internal resistance of the battery?

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 .............. $A$.