When can the internal resistance of a cell be neglected? Explain.

(N/A) The internal resistance $r$ of a cell can be neglected when the external resistance $R$ connected in the circuit is very large compared to the internal resistance $(R \gg r)$.
Explanation:
The terminal voltage $V$ of a cell is given by the formula $V = E - Ir$,where $E$ is the electromotive force $(EMF)$,$I$ is the current,and $r$ is the internal resistance.
Since $I = E / (R + r)$,we can write $V = E - [E / (R + r)] \cdot r = E \cdot [R / (R + r)]$.
If $R \gg r$,then $(R + r) \approx R$.
Substituting this into the equation,we get $V \approx E \cdot (R / R) = E$.
In this condition,the potential drop across the internal resistance is negligible,and the terminal voltage is approximately equal to the $EMF$ of the cell.