Q.State the difference between the EMF and the terminal potential difference of a cell. Under what condition are the two equal in magnitude?

The EMF, $\varepsilon$, of a cell is defined as the potential difference between its terminals when it is delivering NO current -- i.e. in an open circuit. It represents the full driving influence of the cell's internal energy source.

The terminal potential difference, $V$, is what is actually measurable across the cell's terminals once it IS delivering current $I$ through some external circuit. Because part of the EMF is used up driving current through the cell's own internal resistance $r$, the terminal PD is reduced below the EMF:

$V = \varepsilon - Ir$ …

Recall the definitions of EMF (zero-current condition) and terminal PD (any-current condition), and identify the two sp …

• Treating EMF and terminal PD as always interchangeable, ignoring the internal-resistance drop whenever current is actually flowing. …