and A + Potential difference between A and B + Potential difference between B and N = ε ( . ) Thus, emf ε is the potential difference between the positive and negative electrodes in an open circuit, i.e., when no current is flowing through the cell. If however R is finite, I is not zero. In that case the potential difference between P and N is V = V + + V – – I r = ε – I r ( .
) Note the negative sign in the expression ( I r ) for the potential difference between A and B. This is because the current I flows from B to A in the electrolyte. In practical calculations, internal resistances of cells in the circuit may be neglected when the current I is such that ε >> I r . The actual values of the internal resistances of cells vary from cell to cell.
The internal resistance of dry cells, however, is much higher than the common electrolytic cells. FIGURE . (a) Sketch of an electrolyte cell with positive terminal P and negative terminal N. The gap between the electrodes is exaggerated for clarity.
A and B are points in the electrolyte typically close to P and N. (b) the symbol for a cell, + referring to P and – referring to the N electrode. Electrical connections to the cell are made at P and N.