. C ELLS , EMF , I NTERNAL R ESISTANCE We have already mentioned that a simple device to maintain a steady current in an electric circuit is the electrolytic cell. Basically a cell has two electrodes, called the positive (P) and the negative (N), as shown in Fig. .
. They are immersed in an electrolytic solution. Dipped in the solution, the electrodes exchange charges with the electrolyte. The positive electrode has a potential difference V + (V + > ) between itself and the electrolyte solution immediately adjacent to it marked A in the figure.
Similarly, the negative electrode develops a negative potential – ( V – ) ( V – ≥ ) relative to the electrolyte adjacent to it, marked as B in the figure. When there is no current, the electrolyte has the same potential throughout, so that the potential difference between P and N is V + – (– V – ) = V + + V – . This difference is called the electromotive force (emf) of the cell and is denoted by ε . Thus ε = V + + V – > ( .
) Note that ε is, actually, a potential difference and not a force . The name emf, however, is used because of historical reasons, and was given at a time when the phenomenon was not understood properly. To understand the significance of ε , consider a resistor R connected across the cell (Fig. .
). A current I flows across R from C to D. As explained before, a steady current is maintained because current flows from N to P through the electrolyte. Clearly, across the electrolyte the same current flows through the electrolyte but from N to P, whereas through R , it flows from P to N.
The electrolyte through which a current flows has a finite resistance r , called the internal resistance . Consider first the situation when R is infinite so that I = V / R = , where V is the potential difference between P and N. Now, V = Potential difference between P