CELL AND BATTERY

                                    CELL AND BATTERY

The word cell means one unit or a combination of materials for converting chemical energy into electrical energy. A battery means a combination of these units or cells.

                          E.M.F AND TERMINAL POTENTIAL DIFFERENCE

The e.m.f. of the cell, as said earlier, is the total potential difference established within the cell between the two electrodes when the cell is not supplying any current. The e.m.f. can be measured by connecting a suitable voltmeter across the electrodes. But the terminal potential difference is equal to the e.m.f minus the internal voltage drop. In other words,  the T.P.D is the potential difference available for external circuit. If 'i' is the current applied by the cell and 'r' its internal resistance, then Terminal potential difference V= E.M.F.- ir or  V= E-ir where V= terminal potential difference E=E.M.F. generated within the cell.

It should be noted that whereas e.m.f. E is constant, the terminal potential difference V is not, as it depends on load current supplied by the cell.

Also the term e.m.f. has local significance i.e. it is spoken of with reference to the generator itself, but potential difference is distributive. For example, we cannot say that an e.m.f. of 220 volts exists across an electric bulb. It is the potential difference of 220 V. In other words, p.d. can be distributed away from its secure of generation.