A device which is used as a source of e.m.f. and which woks on the principle of conversion of chemical energy into an electrical energy, is called a cell. But practically the voltage of a single cell is not sufficient to use in any practicale application. Hence various cells are connected in series or parallel to obtain the required voltage level. The combination of various cells, to obtain the desired voltage level is called a battery.
The conductors of electricity can be classified in two categories as,
1. Non electrolytes : Conductors which are not affected by the flow of current through them are nonelectrolytes. The examples are metals, alloys, carbon and some other materials.
2. Electrolytes : Conductors which undergo decomposition due to the flow of current through them are electrolytes. The examples are various acids, bases, salt solutions and molten salts.
In any cell, two different conducting materials are immersed in an electrolyte. The chemical reaction results which separates the charges forming a new solution. The charges get accumulated on the conductors. Such charged conductors are called electrodes. The positively charged conductor is called anode while the negatively charged conductor is called cathode. Thus the charge accumulated on the electrodes creates a potential difference between the two conductors. The conductor ends are brought out as the terminals of the cell, for connecting the cell to an external circuit. The terminals are marked as positive and negative. Thus the chemical energy gets converted to an electrical energy. Hence the cell is an electrochemical device.
Note : the chemical action in the cell continuously separates the charges to maintain the required terminal voltage.