Electromotive force or emf is another term for voltage or potential difference but refers to a power source. Chemical sources of emf make use of the fact that in many chemical reactions, electrons are liberated at one place and absorbed at another. The cycle is frequently closed in a small region, producing no external electric effects. Luigi Galvani and Alessandro Volta discovered that a cell could be a constructed in which electrons were liberated from a solution onto a zinc plate and absorbed into the solution from a copper one causing potential difference to exist between the plates. These galvanic cells were the first continuous sources of current.
The most commonly used galvanic cell is the dry cell. The positive electrode of this cell is a carbon rod and the negative terminal is the zinc container for the cell. A layer of paper moistened with ammonium chloride is placed in contact with zinc, while the space between this and the central carbon rod is filled with manganese dioxide and granulated carbon moistened with ammonium chloride. The ammonium chloride is the electrolyte, and in the chemical reaction, hydrogen is liberated at the carbon electrode. The hydrogen reduces the effectiveness of the cell for two reasons (1) the gas increases the internal resistance of the cell and (2) the hydrogen produces a reverse emf at the carbon electrode. This is countered by the manganese dioxide. With continuous use, dry cells get exhausted. The storage cells differ from the primary cell in that it can be recharged by the use of reverse current from an outside source. This cell transforms electric energy into chemical energy during the charging process. During the discharge, chemical energy is transformed into electrical energy, as in the case of a dry cell. The amount of energy that can be stored depends upon the size of the plates. A large cell can have the same voltage as a small cell but the energy available in it when fully – charged is much greater than that of a small cell.
Posted 2011-01-29 and updated on Jun 07, 2011 11:49pm by crisd