Electrical Engineering ⇒ Topic : Maintaining Potential Difference

Maintaining Potential Difference

A device that maintains potential difference between two points is said to develop electromotive force (e.m.f.). A simple example is that of a cell. Fig.(a) shows the familiar voltaic cell. It consists of a copper plate (called anode) and a zinc rod (called cathode) immersed in dilute H₂SO₄

                                                                     _{figure (a)}

The chemical action taking place in the cell removes electrons from copper plate and transfers them to the zinc rod. This transference of electrons takes place throuah the agencv of dil. H₂SO₄ (called electrolyte). Consequently, the copper plate attains a positive charge of +Q coulombs and zinc rod a charge of Q coulombs. The chemical action of the cell has done a certain amount of work (say W joules) to do so. Clearly, the potential difference between the two plates will be W/Q volts. If the two plates are joined through a wire, some electrons from zinc rod will be attracted through the wire to copper plate.The chemical action of the cell now transfers an equal amount of electrons from copper plate to zinc rod internally through the cell to maintain original potential difference (i. e. WIQ). This process continues so long as the circuit is complete or so long as there is chemical energy. The flow of electrons through the external wire from zinc rod to copper plate is the electric current.

Thus potential difference causes cun-ent to flow while an e.m.f. maintains the potential difference.Although both e.m.f and p. d. are measured in volts, they do not mean exactly the same thing