The Idea of Electric Potential and Resistance

The Idea of Electric Potential

In Fig. 1.1, a simple voltaic cell is shown. It consists of a copper plate (known as anode) and a zinc rod (i.e. cathode) immersed in dilute sulphuric acid (H2SO4) contained in a suitable vessel. The chemical action taking place within the cell causes the electrons to be removed from the copper plate and to be deposited on the zinc rod at the same time. This transfer of electrons is accomplished through the agency of the diluted H2SO4 which is known as the electrolyte. The result is that the zinc rod becomes negative due to the deposition of electrons on it and the copper plate becomes positive due to the removal of electrons from it. The large number of electrons collected on the zinc rod is being attracted by anode but is prevented from returning to it by the force set up by the chemical action within the cell.

a simple voltaic cell

But if the two electrodes are joined by a wire externally, then electrons rush to the anode thereby equalizing the charges of the two electrodes. However, due to the continuity of chemical action, a continuous difference in the number of electrons on the two electrodes is maintained which keeps up a continuous flow of current through the external circuit. The action of an electric cell is similar to that of a water pump which, while working, maintains a continuous flow of water i.e., water current through the pipe (Fig. 1.2).
It should be particularly noted that the direction of electronic current is from zinc to copper in the external circuit. However, the direction of conventional current (which is given by the direction of flow of positive charge) is from copper to zinc. In the present case, there is no flow of positive charge as such from one electrode to another. But we can look upon the arrival of electrons on the copper plate (with a subsequent decrease in its positive charge) as equivalent to an actual departure of positive charge from it.
When zinc is negatively charged, it is said to be at a negative potential concerning the electrolyte, whereas anode is said to be at positive potential relative to the electrolyte. Between themselves, the copper plate is assumed to be at a higher potential than the zinc rod. The difference in potential is continuously maintained by the chemical action going on in the cell which supplies energy to establish this potential difference.

Resistance

It may be defined as the property of a substance due to which it opposes (or restricts) the flow of electricity (i.e., electrons) through it.
Metals (as a class), acids and salts solutions are good conductors of electricity. Amongst pure metals, silver, copper, and aluminum are very good conductors in the given order.* This, as discussed earlier, is due to the presence of a large number of free or loosely-attached electrons in their atoms. These vagrant electrons assume a directed motion on the application of an electric potential difference. These electrons while flowing pass through the molecules or the atoms of the conductor, collide and other atoms and electrons, thereby producing heat.


Those substances which offer relatively greater difficulty or hindrance to the passage of these electrons
are said to be relatively poor conductors of electricity like bakelite, mica, glass, rubber, p.v.c. (polyvinyl chloride) and dry wood etc. Amongst good insulators can be included fibrous substances such as paper and cotton when dry, mineral oils free from acids and water, ceramics like hard porcelain and asbestos and many other plastics besides p.v.c. It is helpful to remember that electric friction is similar to friction in Mechanics.

The Unit of Resistance

The practical unit of resistance is the ohm.** A conductor is said to have a resistance of one ohm if it permits one ampere current to flow through it when one volt is impressed across its terminals.
For insulators whose resistances are very high, a much bigger unit is used i.e., mega-ohm = 106 ohm (the prefix ‘mega’ or mego meaning a million) or kilo-ohm = 103 ohm (kilo means thousand). In the case of very small resistances, smaller units like milli-ohm = 10−3 ohm or micro- ohm = 10−6 ohm are used. The symbol for ohm is Ω.

** However, for the same resistance per unit length, the cross-sectional area of aluminum conductor has to be 1.6 times that of the copper conductor but it weighs only half as much. Hence, it is used where the economy of weight is more important than the economy of space.
** After George Simon Ohm (1787-1854), a German mathematician who is about 1827 formulated the law known after his name as Ohm’s Law.

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