POTENTIAL DIFFERENCE, ELECTROMOTIVE FORCE AND RESISTANCE

The amount of electrons at a point is known as the poten­tial at that point. As was previously stated, there is always a disorderly movement of free electrons within all substances, especially metals. Thus, if at one end of a conductor there is an excess of electrons and at the other end—a deficiency, the excess of electrons will flow in a disorderly manner towards the point of deficiency. It is the difference of potential that caus­es the free electrons to flow from one point of the conductor to another. Of course, the greater the difference, the greater is the flow.

Let us suppose that there is a movement of electrons through the wire, say, from point A to point B! What does it mean? It means that there is an excess of electrons at point A. Unless there were a flow of electric current between A and В in any direction, it would mean that both the former and the latter were at the same potential.

The electromotive force is the very force that makes the electrons move along a conductor, the letters emf standing for the above term. In effect, it is the emf that moves electri­cal charges from one point in an electric circuit towards anoth­er. In case this emf is direct, the current, too, is direct. On the other hand, were the electromotive force alternating, the current would be alternating, too.

One need not explain to the reader that a current is unable flow in a circuit consisting of metallic wires without a source of emf. The source under consideration may be a cell, a battery, a generator, a thermocouple or a photo cell, с Needless to say, the emf is measurable and it is the volt that is the unit to be used for measuring it.

In addition to electromotive force and potential difference Terence should be made here to another important factor lat greatly influences electrical flow, namely, resistance.

So, to resistance shall we turn our attention now. The reader most probably remembers that all substances offer a certain amount of opposition, that is to say resistance, to the passage of current. This resistance may be high or low depending on the type of circuit and the material employed. Take glass and rubber as an example! They offer a very high resist­ance and, hence, are considered to be good insulators. Never­theless, one must not .forget, too, that all substances do allow the passage of some cur­rent, provided the potential difference is high enough.

In Fig. 9 two oppositely charged balls are suspended far apart in the air. In spite of our having a difference" of poten­tial, no current flows. How can we explain this strange be­haviour? The simple reason is that the air between the balls offers too great a resistance to current flow. However, the electrons could certainly flow from the negatively charged ball towards the positively charged one, provided we connected them by a metal wire. As a matter of fact, it is not necessary at all to connect both balls in the manner described in order to obtain similar result. All that we have to do, is to increase the charges. The potential difference becoming great enough, the electrons will jump through the air forming an electric spark.

One should mention in this connection that certain factors can greatly influence the resistance of an electric circuit. Among them we find its size, length, type and temperature. In short, the thinner or longer the wire, the greater is the offered resistance. Besides, could we use a silver wire, it would offer less resistance than an iron one. The higher the temperature of a metal, the higher is its resistance.

Exercises

1.Learn the following active words:

Deficiency недостаток, дефицит

Disorderly беспорядочный

electromotive force электродвижущая сила

(e.m.f.) (ЭДС)

excess (n,a) избыток, избыточный

influence (v) влиять

offer (v) оказывать

potential difference разница потенциалов

photocell фотогальванический элемент

resistance сопротивление

same тот же самый, одинаковый

suspend подвешивать

thermocouple термопара