Техническое чтение для энергетиков. Бухарова Г.П. - 52 стр.

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54
ELECTROMOTIVE FORCE AND RESISTANCE
As was previously stated, there is always a disorderly movement of free
electrons within all substances, especially metals.
Let us assume 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.
Of course, the greater the potential difference, the greater is the electron flow.
The electromotive force (e.m.f.) is the very force that moves the electrons from
one point in an electric circuit towards another. In case this e.m.f. is direct, the current
is direct. On the other hand, were the electromotive force alternating, the current
would be alternating, too. The c.in.f. is measurable and it is the volt that is the unit
used for measuring it.
One need not explain to the reader that a current is unable to flow in a circuit
consisting of metallic wires alone. A source of an e.m.f. should be provided as well.
The source under consideration may be a cell or a battery, n generator, a
thermocouple or a photocell, etc.
In addition to the electromotive force and the potential difference reference
should be made here to another important factor that greatly influences electrical
flow, namely, resistance. So, to resistance shall we turn our attention now. The
student 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, they are considered as good
insulators. Nevertheless, one must not forget that all substances do allow the passage
of some current provided the potential difference is high enough.
Imagine two oppositely charged balls 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 a similar result. All that we have to
do is to increase the charges. If the potential difference becomes 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 the size of the wire, its
length, and type. In short, the thinner or longer the wire, the greater is the resistance
offered. Besides, could we use a silver wire, it would offer less resistance than an iron
one.

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