HOW ELECTRICITY FLOWS

Wehave already seen how electric charges produced by friction can be made to flow from one place to another along a wire.

Electrons, as one knows, are minute negative charges of electгicity. As these minutecharges move along a wire, that wire is said to carry an electric current. In other words, an electric current is the flow of electrons through a metal conductor, these electrons flowing along a wire much thesameas water runs through a pipe.

When connecting the two ends of a conductor two points at different potentials, for example, such as the terminals of a battery, they say that there is an electric cur­rent in the conductor. What actually happens?

The battery causes a potential difference bet­ween the ends of the wire and, thus,

provides forces

that make the electrons move in this wire towards the point of higher potential. It is this electron stream to­wards the positive electrode that represents the electric current.

However, the positive and negative charges had been named by Benjamin Franklin long before the electron was discovered. As nobody knew then whether the positive charges or the negative ones could freely move in a metal conductor, it was assumed that the current consists of moving positive charges. Since that time, it has become the usual thing to speak of the current as flowing from positive to negative. This direction is generally meant when the term “current” is used. As shown in fig. 3-9, the conventional direction of current differs greatly from the actual direction of the electron stream.

In other conductors: non-metals, electrolytes, gases the current may be transmitted by negative charges as well as by positive ones. As for metals, they differ greatly in the ease with which electrons can be made to move from atom to atom.

The magnitude of the current as well as the voltage and resistance may vary from a minute amount to a very large quantity. The electric current passing through a conductor is the amount of electricity (that is, the sum of electron charges) passing through the cross-section of this conduc­tor in a unit of time.

In order to have a steady current, we must have a completed circuit that is also called "closed circuit" and a continuous supply of electric charges. (The terms “clo­sed circuit” and "completed circuit" wing synonyms, we shall meet the one as well as the other while reading tech­nical literature). Another important factor to be taken in­to consideration is that current strength is equal at all points of a series circuit.

Although mentioned above, we should like to stress once more that the electric current flows only in complet­ed circuits. Thus, the current starts flowing at the very moment when we close the circuit, for instance, as soon as we turn the light on in our room.

What do you suppose would happen if one broke one of the wires of the electric circuit? Would that break pro­duce any change? Of course, it would. If broken anywhere, the electric circuit will immediately slop carrying a cur­rent. We can easily observe it many times a day, say, whi­le turning off the light on leaving our room. There is no flow of electrons along a conductor and. therefore, no light as soon as we switch off, that is to say, break the electric circuit.

Date: 2015-02-28; view: 1671