12.How Electric Current Is Conducted

All electric currents consist of charges in motion. However, electric current is conducted differently in solids, gases, and liquids. When an electric current flows in a solid conductor, the flow is in one direction only, because the current is carried entirely by electrons. In liquids and gases, however, a two-directional flow is made possible by the process of ionization (see Electrochemistry).

13.Conduction in Solids

The conduction of electric currents in solid substances is made possible by the presence of free electrons (electrons that are free to move about). Most of the electrons in a bar of copper, for example, are tightly bound to individual copper atoms. However, some are free to move from atom to atom, enabling current to flow.

Ordinarily the motion of the free electrons is random; that is, as many of them are moving in one direction as in another. However, if a voltage is applied to the two ends of a copper bar by means of a battery, the free electrons tend to drift toward one end. This end is said to be at a higher potential and is called the positive end. The other end is said to be at a lower potential and is called the negative end. The function of a battery or other source of electric current is to maintain potential difference. A battery does this by supplying electrons to the negative end of the bar to replace those that drift to the positive end and also by absorbing electrons at the positive end.

Insulators cannot conduct electric currents because all their electrons are tightly bound to their atoms. A perfect insulator would allow no charge to be forced through it, but no such substance is known at room temperature. The best insulators offer high but not infinite resistance at room temperature.

Some substances that ordinarily have no free electrons, such as silicon and germanium, can conduct electric currents when small amounts of certain impurities are added to them. Such substances are called semiconductors. Semiconductors generally have a higher resistance to the flow of current than does a conductor, such as copper, but a lower resistance than an insulator, such as glass.

14.Conduction in Gases

Gases normally contain few free electrons and are generally insulators. When a strong potential difference is applied between two points inside a container filled with a gas, the few free electrons are accelerated by the potential difference and collide with the atoms of the gas, knocking free more electrons. The gas atoms become positively charged ions and the gas is said to be ionized. The electrons move toward the high-potential (more positive) point, while the ions move toward the low-potential (more negative) point. An electric current in a gas is composed of these opposite flows of charges. C.

15.Conduction in Liquid Solutions

Many substances become ionized when they dissolve in water or in some other liquid. An example is ordinary table salt, sodium chloride (NaCl). When sodium chloride dissolves in water, it separates into positive sodium ions, Na+, and negative chlorine ions, Cl-. If two points in the solution are at different potentials, the negative ions drift toward the positive point, while the positive ions drift toward the negative point. As in gases, the electric current is composed of these flows of opposite charges. Thus, while water that is absolutely pure is an insulator, water that contains even a slight impurity of an ionized substance is a conductor.

Since the positive and negative ions of a dissolved substance migrate to different points when an electric current flows, the substance is gradually separated into two parts. This separation is called electrolysis.