The Nature of Electricity
Electricity is most easily explained by the electron theory, developed in the early 20th century. The electron theory, in turn, depends on the atomic theory of matter.
The center, or nucleus, of an atom contains one or more particles called protons. A proton has a type of electric charge that is said to be positive. Circling the nucleus are one or more electrons, which are much smaller than the proton. An electron has a type of charge that is said to be negative.
An electrically neutral atom has one electron for each proton. In such an atom, the positive and negative charges exactly balance. An atom may lose or gain one or more electrons, leaving it with a net positive or negative charge. A charged atom is called an ion.
An electric charge brought near one or more other electric charges will experience an electrical force. One of the fundamental laws of electricity is that like charges (either positive or negative) repel each other, and unlike charges attract each other. The region in which a charge experiences an electrical force is called an electric field.
Electric fields are commonly pictured as consisting of lines, called lines of force. The lines of force indicate the path that a positive electric charge would follow in the field, and normally radiate from or converge on the charged body. Electrically charged bodies—that is, objects in which there is a net electric charge—exert forces on each other at a distance by means of their electric fields.
When the atoms that make up an object lose or gain electrons, the object acquires a net electric charge. An object with a positive charge tends to attract electrons; one with a negative charge tends to repel them.
Static electricity exists when an object has a net electric charge and there is no movement of electrons into or away from the object. Part or all of a static charge is lost when the charged object touches an uncharged or oppositely charged object.
An electric current exists when there is a net flow of electrically charged particles. Most uses of electricity involve the flow of electrons. Some electric currents, such as those that occur in a battery, involve the flow of positive and negative ions. (By convention, the direction of a current in an electric circuit is considered to be the direction in which positive charge would flow, and is opposite the direction of electron flow.) An electric current has energy that can be converted to heat or light, or—as in an electric motor—used to perform mechanical work.
An electric current in a metal wire consists of the movement of electrons from a negatively charged region to a positively charged one. The currents used in everyday electrical devices involve the movement of very large numbers of electrons. For example, every second that a lightbulb is on, some billion billion electrons enter (and leave) the lightbulb filament. Although the individual electrons forming a current move through a wire slowly (typically less than 5.5 inches per hour [14 cm/h]), the force of repulsion between the electrons travels at nearly the speed of light.
There are two basic types of electric current—direct current (DC) and alternating current (AC). In a direct current, the direction of the flow of electric charge does not change, although the current may increase and decrease. Alternating current, in contrast, regularly reverses direction.
The electric current delivered to the home from an electric power company is alternating current. Its main advantage is that its voltage (electrical pressure) can be easily increased or decreased (by devices called transformers). Another advantage is that AC machinery is generally simpler to design and build than DC machinery. Direct current, however, is needed by electronic devices and for such processes as charging storage batteries and electroplating. An advantage of direct current is that it can be readily produced by batteries for use in portable devices.
Electric charges can flow much more easily through some materials than others. A material that has very little resistance to the passage of an electric current is called a conductor. In general, metals are very good conductors, because their atoms contain one or more loosely bound electrons. These electrons are free to move and form part of an electric current. At room temperature, silver offers the least resistance to an electric current followed by copper, gold, and aluminum. A liquid that permits the flow of positive and negative ions is called an electrolyte. An important use of electrolytes is in batteries. Under certain conditions, some materials—called superconductors—have no resistance to an electric current.
Some materials offer a very large resistance to the flow of electric charges. Such a material is called an insulator, or dielectric. Some common insulators include glass, rubber, porcelain, paraffin, mica, and dry air. Insulators are important in the use of electricity because they will confine an electric current to the conductor intended to carry it. For example, wires are usually covered with insulation to help prevent electric charges flowing in the wire from escaping to surrounding materials. Insulators are also important in a type of electrical device called a capacitor.
A semiconductor is a material whose ability to carry an electric current is between that of conductors and that of insulators. Semiconductors such as silicon are essential in many kinds of electronic devices.