Understanding Electricity: Properties, Uses, and Energy

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Introduction to Electricity

Electricity, the phenomena caused by a fundamental property of matter called electric charge. The term is commonly used to refer to electric charge itself, to electric energy, and to electric power. Electric energy, the most versatile form of energy available, is used for lighting, heating, and cooling. It is also used for communications, for running motors, in various kinds of industrial processes, and for many other purposes.

Terms used in electricityAmpere is the unit used to measure the rate of flow of an electric current.Conductor is a material through which electric current flows easily.Electric charge is a basic feature of certain particles of matter that causes them to attract or repel other charged particles.Electric circuit is the path that an electric current follows.Electric current is the flow of electric charges.Electric field is the influence a charged body has on the space around it that causes other charged bodies in that space to experience electric forces.Electrode is a piece of metal or other conductor through which current enters or leaves an electric device.Electromagnetism is a basic force in the universe that involves both electricity and magnetism.Electron is a subatomic particle with a negative electric charge.Insulator is a material that opposes the flow of electric current.Ion is an atom or group of atoms that has either gained or lost electrons, and so has an electric charge.Kilowatt-hour is the amount of electric energy a 1,000-watt device uses in one hour.Neutron is a subatomic particle that has no electric charge.Ohm is the unit used to measure a material's resistance to the flow of electric current.Proton is a subatomic particle with a positive electric charge.Resistance is a material's opposition to the flow of electric current.Static electricity is electric charge that is not moving.Voltage is a type of "pressure" that drives electric charges through a circuit.Watt is the unit used to measure the rate of energy consumption, including electric energy.

The Nature of Electricity

The Electron Theory

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.

Electric Field

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.

Static and Current Electricity

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.

Conductors and Insulators

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.

How Electricity Is Produced

Static Electricity

The most familiar way of producing static electricity is by rubbing, or friction. Rubbing together two different kinds materials that are insulators can transfer electrons from one substance to another. The substance that gains electrons acquires a negative charge, and the one that loses them acquires a positive charge. For example, rubbing a balloon against dry hair produces an opposite electric charge in the balloon and the hair (which will be drawn to the balloon). Similarly, shuffling over a carpet in dry weather will produce a sufficient electrostatic charge on a person's body to give a slight shock when the person touches a conductor.

Objects can also acquire an electric charge through a process called electrostatic induction. In the illustration Electrostatic Induction, a charged object (the negatively charged rod) is brought near an electrically insulated metal sphere, but not into contact with it. The excess electrons in the rod will repel the electrons from the part of the sphere nearest the rod to the part farthest from the rod. If electrons are allowed to escape from the sphere through an electrical connection to the ground, the sphere will be left with a net positive charge.

Current Electricity

is produced by creating a difference in electric potential between two points connected by a conductor. A potential difference exists between two points when one has more electrons than the other. The point with excess electrons is called the negative terminal; the other, the positive terminal. The potential difference between the two terminals creates an electrical pressure called electromotive force (emf), or voltage.

The two most common ways of creating a voltage to produce current are chemically (using batteries) and by electromagnetic induction (using generators). A voltage can also be created by heat, light, or mechanical pressure.

In the chemical method, explained in the article Battery, Electric, complementary chemical reactions cause one terminal to gain electrons, making it negative, and cause the other to lose electrons, making it positive.

The electromagnetic induction method for producing an electric current involves the use of a permanent magnet or an electromagnet. When a wire is moved through a magnetic field, the electrons in the wire are displaced and move toward one end of the wire. This action makes one end negative, the other positive.

Under certain conditions, heat will cause electrons to flow between two different materials. One device for producing this effect is the thermocouple, which is used as a measuring instrument and as a control device.

Light falling on certain metals and semiconductors will release electrons to produce an electric current. This effect is used in certain kinds of batteries.

Some crystals, including quartz crystals cut into certain geometric shapes, will produce a voltage when squeezed and will vibrate when subjected to a voltage. This phenomenon, called the piezoelectric effect is used in a variety of electrical devices, including microphones, radio transmitters, and electronic watches.

How Electricity Works For Us

One of the most useful properties of electricity is its ability to produce heat. Electricity produces heat in a conductor as it overcomes the conductor's resistance to the flow of electrons through it, just as mechanical energy produces heat in overcoming friction. The heat-producing effect of electricity is used in electric ranges, toasters, soldering irons, and many other devices. In incandescent lightbulbs, the effect is used to make a filament glow brightly.

Another very useful property of electricity is that it can be a source of magnetism; electrons flowing through a wire create a magnetic field around the wire. This effect is the basis for the operation of electromagnets, which make possible electric motors, telephones, loudspeakers, and many other devices.

Electricity has several other useful properties. It can be made to jump a narrow gap separating two conductors, creating a spark. An important application of such sparks is in igniting the fuel in the cylinders of gasoline engines. Electricity can also produce chemical changes.

In a gas or vacuum, electrons can be accelerated to high speed and directed in specific directions. Electrons used in this way make possible many kinds of devices, including television picture tubes, fluorescent lights, and X-ray tubes. In certain semiconductor devices, such as transistors, the movement of electrons can be readily controlled by electrical means. Such devices are used in computers, radios, tape recorders, and many other products.

Units of Electricity

Five common units used in working with electricity and electric circuits are the volt, ampere, watt, ohm, and hertz.

The Volt (V)

is a unit for measuring both electric potential difference and electromotive force. The voltage supplied by most automobile storage batteries is 12 volts. In many countries, including most of those of Europe, electricity is supplied to homes at 220 or 240 volts. In the United States and Canada, homes are typically supplied with electricity at around 120 volts for ordinary use and 240 volts for such appliances as electric ranges and electric water heaters. High-tension power lines have voltages of hundreds of thousands of volts. These lines are used for the transmission of electric energy over long distances.

The Ampere (A)

is a unit for measuring electric current—the flow of electric charges. The ampere (or amp) is a base unit of the SI (metric system) and is defined in terms of the magnetic force produced between two parallel wires carrying an electric current. Houses are typically wired to provide a total of 60 or more amperes. The amount of electric charge transferred by a current of one ampere in one second is one coulomb.

The Watt (W)

Electric power—that is, the rate at which electric energy is used to perform work—is measured in watts. Lightbulbs and appliances are usually marked with their wattage, indicating the rate at which they consume energy. The normal 120-volt, 15-ampere household circuit can safely handle electrical devices drawing a total of 1,800 watts (1.8 kilowatts). The mechanical power an electric motor can provide is usually given in watts or horsepower. (One horsepower equals 746 watts.) Consumption of electricity is measured in terms of the kilowatt-hour—the work done by 1,000 watts in one hour.

The relationship of voltage (in volts) to current (in amperes) and resistance (in ohms) is expressed by Ohm's Law:

Voltage = current X resistance

The Ohm (W)

is a unit for measuring the resistance a material has to the flow of an electric current.

The relationship between power (in watts), voltage (in volts), and current (in amperes) is:

Power = voltage X current

The Hertz (Hz)

is a measure of the rate at which an alternating current reverses direction. Each two consecutive reversals in an alternating current are called a cycle. Commercially generated alternating current in the United States has a frequency of 60 hertz (cycles per second). In many countries, including European countries, alternating current has a frequency of 50 hertz.

Electric Circuits

An electric circuit consists of the various conductors that lead from the negative to the positive terminal of a source of electricity. The various parts of a typical house circuit include a fuse or circuit breaker, wires, switches, wall outlets, and light sockets and bulbs.

A circuit through which electricity is flowing is said to be closed. The circuit can be opened, or broken, by turning off a switch or by removing a fuse, pulling out a plug, or disconnecting the wires. A circuit generally contains a load, a device such as a lightbulb or appliance that provides resistance in the circuit. If a current is allowed to flow from one terminal to another with very little resistance, a short circuit exists. Unless such a current is quickly stopped by a fuse or circuit breaker, the wires may heat up enough to start a fire.

There are two basic methods of wiring a circuit—in series and in parallel. In the series circuit the current flows through one device (such as a lightbulb) to reach the next. In the parallel circuit the current enters and leaves each device separately. Devices connected in series each carry the same amount of current; devices connected in parallel are each subjected to the same voltage. Many electrical applications use a combination of these two types of circuits.

History

Static electricity was observed by Thales, a Greek philosopher who lived in the sixth century B. C. He thought static electricity was a form of magnetism, and it was not until 1600 that electricity was recognized as something different from magnetism. This discovery was made by William Gilbert, an English physician. He called the force involved “electric,”; taken from elektron, the Greek word for amber. (Static electricity was first observed in amber.)

In the mid-1700's, E. Georg von Kleist of Prussia and Pieter van Musschenbroek of Holland, working independently, invented a device for storing electricity. In 1780 Luigi Galvani, an Italian physiologist, found that a severed frog's leg hung by a copper hook would twitch when touched with iron. Galvani had discovered current electricity (which for many years was called galvanism). Another Italian, Alessandro Volta, used Galvani's discovery when he invented the first electric battery in 1800. His studies led to the discovery of electrolysis, the decomposition of a substance with an electric current.

In his kite experiment of 1752 Benjamin Franklin proved that lightning is an electrical discharge. He suggested, incorrectly, that electricity is a kind of fluid and that it flows from a point where there is a surplus, or positive amount, to a point where there is a deficiency, or negative amount. This theory, called the conventional (or fluid) theory of current, persisted even after the development of the electron theory early in the 20th century. Today, the direction of a current is still considered to be from the positive terminal to the negative, even though electrons flow in the opposite direction.

In 1820, Hans Christian Oersted, a Danish physicist, discovered that an electric current can produce magnetic effects. In 1831 Michael Faraday, an English physicist and chemist, discovered the complementary phenomenon: that a magnet can produce electrical effects. (Joseph Henry, a United States physicist, observed the phenomenon in 1830 but did not publish his findings until after Faraday.) Faraday used this discovery—electromagnetic induction—to invent an experimental electric generator. He also formulated the laws of electrolysis and developed the first transformer.

Between 1870 and 1880, Sir William Crookes, an English scientist, experimenting with vacuum tubes, concluded that the rays produced in these tubes were composed of particles smaller than atoms. J. J. Thomson, an English physicist, demonstrated the existence of these particles (later called electrons) in 1897. Thomson's work led to the theory of electric current being the flow of electrons.

As the basic principles of electricity were discovered and electrical theories were developed, inventors began putting them to use. Among the electrical inventions of the late 19th century are Samuel F. B. Morse's telegraph; Alexander Graham Bell's telephone; Thomas A. Edison's electric light; Guglielmo Marconi's wireless telegraph; and electric generators and motors.

By the 1920's, scientists studying the electrical structure of the atom had developed a complex mathematical description of its nature. On a more practical level, the development during the early 20th century of such electrical appliances as electric refrigerators and washing machines helped raise the standard of living in many countries.

The understanding of the behavior of electrons in a vacuum permitted the development of electronics in the first half of the 20th century. Following the invention of the transistor in 1948, the use of electronic devices based on semiconductors came to dominate the field of electronics. The miniaturization of electronic devices led to the development of a large number of new electrical devices, such as the personal computer, and to the widespread use of electronic controls.