Electricity

Separate-but related articles are Electrical energy, and Electric power

'Electricity' is the term the early scientists used when they were actually referring to 'charge'. Present usage of the term 'electricity' has become confused. In addition to the electric charge definition, the word electricity has several popular definitions which are contradictory. Many sources say that "electricity" is the quantity of electrical energy measured in joules or kilowatt-hours. Other sources call it the motion of charges within a conductor, and they measure the quantity of electricity in terms of amperes. Still others call a wide variety of electrical phenomena by this name, e.g., bioelectricity, piezoelectricity, triboelectricity, etc. It is advisable to be cautious when interpreting the term "electricity" in place of the more precise terms electric charge, electric current, electrical energy, etc.

File:Lightning.jpg

Lightning strikes during a night-time thunderstorm.

Electric charge

Electric charge is a property of certain subatomic particles (e.g., electrons and protons) which interacts with electromagnetic fields and causes attractive and repulsive forces between them. Electric charge gives rise to one of the four fundamental forces of nature, and is a conserved property of matter that can be quantified. In this sense, the phrase "quantity of electricity" is used interchangeably with the phrases "charge of electricity" and "quantity of charge." There are two types or charge: we call one kind of charge positive and the other negative. Through experimentation, we find that like-charged objects repel and opposite-charged objects attract one another. The magnitude of the force of attraction or repulsion is given by Coulomb's law. Some electrical effects are discussed under electrical phenomenon and electromagnetism.

The SI unit of charge is the coulomb, which has the abbreviation "C". The symbol Q is used in equations to represent the quantity of electricity or charge. For example, "Q = 0.5 C" means "the quantity of electric charge is 0.5 coulomb."

History

Ancient

According to Thales of Miletus, writing circa 600 B.C.E., a form of electricity was known to the Ancient Greeks, who found that rubbing fur on various substances, such as amber, would cause a particular attraction between the two. The Greeks noted that the amber buttons could attract light objects such as hair, and that if they rubbed the amber for long enough, they could even get a spark to jump. This is the origin of the word "electricity", from the Greek ēlektron = "amber", which came from an old root ēlek- = "shine".

An object found in Iraq in 1938, dated to about 250 B.C.E. and called the Baghdad Battery, resembles a galvanic cell and is believed by some to have been used for electroplating. The conjecture that this or other ancient artifacts had an electrical function remains unproven, and such proposed ancient knowledge bears no known continuous relationship to the development of modern electrical technology.

Modern

In 1600 the English scientist William Gilbert returned to the subject in De Magnete, and coined the modern Latin word electricus from ηλεκτρον (elektron), the Greek word for "amber", which soon gave rise to the English words electric and electricity. He was followed in 1660 by Otto von Guericke, who is regarded as having invented an early electrostatic generator. Other European pioneers were Robert Boyle, who in 1675 stated that electric attraction and repulsion can act across a vacuum; Stephen Gray, who in 1729 classified materials as conductors and insulators; and C. F. Du Fay, who first identified the two types of electricity that would later be called positive and negative. The Leyden jar, a type of capacitor for electrical energy in large quantities, was invented at Leiden University by Pieter van Musschenbroek in 1745. William Watson, experimenting with the Leyden jar, discovered in 1747 that a discharge of static electricity was equivalent to an electric current.

In June, 1752, Benjamin Franklin promoted his investigations of electricity and theories through the famous, though extremely dangerous, experiment of flying a kite during a thunderstorm. Following these experiments he invented a lightning rod and established the link between lightning and electricity. If Franklin did fly a kite in a storm, he did not do it the way it is often described (as it would have been dramatic but fatal). It was either Franklin (more frequently) or Ebenezer Kinnersley of Philadelphia (less frequently) who created the convention of positive and negative electricity.

Franklin's observations aided later scientists such as Michael Faraday, Luigi Galvani, Alessandro Volta, André-Marie Ampère, and Georg Simon Ohm whose work provided the basis for modern electrical technology. The work of Faraday, Volta, Ampere, and Ohm is honored by society, in that fundamental units of electrical measurement are named after them.

Volta worked with chemicals and discovered that chemical reactions could be used to create positively charged anodes and negatively charged cathodes. When a conductor was attached between these, the difference in the electrical potential (also known as voltage) drives a current between them through the conductor. The potential difference between two points is measured in units of volts in recognition of Volta's work.

The late 19th and early 20th century produced such giants of electrical engineering as Nikola Tesla, inventor of the induction motor and the fundamental alternating current transmission system, Samuel Morse, inventor of the telegraph; Antonio Meucci, inventor of the telephone; Thomas Edison inventor of the phonograph and a practical incandescent light bulb; George Westinghouse, inventor of the electric locomotive; Charles Steinmetz, theoretician of alternating current.

Nikola Tesla performed experiments with very high voltages that are the stuff of legend, involving ball lightning and other effects (some have been duplicated or explained; and others which have not). Nikola Tesla, inventor of the induction motor and developer of polyphase systems, contributed to the world of electrodynamics the theory of polyphase alternating current, which he used to build the first induction motor, invented in 1882. In May 1885, Westinghouse, then president of the Westinghouse Electric Company in Pittsburgh, Pennsylvania, bought the rights to Tesla's patents for polyphase alternating-current dynamos. This led to a contest in the so-called court of public opinion as to which system would be adopted as the standard for power transmission (known as the War of Currents), Edison's direct-current system or Westinghouse's alternating-current method.

Edison conducted a spirited public relations campaign which included his promotion of the electric chair as a method of execution. The electric chair ran on Westinghouse's AC; Edison wanted to prove that AC power was capable of killing, and should therefore be viewed by the public as inherently dangerous. This fear, uncertainty and doubt campaign included the electrocution of Topsy the Elephant. AC power was eventually adopted as the standard.

Electric power

File:Power pole.jpg

An electric power pole

See main article: Electric power.

"Electric power" is the popular name given to electrical energy production and distribution. For most consumers, electrical energy is generated centrally by utility companies using coal, natural gas, hydropower, nuclear power or petroleum. In 2000, U.S. electric utilities had 600 gigawatts of maximum summer generating capacity including 261 GW from coal, 118 GW from natural gas, 92 GW from hydropower, 86 GW from nuclear and 41 GW from petroleum. Little generating capacity is presently based on renewable energy sources such as solar power and wind power. Some individuals and communities prefer renewable sources because there is less pollution and because users of renewable energy sources can sometimes gain a measure of economic independence from the electrical utilities.

Devices powered by electrical energy include lamps, computers and the Internet, radio and television, refrigeration, air conditioning, traffic signals, electric guitars and other electronic musical instruments, and the spark plugs in automobiles. A summary of the applications of electric energy (written for non-specialists, and briefly explaining motors, transformers, transistors, digital, etc.) is the book, Industrial Electronics for Engineers, Chemists, and Technicians, by D. J. Shanefield, William Andrew Publishing (Norwich, NY), 2001.

Safety in power systems

It is often important that one side of a circuit be electrically bonded to an earth terminal. Such an earth terminal is usually connected to an electrode buried in the ground. The potential of earth (ground) is defined as zero by convention, and the electrical resistance between similarly buried electrodes is usually low enough that all earth terminals within a given area can be considered to be at the same voltage.

Applications

In electrical engineering, the energy in electromagnetic fields is harnessed to perform useful work—either as a method to transmit energy to the appropriate place and then convert it back into a different, useful form of energy (for instance, heat, light, or motion), or by using the presence or level of electricity to convey information.

Producing electrical energy

See also: Electricity generation

There are many different types of generators to produce electrical energy: Hydropower, nuclear power, solar power, and wind power. These different types of generators use different natural and man-made materials to liberate energy.

Hydropower

See main article: Hydropower

Hydropower consists in taking advantage of water falling or flowing, most of the time at waterfalls or rivers, to convert the mechanical energy into electrical energy by using generators coupled to turbines.

Nuclear power

See main article: nuclear power

Nuclear power uses heat generated by nuclear fission of either uranium or plutonium to heat a liquid, usually water. This heat then drives a steam engine to turn a generator and produce electrical energy. Nuclear fusion generates greater energy than fission. However, a convenient and cheaper way of harnessing fusion is still being researched.

Solar power

See main article: Solar power Solar power is created by the sun's radiant energy falling on the earth. The energy from the sun excites electrons in a semiconductor material which gives rise to a photo voltaic potential.

Wind power

See main article: wind power

Energy is captured from the wind by the use of wind turbines. The most common type of wind turbine is three-bladed, though two- and four-bladed ones can be found. Most turbines have blades that rotate perpendicular to the wind, though there is a variety that rotates parallel to it.

Wind turbines must be placed in an area where there is an almost constant source of wind. The most productive locations are on or near the shore of a large lake, sea or ocean, though they can be placed anywhere that there is a breeze.

Burning of fuel

See main article: Fossil fuel power plant

Energy is captured from the heat given off by coal, oil, wood and fossil fuels. These are often used when no other means of energy generation can be used, either because of the local geography or technical issues. It is also one of the worst polluting type of generator.

Electric current

The electric charge which occurs naturally within conductors can be forced to flow, while the charges within insulators are locked in place and cannot be moved. Devices that use charge flow principles in materials are called electronic devices. A flow of electric charge is called an electric current. A direct current (DC) is a unidirectional flow; alternating current (AC) is a flow whose time average is zero, but whose energy capability (RMS level) is not zero. With AC the electric current repeatedly changes direction.

See electrical conduction

Ohm's Law is an important relationship describing the behaviour of electric currents:

V=I\cdot R\,

where

$V$ is the applied voltage, measured in volts

$I$ is the current, measured in amperes

$R$ is the resistance, measured in ohms

For historical reasons, electric current is said to flow from the most positive part of a circuit to the most negative part. The electric current thus defined is called conventional current. It is now known that, depending on the type of conductor, an electric current can consist of a flow of charged particles in either direction, or even in both directions at once. The positive-to-negative convention is widely used to simplify this situation. If another definition is used - for example, "electron current" - it should be explicitly stated.

SI electricity units

SI electromagnetism units
Symbol	Name of Quantity	Derived Units	Unit	Base Units
I	Current	ampere (SI base unit)	A	A = W/V = C/s
q	Electric charge, Quantity of electricity	coulomb	C	A·s
V	Potential difference	volt	V	J/C = kg·m²·s⁻³·A⁻¹
R, Z, X	Resistance, Impedance, Reactance	ohm	Ω	V/A = kg·m²·s⁻³·A⁻²
ρ	Resistivity	ohm metre	Ω·m	kg·m³·s⁻³·A⁻²
P	Power, Electrical	watt	W	V·A = kg·m²·s⁻³
C	Capacitance	farad	F	C/V = kg⁻¹·m⁻²·A²·s⁴
	Elastance	reciprocal farad	F⁻¹	V/C = kg·m²·A⁻²·s⁻⁴
ε	Permittivity	farad per metre	F/m	kg⁻¹·m⁻³·A²·s⁴
χ_e	Electric susceptibility	(dimensionless)	-	-
G, Y, B	Conductance, Admittance, Susceptance	siemens	S	Ω⁻¹ = kg⁻¹·m⁻²·s³·A²
σ	Conductivity	siemens per metre	S/m	kg⁻¹·m⁻³·s³·A²
H	Auxiliary magnetic field, magnetic field intensity	ampere per metre	A/m	A·m⁻¹
Φ_m	Magnetic flux	weber	Wb	V·s = kg·m²·s⁻²·A⁻¹
B	Magnetic field, magnetic flux density, magnetic induction, magnetic field strength	tesla	T	Wb/m² = kg·s⁻²·A⁻¹
	Reluctance	ampere-turns per weber	A/Wb	kg⁻¹·m⁻²·s²·A²
L	Inductance	henry	H	Wb/A = V·s/A = kg·m²·s⁻²·A⁻²
μ	Permeability	henry per metre	H/m	kg·m·s⁻²·A⁻²
χ_m	Magnetic susceptibility	(dimensionless)	-	-

External links

What is electricity?
Merriam-Webster: Electricity
Tyndall: Faraday as Discovery: Identity of Electricities
US Energy Department Statistics
How to save on your electricity bills
Electricity around the world
A Comprehensive Collection of Franklin’s Electrical Works: The Electrical Writings of Benjamin Franklin, Created and Collected by Robert A. Morse (2004)

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