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Electricity
Electricity
In
physics, the
electromagnetic
phenomenon of electricity (or
electric charge) 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 of electricity or charge: we call
one kind of charge positive and the other negative. Through experiment, 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.
The
SI
unit of electrical charge is the
coulomb.
According to
Thales of Miletus, writing circa 600 BC, electricity was known to
the
Ancient Greeks, who found that rubbing
fur
on various substances, such as
amber, would cause an electric charge imbalance. The Greeks noted that the
charged 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.
An object found in Iraq in 1938, dated to about 250 BC and
called the
Baghdad Battery, resembles an electrochemical cell and is believed by some
to have been used for electroplating. There is no "firm" documentary
evidence to indicate what the object was used for, though there are other
anachronistic descriptions of electrical devices on Egyptian walls and in
ancient writings.
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 invented an early electrostatic generator. Other
European pioneers were
Robert Boyle, who stated in 1675 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 electric charge that
would later be called positive and negative. The
Leyden jar, a type of
capacitor for storing electric charge in large quantities, was invented at
Leyden 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 charge. 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
Samuel Morse, inventor of the telegraph;
Alexander Graham Bell, inventor of the telephone;
Thomas Edison (inventor of the
phonograph,
motion pictures and a practical
incandescent light bulb) ;
George Westinghouse, inventor of the electric
locomotive;
Charles Steinmetz, inventor of alternating current; and
Nikola Tesla, inventor of the induction motor and developer of
polyphase systems.
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). He contribution to the world of electrodynamics the
theory of
polyphase
alternating current electricity, 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 transmission was eventually adopted as the
standard.
Electric power, for most consumers, is generated centrally by
utility companies using
coal,
oil,
hydropower, or
nuclear power. In 2000, U.S. electric utilities had 600 gigawatts of maximum
summer generating capacity including 261 GW from coal, 41 GW from petroleum, 118
GW from natural gas, 92 GW from
hydropower and 86 GW from nuclear fuels. Little generating capacity is
presently based on
renewable 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.
Things that are powered by electricity include lamps;
computers and the
internet;
radio and
television;
refrigeration;
air conditioning; traffic signals; electric guitars and other
electronic musical instruments; the
spark plugs in
automobiles.
Today, for residents of most developed countries, 24-hour,
on-demand, access to electrical power is taken for granted. People gripe about
their electric bill and about electric power monopolies and utility pricing, but
by any comparison with pre-industrial standards of living, electricity is still
a bargain. Few would want to go back to life without it.
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.
Today's electrical engineers enjoy the ability to design
circuits using pre-manufactured building blocks such as
power supplies,
resistors,
capacitors,
semiconductors such as
transistors, and
integrated circuits. An
integrated circuit inside a
computer, a
microprocessor, performs millions of computations per second.
A flow of electric charge is called an electric current. A
direct current (DC) is a steady flow;
alternating current (AC) is a flow whose time average is zero, but is not
zero at all times. That definiation of AC implies that the flow repeatedly
changes direction. (Polarity and numerical sign (i.e. negative vs. positive) are
additional terms for direction in this sense).
Flows of electric charge can be produced within
conductors and cannot exist within
insulators. Some electrical devices that use electrical physics are called
electronic devices. See
electrical conduction for more information about current flow in materials.
Ohm's Law is an important relationship describing the behaviour of electric
currents: voltage potential difference = current * resistance, or:
V = IR
-
lightning
-
bioelectricity - Many animals are sensitive to electric fields, some
(e.g., sharks) more than others (e.g., people). A few, such as the
electric eel, generate their own electric fields.
-
matter - since
atoms and
molecules are held together by electric forces.
-
the
Earth's magnetic field - which is created by electric currents circulating
in the planet's core.
In addition to its definition by physicists, the word
electricity has several popular definitions which are contradictory. Rather
than using the word electricity to refer to the quantity of
electric charge, many sources instead say that electricity is the quantity
of electromagnetic energy measured in
joules or
kilowatt-hours. Other sources call the flow of charges within a conductor by
the name electricity and they measure the quantity of electricity in
terms of
amperes. Still others call a wide variety of electrical phenomena by the
name electricity, e.g. bioelectricity, piezoelectricity,
triboelectricity, etc. It is advisable to be extremely careful when interpreting
texts which use the frequently misused term electricity in place of the
more accurate terms
electric charge,
electric current,
electrical energy, etc.
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