Electricity

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Electricity is a general term applied to phenomena involving a fundamental property of matter called an electric charge. This article will introduce and explain some of the basic principles of electricity. In casual usage, the term electricity is applied to several related concepts that are better identified by more precise terms. Some of these concepts are the subjects of other articles:

  • Electric charge: the basic concept involved in electricity, introduced in this article, but discussed more extensively in a separate article.
  • Electric current: a movement or flow of electrically charged particles, introduced in this article, but discussed more extensively in a separate article.
  • Electrical energy: a form of energy related to the position of an electric charge in an electric field.
  • Electric power: electrical energy as it is delivered to individual, commercial and industrial customers for use in operating equipment, and providing heat and illumination.
  • Electric potential or voltage: a measure of the force available to cause electric current to flow.
  • Bioelectricity: electrical phenomena within living organisms.
  • Piezoelectricity: the ability of certain crystals to generate a voltage in response to applied mechanical stress.
  • Triboelectricity: electric charge taken on by contact or friction between two different materials.
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Contents

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 of 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 BCE, 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 BCE 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 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:

<math>V = I \cdot R \,</math>

where

<math>V</math> is the applied voltage, measured in volts

<math>I</math> is the current, measured in amperes

<math>R</math> is the resistance, measured in ohms

(Therefore it is "Voltage equals Ampere multiplied by 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 electromagnetic units

edit

Quantity Name Symbol Dimensions
Current ampere (SI base unit) A A
Electric charge, Quantity of electricity coulomb C A·s
Potential difference volt V J/C = kg·m2·s−3·A−1
Resistance, Impedance, Reactance ohm Ω V/A = kg·m2·s−3·A−2
Resistivity ohm metre Ω·m kg·m3·s−3·A−2
Electrical power watt W V·A = kg·m2·s−3
Capacitance farad F C/V = kg−1·m−2·A2·s4
Elastance reciprocal farad F−1 kg·m2·A−2·s−4
Permittivity farad per metre F/m kg−1·m−3·A2·s4
Conductance, Admittance, Susceptance siemens S Ω−1 = kg−1·m−2·s3·A2
Conductivity siemens per metre S/m kg−1·m−3·s3·A2
Magnetic flux weber Wb V·s = kg·m2·s−2·A−1
Magnetic flux density tesla T Wb/m2 = kg·s−2·A−1
Magnetic induction ampere per metre A/m A·m−1
Reluctance ampere-turns per weber A/Wb kg−1·m−2·s2·A2
Inductance henry H Wb/A = V·s/A = kg·m2·s−2·A−2
Permeability henry per metre H/m kg·m·s−2·A−2
Magnetic susceptibility (dimensionless) χ -

See also

Devices

Engineering

Safety

Electrical phenomena in nature

External links

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