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History of electricity

Static electricity produced by rubbing objects against fur was known to the ancient Greeks, Phoenicians, Parthians and Mesopotamians. The Parthians and Mesopotamians may have had some knowledge of electroplating, based on the discovery of the Baghdad Battery, which resembles a Galvanic cell.

Benjamin Franklin conducted extensive research in electricity. He had theories on the relationship between lightning and static electricity, including his famous kite-flying experiment,which was a key attached to a wet string and kite. During a lightning storm a small spark struck his finger showing that lightning is electricity. It sparked the interest of later scientists whose work provided the basis for modern electrical technology. Most notably these include Luigi Galvani (1737–1798), Alessandro Volta (1745-1827), Michael Faraday (1791–1867), André-Marie Ampère (1775–1836), and Georg Simon Ohm (1789-1854). The late 19th and early 20th century produced such giants of electrical engineering as Nikola Tesla, Antonio Meucci, Thomas Edison, George Westinghouse, Werner von Siemens, Charles Steinmetz, Alexander Graham Bell and William Thomson, 1st Baron Kelvin


Electric potential

The electric potential difference between two points is defined as the work done (against electrical forces) per unit of charge in moving a positive point charge slowly between two points. If one of the points is taken to be a reference point with zero potential, then the electric potential at any point can be defined in terms of the work done per unit charge in moving a positive point charge from that reference point to the point at which the potential is to be determined. For isolated charges, the reference point is usually taken to be infinity. The potential is measured in volts. (1 volt = 1 joule/coulomb) The electric potential is analogous to temperature: there is a different temperature at every point in space, and the temperature gradient indicates the direction and magnitude of the driving force behind heat flow. Similarly, there is an electric potential at every point in space, and its gradient indicates the direction and magnitude of the driving force behind charge movement.

Electric current

Nikola TeslaAn electric current is a flow of electric charge, and its intensity is measured in amperes. Examples of electric currents include metallic conduction, where electrons flow through a conductor or conductors such as a metal wire, and electrolysis, where ions (charged atoms) flow through liquids. The particles themselves often move quite slowly, while the electric field that drives them propagates at close to the speed of light. See electrical conduction for more information.

Devices that use charge flow principles in materials are called electronic devices.

A direct current (DC) is a unidirectional flow, while an alternating current (AC) reverses direction repeatedly. The time average of an alternating current is zero, but its energy capability (RMS value) is not zero.

Ohm's law is an important relationship describing the behaviour of electric currents, relating them to voltage.

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 conditions, 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.

2007-07-20 23:24:42 補充：
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History of electricity
Main article: History of electricity
Static electricity produced by rubbing objects against fur was known to the ancient Greeks, Phoenicians, Parthians and Mesopotamians. The Parthians and Mesopotamians may have had some knowledge of electroplating, based on the discovery of the Baghdad Battery, which resembles a Galvanic cell.

Benjamin Franklin conducted extensive research in electricity. He had theories on the relationship between lightning and static electricity, including his famous kite-flying experiment,which was a key attached to a wet string and kite. During a lightning storm a small spark struck his finger showing that lightning is electricity. It sparked the interest of later scientists whose work provided the basis for modern electrical technology. Most notably these include Luigi Galvani (1737–1798), Alessandro Volta (1745-1827), Michael Faraday (1791–1867), André-Marie Ampère (1775–1836), and Georg Simon Ohm (1789-1854). The late 19th and early 20th century produced such giants of electrical engineering as Nikola Tesla, Antonio Meucci, Thomas Edison, George Westinghouse, Werner von Siemens, Charles Steinmetz, Alexander Graham Bell and William Thomson, 1st Baron Kelvin.

History of Magnetism
In the natural world, magnetotactic bacteria had evolved miniature magnets inside themselves and used them to establish their orientation relative to the Earth's magnetic field [1].

Aristotle attributes the first scientific theory on magnetism to Thales, who lived from about 625 BC to about 545 BC. [2] In China, the earliest literary reference to magnetism lies in a 4th century BC book called Book of the Devil Valley Master (鬼谷子): "The lodestone makes iron come or it attracts it."[1] The earliest mention of the attraction of a needle appears in a work composed between 20 and 100 AD (Louen-heng): "A lodestone attracts a needle."[2] The scientist Shen Kuo (1031-1095) wrote of the magnetic needle compass and improved the accuracy of navigation by employing the astronomical concept of true north (Dream Pool Essays, 1088 AD), and by the 12th century the Chinese were known to use the lodestone compass for navigation. Alexander Neckham, by 1187, was the first in Europe to describe the compass and its use for navigation.

The Epic of Gilgamesh also mentions the use of Directional stones, 3700 BC.

Magnetism is the oldest branch of physical science. Theories concerning magnetism were advanced by the Greeks and the first work of experimental science appeared in 1269 A.D. as a treatise on magnetism. See the external links below for more historical information