Electrons
SOURCE
#4.
Electrons
Matter consists of atoms, and atoms consist of
electrically charged components--lightweight negative electrons,
and positive nuclei.
How do we know?
One clue comes from the "Edison effect,"
discovered by Thomas Alva Edison. Imagine a glass bulb from which air
has been pumped, until hardly any of it remains. In one end we embed a
metal coil of wire (like that of a flashlight bulb) in the other a metal
plate, as drawn. Connect now a battery between the coil and the plate,
so that the former is negative and the latter is positive.
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No current will flow in this circuit: some atoms or
molecules may be left inside the bulb, but they are electrically
neutral, and can carry no electric current. Air is an excellent
insulator: electric companies can string power lines in the open air and
never have to worry about currents dribbling out on their way from the
power station to consumers.
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Now connect a second battery to the end of the coil, so
that a current flows through the coil and heats it up. As the wire
begins to glow, a current begins to flow, because now negatively
charged particles are emitted from the hot wire, are attracted to
the positive charge on the plate and by doing so, complete the
electrical circuit.
Suppose the connections of the first battery are reversed,
so that now the coil is positive and the plate is negative. Then no
current flows, showing that the hot wire releases only
negative particles, not positive ones. These particles were named electrons
In laboratory experiments these particles were directed
by electrically charged structures (similar to the "electron
guns" inside TV picture tubes) to form beams. Those beams were then
bent by magnets and by electrified plates, and their behavior was
studied. From such experiments and others the mass of the emitted
particles, which became known as "electrons", could be
determined. It turned out that they were rather lightweight. The
simplest atom, that of hydrogen, contains a central positive particle, a
proton, and a single electron, and the proton is nearly 2000 times
heavier.
The same process will charge a spacecraft orbiting in
the sunlight positively, to a few volts. Sunlight knocks out electrons
from the surface and a few manage to escape, leaving the spacecraft
positively charged; the situation then stabilizes, because the positive
charge prevents any more electrons from leaving.
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#4H. History of the Electron
The experiment with a pumped-out glass bulb, in which an electric
circuit is completed by electrons emitted from a hot wire, is credited
to the US inventor Thomas Alva Edison (1847-1931), who patented it in
1883. The phenomenon is known as the "Edison effect" and many
electronic devices use it nowadays.
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Experiments with beams of negative particles were performed in
Britain by Joseph John ("J.J.") Thomson, and led to his
conclusion in 1897 that they consisted of lightweight particles with a
negative electric charge, nowadays known as electrons. Thomson was
awarded the 1906 Nobel Prize.
The word "elektron" in Greek means amber, the yellow
fossilized resin of evergreen trees, a "natural plastic
material" already known to the ancient Greeks. It was known that
when amber was rubbed with dry cloth--producing what now one would call
static electricity--it could attract light objects, such as bits of
paper.
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J.J. Thomson
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William
Gilbert, a physician who lived in London at the time of Queen
Elizabeth I and Shakespeare, studied magnetic phenomena and demonstrated
that the Earth itself was a huge magnet, by means of his "terrella"
experiment. But he also studied the attraction produced when materials
such as amber were rubbed, and named it the "electric"
attraction. From that came the word "electricity" and all
others derived from it.
During the 1800s it became evident that electric charge had a
natural unit, which could not be subdivided any further, and in 1891
Johnstone Stoney proposed to name it "electron." When
J.J. Thomson discovered the light particle which carried that charge,
the name "electron" was applied to it. The many applications
of electrons moving in a near-vacuum or inside semiconductors were later
dubbed "electronics."
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Further reading:
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To mark the 100th anniversary of the discovery of the electron, the
Center for History of Physics of the American Institute of Physics has
created on the world wide web a suitable exhibit: http://www.aip.org/history/electron.
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Sir George Thomson, J. J. Thomson's son, was a renowned physicist in his
own right and won the Nobel prize in 1937. On the 70th anniversary of the
discovery of the electron he wrote about that discovery and about later
developments where electrons were discovered to act sometimes like waves:
"The Septuagenarian Electron", Physics Today, May 1967,
55-61
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The story of J. J. Thomson is also briefly given in the first chapter of
"From X-Rays to Quarks" by Emilio Segre (W.H. Freeman and
Co., 1980). Segre was a physicist who won the 1959 Nobel Prize and his
concise history of modern physics is filled with insights and stories, some
of them drawn from his own experience.
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Electron
From Wikipedia, the free encyclopedia.
Electron
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The electrons move rapidly around the nucleus.
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Classification
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First Generation
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Electron
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Properties
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Mass:
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9.10 × 10-31 kg
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Electric Charge:
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-1.6 × 10-19C
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Spin:
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1/2
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Color Charge:
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none
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Interaction:
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The electron (also called negatron,
commonly represented as e−) is a subatomic particle.
In an atom the
electrons surround the nucleus
of protons
and neutrons.
The electron is one of a class of subatomic particles called leptons
which are believed to be fundamental
particles (that is, they cannot be broken down into smaller constituent
parts).
In quantum
mechanics the electron is described by the Dirac
Equation. In the Standard
Model it forms a doublet in SU(2) with the electron
neutrino, as they interact through the weak
interaction. The electron has two more massive partners, with the same
charge but different masses: the muon
and the Tauon.
The antimatter
counterpart of the electron is its antiparticle, the positron.
The positron has the same amount of electrical charge as the electron, except
that the charge is positive. It has the same mass and spin as the electron.
When an electron and a positron meet, they may annihilate
each other, giving rise to two gamma-ray
photons, each having an energy of 0.511 MeV
(511 keV). See also
Electron-positron
annihilation.
Some theorists believe the electron may be a very small black hole.
Dual nature
Electrons can exhibit properties of both particles and waves. An electron
bound to a nucleus behaves as a standing
wave.
Details
The electron has a negative electric
charge of -1.6 × 10-19 coulombs,
and a mass of about 9.10
× 10-31 kg (0.51 MeV/c2), which is 1/1800 of the proton
mass.
It is believed that the number of electrons that would fit in the known universe
is 10 followed by 130 zeros.
Electricity
When electrons move, free of the nuclei of atoms, and there is a net flow,
this flow is called electricity,
or an electric
current. This might be compared to a flock of sheep moving north together,
while the shepherds do not. Electric charge can be directly measured with an electrometer.
Electric current can be directly measured with a galvanometer.
So-called "static electricity" is not a flow of electrons at
all. More correctly called a "static charge", it refers to a body
that has more or fewer electrons than are required to balance the positive
charge of the nuclei. When there is an excess of electrons, the object is said
to be "negatively charged". When there are fewer electrons than protons,
the object is said to be "positively charged". When the number of
electrons and the number of protons are equal, the object is said to be
electrically "neutral".
History
The electron had been posited by G.
Johnstone Stoney, as a unit of charge in electrochemistry, but Thompson
realised that it was also a subatomic
particle.
See also
External links
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Stoney, G. Johnstone, "Of
the 'Electron,' or Atom of Electricity (http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Stoney-1894.html)".
Philosophical Magazine. Series 5, Volume 38, p. 418-420 October 1894. ()
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