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#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.



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.


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.

Light, like heat, can also knock electrons out of a metal. If the heated coil in the drawing is replaced by a clean metal plate, and light shines onto it, electrons are again released, and current will flow in the circuit. The explanation of this phenomena, called the photoelectric effect, earned Albert Einstein the 1921 Nobel Prize.

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.


#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.


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.


J.J. Thomson

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."


Further reading:

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.


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


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.



From Wikipedia, the free encyclopedia.


Helium atom (not to scale)
The electrons move rapidly around the nucleus.


Elementary particle



First Generation






9.10 10-31 kg

Electric Charge:

-1.6 10-19C



Color Charge:



Gravity, Electromagnetic,Weak

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).

The electron has spin 1/2, which implies it is a fermion, i.e., follows the Fermi-Dirac statistics.

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.

Table of contents [hide]


Dual nature

Electrons can exhibit properties of both particles and waves. An electron bound to a nucleus behaves as a standing wave.



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.



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".



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.

The electron was discovered by J.J. Thomson in 1897 at the Cavendish Laboratory at Cambridge University, while studying "cathode rays." Influenced by the work of James Clerk Maxwell, and the discovery of the X-ray, he deduced that cathode rays existed and were negatively charged "particles", which he called "corpuscles".


See also

Standard model

Subatomic particle



Photoelectric Effect


List of particles

Cathode rays


External links

Particle Data Group (http://pdg.lbl.gov/)

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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