Electron

Electron
The first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
Classification
Elementary particle Fermion Lepton First Generation Electron
Properties
Mass: 9.109 3826(16) × 10−31 kg 1⁄1836.152 672 61(85) amu 0.510 998 918(44) MeV/c2 Electric Charge: −1.602 176 53(14) × 10−19 C Spin: ½ Color Charge: none Interaction: Gravity, Electromagnetic, Weak

The electron is a fundamental subatomic particle which carries a negative electric charge. They are found within atoms and are the carriers of electrical current in metals. Within an atom the electrons surround the nucleus of protons and neutrons in an electron configuration. It is this electonic configuration of atoms that determines an elements physical and chemical properties. The word electron was coined in 1894 and is derived from the term electric, whose ultimate origin is the Greek word 'ηλεκτρον, meaning amber.

Electrons generate an electric field. In organized motion they constitute electric current and generate a magnetic field. Electric current over time is a form of energy (electricity) that may be harnessed as a practical means to perform work.

Characteristics

The electron is one of a class of subatomic particles called leptons which are believed to be fundamental particles. As an elementary particle it is not considered to have any substructure (at least, experiments have not found any so far) and there is good reason to believe that there is not any. Hence, it is usually described as point-like, i.e. with no spatial extension. However, if one gets very near an electron, one notices that its properties (charge and mass) seem to change. This is an effect common to all elementary particles: the particle influences the vacuum fluctuations in its vicinity, so that the properties one observes from far away are the sum of the bare properties and the vacuum effects (see renormalization).

Charged particles, monatomic ions and larger particles, arise from an imbalence in the total number of electons and protons in the particle. 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. A macroscopic body can aquire charge through rubbing, i.e. the phenomena of triboelectricity.

Electrons have a negative electric charge of −1.6 × 10⁻¹⁹ coulombs (this is usually just stated as a charge of −1) and a mass of about 9.11 × 10⁻³¹ kg (0.51 MeV/c²), which is approximately ¹⁄₁₈₃₆ of the mass of the proton. These are commonly represented as e⁻. The electron has spin ½, which implies it is a fermion, i.e., it follows the Fermi-Dirac statistics. While most electrons are found in atoms, others move independently in matter, or together as an electron beam in a vacuum. In some superconductors, electrons move in Cooper pairs, in which their motion is coupled to nearby matter via lattice vibrations called phonons. When electrons move, free of the nuclei of atoms, and there is a net flow of charge, this flow is called electricity, or an electric current. There is also a physical constant called the classical electron radius, with a value of 2.8179 × 10⁻¹⁵ m. Note that this is the radius that one could infer from its charge if the physics were only described by the classical theory of electrodynamics and there were no quantum mechanics (hence, it is an outdated concept that nevertheless sometimes still proves useful in calculations).

Electrons in theory

As applied to electrons word "particle" is somewhat misleading however. This is because electrons can also behave like a wave; that is they exhibit wave-particle duality. The wave behaviour of electrons can be demonstrated in the interference patterns produced in a double-slit experiment, and is employed in the electron microscope. The wave nature of electrons is essential to the quantum mechanics of the electromagnetic interaction, where electrons are represented by wavefunctions. From the square of the wavefunction the electron density can be determined. Also, the exact momentum and position of an electron cannot be simultaneously determined. This is a limitation described by the Heisenberg uncertainty principle, which, in this instance, simply states that the more accurately we know a particle's position, the less accurately we can know its momentum and vice versa.

In relativistic quantum mechanics, the electron is described by the Dirac Equation. Quantum electrodynamics (QED) models an electron as a charged particle surrounded a sea of interacting virtual particles, modifying the sea of virtual particles which makes up a vacuum. Although this theory involves difficult theoretical problems where calculations produce infinite terms, a practical (although mathematically dubious) method called renormalization was discovered whereby infinite terms can be cancelled to produce finite predictions about the electron. The correction of just over 0.1% to the predicted value of the electron's gyromagnetic ratio from exactly 2 (as predicted by Dirac's single particle model), and it's extraordinarily precise agreement with the experimentally determined value is viewed as one of the pinnacles of modern physics. There are now indications that string theory and its descendants may provide a model of the electron and other fundamental particles where the infinities in calculations do not appear, because the electron is no longer seen as a dimensionless point. At present, string theory is very much a 'work in progress' and lacks predictions analogous to those made by QED that can be experimentally verified.

In the Standard Model of particle physics, it forms a doublet in SU(2) with the electron neutrino, as they interact through the weak interaction. The standard model contains three generations of matter particles, where the muon and the tauon correspond to the electron in other generations of particles.

Electrons are also a key element in electromagnetism, an approximate theory that is adequate for macroscopic systems, and for classical modelling of microscopic systems.

History

The electron as a unit of charge in electrochemistry had been posited by G. Johnstone Stoney in 1874. In 1894, he also invented the word itself.

The discovery that the electron was a subatomic particle was made in 1897 by J.J. Thomson at the Cavendish Laboratory at Cambridge University, while he was 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". He published his discovery in 1897.

The periodic law states that the chemical properties of elements largely repeat themselves periodically and is the foundation of the periodic table of elements. The law itself was initially explained by the atomic mass of the elements. However, as there were anomalies in the periodic table, efforts were made to find a better explanation for it. In 1913, Henry Moseley introduced the concept of the atomic number and explained the periodic law with the number of protons each element has. In the same year, Niels Bohr showed that electrons are the actual foundation of the table. In 1916, Gilbert Newton Lewis and Irving Langmuir explained the chemical bonding of elements by electronic interactions.

Electrons in the universe

It is believed that the number of electrons existing in the known universe is at least 10⁷⁹. This number amounts to a density of about one electron per cubic metre of space.

Based on the classical electron radius and assuming a dense sphere packing, it can be calculated that the number of electrons that would fit in the observable universe is on the order of 10¹³⁰. Of course, this number is even less meaningful than the classical electron radius itself.

Electrons in industry

Electron beams are used in welding as well as lithography.

Electrons in the laboratory

Early experiments

The quantum or discrete nature of electron's charge was observed by Robert Millikan in the Oil-drop experiment of 1909.

Use of electrons in the laboratory

Electron microscopes are used to magnify details up to 500,000 times. Quantum effects of electrons are used in Scanning tunneling microscope to study features at the atomic scale.

See also

• Standard model
• Subatomic particle
• Proton
• Positron
• Neutron
• Photoelectric Effect
• Lightning
• List of particles
• Cathode rays
• Electricity
• Fermion field

External links

• The Discovery of the Electron from the American Institute of Physics History Center
• Particle Data Group
• Stoney, G. Johnstone, "Of the 'Electron,' or Atom of Electricity". Philosophical Magazine. Series 5, Volume 38, p. 418-420 October 1894.
• Eric Weisstein's World of Physics: Electron

References

ISBN links support NWE through referral fees

• Griffiths, David J. (2004). Introduction to Quantum Mechanics (2nd ed.). Prentice Hall. ISBN 013805326X.
• Tipler, Paul; Llewellyn, Ralph (2002). Modern Physics (4th ed.). W. H. Freeman. ISBN 0716743450.
• Brumfiel, G. (6 January 2005). Can electrons do the splits? In Nature, 433, 11.

Positron

This section was copied from the  
Positron article at Wikipedia.

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File:Cloud chamber - visible trace of positron.JPG

The first detection of the positron in 1932 by Carl D. Anderson

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 a positron annihilates with an electron, their mass is converted into energy in the form of two gamma ray photons. (See electron-positron annihilation)

A positron may be generated by positron emission radioactive decay, or the interaction of photon with a charged particle (such as an atom's nucleus) with energy greater than 2 m_ec² = 2×0.511 MeV = 1.022 MeV with matter (m_e represents the mass of one electron and c is the speed of light in vacuum). This process is called pair production, as it generates one electron and one positron from the energy of the photon.

The existence of positrons was first postulated in 1928 by Paul Dirac as an inevitable consequence of the Dirac equation. In 1932, positrons were observed by Carl D. Anderson, who gave the positron its name. Anderson also unsuccessfully suggested renaming electrons "negatrons." The positron was the first evidence of antimatter and was discovered by passing cosmic rays through a gas chamber and a lead plate surrounded by a magnet to distinguish the particles by bending differently charged particles in different directions.

Today, positrons are produced in enormous numbers in accelerator physics laboratories and used in electron-positron colliders.

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