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Electron

2007 Schools Wikipedia Selection. Related subjects: Electricity and
Electronics

   Electron
   Theoretical estimates of the electron density for the first few
   hydrogen atom electron orbitals shown as cross-sections with
   colour-coded probability density
   Composition: Elementary particle
   Family: Fermion
   Group: Lepton
   Generation: First
   Interaction: Gravity, Electromagnetic, Weak
   Antiparticle: Positron
   Theorized: G. Johnstone Stoney ( 1874)
   Discovered: J.J. Thomson ( 1897)
   Mass: 9.109 3826(16) × 10^–31  kg

   5.485 799 0945(24) × 10^–4  u

   ^1⁄[1822.888 4849(8)]  u
   0.510 998 918(44)  MeV/c^2
   Electric charge: –1.602 176 53(14) × 10^–19  C
   Spin: ½

   The Electron is a fundamental subatomic particle that carries an
   electric charge. It is a spin-½ lepton that participates in
   electromagnetic interactions, and its mass is less than one thousandth
   of that of the smallest atom. Its electric charge is defined by
   convention to be negative, with a value of −1 in atomic units. Together
   with atomic nuclei, electrons make up atoms; their interaction with
   adjacent nuclei is the main cause of chemical bonding.

Overview

   The word electron was coined in 1891 by George Johnstone Stoney and is
   derived from the term electric force introduced by William Gilbert. Its
   origin is in Greek: ήλεκτρον (elektron), meaning amber. J.J. Thomson is
   credited with having first measured the charge/mass ratio and is
   considered to be the discoverer of the electron.

   Within an atom, electrons surround a nucleus composed of protons and
   neutrons in an electron configuration. The variations in electric field
   generated by differing numbers of electrons and their configurations in
   atoms determine the chemical properties of the elements. These fields
   play a fundamental role in chemical bonds and chemistry.

   Electrons in motion produce an electric current and a magnetic field.
   Some types of electric currents are termed electricity.

   Our understanding of how electrons behave has been significantly
   modified during the past century, the greatest advances being the
   development of quantum mechanics in the 20th century. This brought the
   idea of wave-particle duality, that is, that electrons show both
   wave-like and particle-like properties, to varying degrees. Equally
   important, particle physics has furthered our understanding of how the
   electron interacts with other particles.

Classification

   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).

   As with all particles, electrons can also act as waves. This is called
   the wave-particle duality, also known by the term complementarity
   coined by Niels Bohr and can be demonstrated using the double-slit
   experiment.

   The antiparticle of an electron is the positron, which has the same
   mass but positive rather than negative charge. The discoverer of the
   positron, Carl D. Anderson, proposed calling standard electrons
   negatrons, and using electron as a generic term to describe both the
   positively and negatively charged variants. This usage never caught on
   and is rarely if ever encountered today.

Properties and behaviour

   Electrons have a negative electric charge of −1.6022 × 10^−19  coulomb,
   a mass of 9.11 × 10^−31 kg based on charge/mass measurements and a
   relativistic rest mass of about 0.511  MeV/c^2. The mass of the
   electron is approximately ^1/[1836] of the mass of the proton. The
   common electron symbol is e^−.

   According to quantum mechanics, electrons can be represented by
   wavefunctions, from which a calculated probabilistic electron density
   can be determined. The orbital of each electron in an atom can be
   described by a wavefunction. Based on the Heisenberg uncertainty
   principle, the exact momentum and position of the actual electron
   cannot be simultaneously determined. This is a limitation 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.

   The electron has spin ½ and is a fermion (it follows Fermi-Dirac
   statistics). In addition to its intrinsic angular momentum, an electron
   has an intrinsic magnetic moment along its spin axis.

   Electrons in an atom are bound to that atom; electrons moving freely in
   vacuum, space or certain media are free electrons that can be focused
   into an electron beam. When free electrons move, there is a net flow of
   charge, this flow is called an electric current. The drift velocity of
   electrons in metal wires is on the order of mm/hour. However, the speed
   at which a current at one point in a wire causes a current in other
   parts of the wire is typically 75% of light speed.

   In some superconductors, pairs of electrons move as Cooper pairs in
   which their motion is coupled to nearby matter via lattice vibrations
   called phonons. The distance of separation between Cooper pairs is
   roughly 100 nm. (Rohlf, J.W.)

   A body has an electric charge when that body 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, their charges cancel each other
   and the object is said to be electrically neutral. A macroscopic body
   can develop an electric charge through rubbing, by the phenomenon of
   triboelectricity.

   When electrons and positrons collide, they annihilate each other and
   produce pairs of high energy photons or other particles. On the other
   hand, high-energy photons may transform into an electron and a positron
   by a process called pair production, but only in the presence of a
   nearby charged particle, such as a nucleus.

   The electron is currently described as a fundamental particle or an
   elementary particle. It has no substructure (although British physicist
   Humphrey Maris claims to have found a way to split the electron into
   "electrinos" using an electron bubble). Hence, for convenience, it is
   usually defined or assumed to be a point-like mathematical point
   charge, with no spatial extension. However, when a test particle is
   forced to approach an electron, we measure changes in its properties
   (charge and mass). This effect is common to all elementary particles:
   Current theory suggests that this effect is due to the influence of
   vacuum fluctuations in its local space, so that the properties measured
   from a significant distance are considered to be the sum of the bare
   properties and the vacuum effects (see renormalization).

   The classical electron radius is 2.8179 × 10^−15 m. This is the radius
   that is inferred from the electron's electric charge, by using the
   classical theory of electrodynamics alone, ignoring quantum mechanics.
   Classical electrodynamics ( Maxwell's electrodynamics) is the older
   concept that is widely used for practical applications of electricity,
   electrical engineering, semiconductor physics, and electromagnetics;
   quantum electrodynamics, on the other hand, is useful for applications
   involving modern particle physics and some aspects of optical, laser
   and quantum physics.

   Based on current theory, the speed of an electron can approach, but
   never reach, c (the speed of light in a vacuum). This limitation is
   attributed to Einstein's theory of special relativity which defines the
   speed of light as a constant within all inertial frames. However, when
   relativistic electrons are injected into a dielectric medium, such as
   water, where the local speed of light is significantly less than c, the
   electrons will (temporarily) be traveling faster than light in the
   medium. As they interact with the medium, they generate a faint bluish
   light, called Cherenkov radiation.

   The effects of special relativity are based on a quantity known as γ or
   the Lorentz factor. γ is a function of v, the velocity of the particle,
   and c. It is defined as:

          \gamma = 1 / \sqrt{1 - (v^2/c^2)}

   The energy necessary to accelerate a particle is γ minus one times the
   rest mass. For example, the linear accelerator at Stanford can
   accelerate an electron to roughly 51 GeV . This gives a gamma of
   100,000, since the rest mass of an electron is 0.51 MeV/c² (the
   relativistic mass of this electron is 100,000 times its rest mass).
   Solving the equation above for the speed of the electron (and using an
   approximation for large γ) gives:

          v = \left(1-\frac {1} {2} \gamma ^{-2}\right)c =
          0.999\,999\,999\,95\,c.

In practice

In the universe

   Scientists believe that the number of electrons existing in the known
   universe is at least 10^79. This number amounts to an average density
   of about one electron per cubic metre of space. Astronomers have
   determined that 90% of all of the detectable mass in the universe is
   hydrogen, which is made of one electron and one proton.

   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 10^130.

In industry

   Electron beams are used in welding, lithography, scanning electron
   microscopes and transmission electron microscopes. LEED and RHEED are
   also important tools where electrons are used.

   They are also at the heart of cathode ray tubes, which are used
   extensively as display devices in laboratory instruments, computer
   monitors and television sets. In photomultiplier tubes, one photon
   strikes the photocathode, initiating an avalanche of electrons that
   produces a detectable current.

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.

In theory

   In relativistic quantum mechanics, the electron can be described by the
   Dirac Equation which defines the electron as a (mathematical) point. In
   quantum field theory, the behaviour of the electron can be described by
   quantum electrodynamics (QED), a U(1) gauge theory. In Dirac's model,
   an electron is defined to be a mathematical point, a point-like,
   charged "bare" particle surrounded by a sea of interacting pairs of
   virtual particles and antiparticles . These provide a 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).
   The extraordinarily precise agreement of this prediction with the
   experimentally determined value is viewed as one of the great
   achievements of modern physics.

   In the Standard Model of particle physics, the electron is the first-
   generation charged lepton. It forms a weak isospin doublet with the
   electron neutrino; these two particles interact with each other through
   the both the charged and neutral current weak interaction. The electron
   is very similar to the two more massive particles of higher
   generations, the muon and the tau lepton, which are identical in
   charge, spin, interaction but differ in mass.

   The antimatter counterpart of the electron is 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. If the electron and
   positron had negligible momentum, each gamma ray will have an energy of
   0.511 MeV. See also Electron-positron annihilation.

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

History

   The electron as a unit of charge in electrochemistry was posited by G.
   Johnstone Stoney in 1874, who also coined the term electron in 1894.
   During the late 1890s a number of physicists posited that electricity
   could be conceived of as being made of discrete units, which were given
   a variety of names, but their reality had not been confirmed in a
   compelling way.

   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 ray tubes. A cathode ray tube
   is a sealed glass cylinder in which two electrodes are separated by a
   vacuum. When a voltage is applied across the electrodes, cathode rays
   are generated, causing the tube to glow. Through experimentation,
   Thomson discovered that the negative charge could not be separated from
   the rays (by the application of magnetism), and that the rays could be
   deflected by an electric field. He concluded that these rays, rather
   than being waves, were composed of negatively charged particles he
   called "corpuscles". He measured their mass-to-charge ratio and found
   it to be over a thousand times smaller than that of a hydrogen ion,
   suggesting that they were either very highly charged or very small in
   mass. Later experiments by other scientists upheld the latter
   conclusion.

   The electron's charge was carefully measured by Robert Millikan in his
   oil-drop experiment of 1909.

   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 in terms of 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 explained
   the chemical bonding of elements by electronic interactions.

        Quantum electrodynamics

                        electron |  positron | photon
            self-energy |  vacuum polarization |  vertex function
           Gupta-Bleuler formalism |  ξ gauge |  Ward identities
        Compton scattering |  Bhabha scattering |  Moeller scattering
                      anomalous magnetic dipole moment
                        bremsstrahlung |  positronium

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