Electron

Related Topics:
Electronica :: Electron :: Electronics_and_Electrical :: Electronic :: Electronic_Products :: Electronic_Text_Archives :: Electronic_Design_Automation :: Electronic_Postcards :: Electronic_Structure :: Electronic_Organs

Electron
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 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

Within an atom, electrons surround a nucleus composed of protons and neutrons in an electron configuration. The word electron was coined in 1894 by George Stoney and is derived from the term electric force introduced by William Gilbert. Its origin is the Greek: ήλεκτρον (elektron), meaning amber.

Electrons in motion produce an electric current, which is used by scientists and engineers to measure many physical properties. Electric current existing for a finite time gives rise to a movement of (charged) particles that may be harnessed as a practical means to perform work in the form known as 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 and the idea of particle-wave duality, that is, that electrons show either wave-like or particle-like properties. Equally important, particle physics has furthered our understanding of how the electron interacts with other particles.

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.

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). The word "particle" is somewhat misleading, however, because experiments show that electrons also behave like a wave, e.g., in the double-slit experiment; this is called wave-particle duality, also known by the term complementarity coined by Niels Bohr.

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 behavior

Electrons have a negative electric charge of −1.6022 × 10⁻¹⁹ coulombs, 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. The common electron symbol is e⁻. ^*

According to quantum mechanics, electrons can be represented by wavefunctions, from which the electron density can be determined. The orbital of each electron has its own wavefunction. Based on the Heisenberg uncertainty principle, the exact momentum and position of an 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. 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, this flow is called an electric current.

A body has a static 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 acquire charge through rubbing, by the phenomenon of triboelectricity. Electrons and positrons can annihilate each other and produce a pair of photons. However, high-energy photons may transform into an electron and a positron by a process called pair production or pair creation, but only in the presence of a nearby nucleus.

The electron is currently described as a fundamental particle or an elementary particle. It has no substructure (at least, experiments have found none so far, and there is good reason to think that there is none). Hence, for convenience, it is usually defined or assumed to be point-like, 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 is understood to influence 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).

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. It 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, optical, laser and quantum physics.

The speed of an electron can approach, but never reach c (the speed of light in a vacuum). This is due to an effect of special relativity. However, if 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 gamma or the Lorentz factor. Gamma is a function of v, the velocity of the particle, and c. The following is the formula for gamma:

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

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

\left(1-\frac {1} {2} \gamma ^{-2}\right)c

= 0.999 999 999 95 c.

(The formula applies for large γ.)

In practice

In the universe

Scientists believe that the number of electrons existing in the known universe is at least 10⁷⁹. This number amounts to an average 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.

In industry

Electron beams are used in welding and lithography.

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 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.

Usage

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 behavior of the electron can be described by quantum electrodynamics (QED), a U(1) gauge theory. In Dirac's model, an electron is 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. Interpretation of experiments indicates it forms a weak isospin doublet with the electron neutrino; the two particles interacting through the 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 with negligible momentum, 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.

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 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.

References

Brumfiel, G. (6 January 2005). Can electrons do the splits? In Nature, 433, 11.

External links

CODATA 2002 electron mass

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

Electron | Leptons

Gourt Home::Gourt :: Electron