Electron

Experiments with a Crookes tube first demonstrated the particle nature of electrons. In this illustration, the profile of the Maltese-cross-shaped
target is projected against the tube face at right by a beam of electrons.
Composition Elementary particle
Statistics Fermionic
Generation First
Interactions Gravity, Electromagnetic, Weak
Symbol e−, β−
Antiparticle Positron (also called antielectron)
Theorized Richard Laming (1838–1851),
G. Johnstone Stoney (1874) and others.
Discovered J. J. Thomson (1897)
Mass 9.10938291(40)×10−31 kg
5.4857990946(22)×10−4 u
[1822.8884845(14)]−1 u[1]
0.510998928(11) MeV/c2
Electric charge −1 e[2]
−1.602176565(35)×10−19 C
−4.80320451(10)×10−10 esu
Magnetic moment −1.00115965218076(27) μB
Spin 1⁄
2
The electron (symbol: e−) is a subatomic particle with a negative elementary electric charge. Electrons belong to the
first generation of the lepton particle family, and are generally thought to be elementary particles because they have
no known components or substructure. The electron has a mass that is approximately 1/1836 that of the proton.
Quantum mechanical properties of the electron include an intrinsic angular momentum (spin) of a half-integer value
in units of ħ, which means that it is a fermion. Being fermions, no two electrons can occupy the same quantum state,
in accordance with the Pauli exclusion principle. Electrons also have properties of both particles and waves, and so
can collide with other particles and can be diffracted like light. Experiments with electrons best demonstrate this
duality because electrons have a tiny mass.
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Many physical phenomena involve electrons in an essential role, such as electricity, magnetism, and thermal
conductivity, and they also participate in gravitational, electromagnetic and weak interactions. An electron in space
generates an electric field surrounding it. An electron moving relative to an observer generates a magnetic field;
external magnetic fields will deflect an electron. Electrons radiate or absorb energy in the form of photons when
accelerated. Laboratory instruments are capable of containing and observing individual electrons as well as electron
plasma using electromagnetic fields, whereas dedicated telescopes can detect electron plasma in outer space.
Electrons have many applications, including electronics, welding, cathode ray tubes, electron microscopes, radiation
therapy, lasers, gaseous ionization detectors and particle accelerators.
Interactions involving electrons and other subatomic particles are of interest in fields such as chemistry and nuclear
physics. The Coulomb force interaction between positive protons inside atomic nuclei and negative electrons
composes atoms. Ionization or changes in the proportions of particles changes the binding energy of the system. The
exchange or sharing of the electrons between two or more atoms is the main cause of chemical bonding. British
natural philosopher Richard Laming first hypothesized the concept of an indivisible quantity of electric charge to
explain the chemical properties of atoms in 1838; Irish physicist George Johnstone Stoney named this charge
'electron' in 1891, and J. J. Thomson and his team of British physicists identified it as a particle in 1897.[3] Electrons
can also participate in nuclear reactions, such as nucleosynthesis in stars, where they are known as beta particles.
Electrons may be created through beta decay of radioactive isotopes and in high-energy collisions, for instance when
cosmic rays enter the atmosphere. The antiparticle of the electron is called the positron; it is identical to the electron
except that it carries electrical and other charges of the opposite sign. When an electron collides with a positron, both
particles may be totally annihilated, producing gamma ray photons.
History
The ancient Greeks noticed that amber attracted small objects when rubbed with fur. Along with lightning, this
phenomenon is one of humanity's earliest recorded experiences with electricity. In his 1600 treatise De Magnete, the
English scientist William Gilbert coined the New Latin term electricus, to refer to this property of attracting small
objects after being rubbed. Both electric and electricity are derived from the Latin ēlectrum (also the root of the alloy
of the same name), which came from the Greek word for amber, ήλεκτρον (ēlektron).
In the early 1700s, Francis Hauksbee and French chemist Charles François de Fay independently discovered what
they believed to be two kinds of frictional electricity; one generated from rubbing glass, the other from rubbing resin.
From this, Du Fay theorized that electricity consists of two electrical fluids, "vitreous" and "resinous", that are
separated by friction and that neutralize each other when combined. A decade later Benjamin Franklin proposed that
electricity was not from different types of electrical fluid, but the same electrical fluid under different pressures. He
gave them the modern charge nomenclature of positive and negative respectively. Franklin thought of the charge
carrier as being positive, but he did not correctly identify which situation was a surplus of the charge carrier, and
which situation was a deficit.
Between 1838 and 1851, British natural philosopher Richard Laming developed the idea that an atom is composed of
a core of matter surrounded by subatomic particles that had unit electric charges. Beginning in 1846, German
physicist William Weber theorized that electricity was composed of positively and negatively charged fluids, and
their interaction was governed by the inverse square law. After studying the phenomenon of electrolysis in 1874,
Irish physicist George Johnstone Stoney suggested that there existed a "single definite quantity of electricity", the
charge of a monovalent ion. He was able to estimate the value of this elementary charge e by means of Faraday's
laws of electrolysis. However, Stoney believed these charges were permanently attached to atoms and could not be
removed. In 1881, German physicist Hermann von Helmholtz argued that both positive and negative charges were
divided into elementary parts, each of which "behaves like atoms of electricity".
In 1891 Stoney coined the term electron to describe these elementary charges, writing later in 1894: "... an estimate
was made of the actual amount of this most remarkable fundamental unit of electricity, for which I have since
Electron 3
ventured to suggest the name electron". The word electron is a combination of the words electr(ic) and (i)on.[4] The
suffix -on which is now used to designate other subatomic particles, such as a proton or neutron, is in turn derived
from electron.
Discovery
A beam of electrons deflected in a circle by a
magnetic field
The German physicist Johann Wilhelm Hittorf studied electrical
conductivity in rarefied gases: in 1869, he discovered a glow emitted
from the cathode that increased in size with decrease in gas pressure. In
1876, the German physicist Eugen Goldstein showed that the rays from
this glow cast a shadow, and he dubbed the rays cathode rays.[5]
During the 1870s, the English chemist and physicist Sir William
Crookes developed the first cathode ray tube to have a high vacuum
inside. He then showed that the luminescence rays appearing within the
tube carried energy and moved from the cathode to the anode.
Furthermore, by applying a magnetic field, he was able to deflect the
rays, thereby demonstrating that the beam behaved as though it were negatively charged.[6] In 1879, he proposed that
these properties could be explained by what he termed 'radiant matter'. He suggested that this was a fourth state of
matter, consisting of negatively charged molecules that were being projected with high velocity from the cathode.
The German-born British physicist Arthur Schuster expanded upon Crookes' experiments by placing metal plates
parallel to the cathode rays and applying an electric potential between the plates. The field deflected the rays toward
the positively charged plate, providing further evidence that the rays carried negative charge. By measuring the
amount of deflection for a given level of current, in 1890 Schuster was able to estimate the charge-to-mass ratio of
the ray components. However, this produced a value that was more than a thousand times greater than what was
expected, so little credence was given to his calculations at the time.[7]
In 1892 Hendrik Lorentz suggested that the mass of these particles (electrons) could be a consequence of their
electric charge.[8]
In 1896, the British physicist J. J. Thomson, with his colleagues John S. Townsend and H. A. Wilson, performed
experiments indicating that cathode rays really were unique particles, rather than waves, atoms or molecules as was
believed earlier. Thomson made good estimates of both the charge e and the mass m, finding that cathode ray
particles, which he called "corpuscles," had perhaps one thousandth of the mass of the least massive ion known:
hydrogen. He showed that their charge to mass ratio, e/m, was independent of cathode material. He further showed
that the negatively charged particles produced by radioactive materials, by heated materials and by illuminated
materials were universal. The name electron was again proposed for these particles by the Irish physicist George F.
Fitzgerald, and the name has since gained universal acceptance.
While studying naturally fluorescing minerals in 1896, the French physicist Henri Becquerel discovered that they
emitted radiation without any exposure to an external energy source. These radioactive materials became the subject
of much interest by scientists, including the New Zealand physicist Ernest Rutherford who discovered they emitted
particles. He designated these particles alpha and beta, on the basis of their ability to penetrate matter. In 1900,
Becquerel showed that the beta rays emitted by radium could be deflected by an electric field, and that their
mass-to-charge ratio was the same as for cathode rays. This evidence strengthened the view that electrons existed as
components of atoms.[9]
The electron's charge was more carefully measured by the American physicists Robert Millikan and Harvey Fletcher
in their oil-drop experiment of 1909, the results of which were published in 1911. This experiment used an electric
field to prevent a charged droplet of oil from falling as a result of gravity. This device could measure the electric
charge from as few as 1–150 ions with an error margin of less than 0.3%. Comparable experiments had been done
Electron 4
earlier by Thomson's team, using clouds of charged water droplets generated by electrolysis, and in 1911 by Abram
Ioffe, who independently obtained the same result as Millikan using charged microparticles of metals, then published
his results in 1913.[10] However, oil drops were more stable than water drops because of their slower evaporation
rate, and thus more suited to precise experimentation over longer periods of time.
Around the beginning of the twentieth century, it was found that under certain conditions a fast-moving charged
particle caused a condensation of supersaturated water vapor along its path. In 1911, Charles Wilson used this
principle to devise his cloud chamber, allowing the tracks of charged particles, such as fast-moving electrons, to be
photographed.
Atomic theory
The Bohr model of the atom, showing states of
electron with energy quantized by the number n.
An electron dropping to a lower orbit emits a
photon equal to the energy difference between the
orbits.
By 1914, experiments by physicists Ernest Rutherford, Henry Moseley,
James Franck and Gustav Hertz had largely established the structure of
an atom as a dense nucleus of positive charge surrounded by
lower-mass electrons. In 1913, Danish physicist Niels Bohr postulated
that electrons resided in quantized energy states, with the energy
determined by the angular momentum of the electron's orbits about the
nucleus. The electrons could move between these states, or orbits, by
the emission or absorption of photons at specific frequencies. By
means of these quantized orbits, he accurately explained the spectral
lines of the hydrogen atom. However, Bohr's model failed to account
for the relative intensities of the spectral lines and it was unsuccessful
in explaining the spectra of more complex atoms.
Chemical bonds between atoms were explained by Gilbert Newton
Lewis, who in 1916 proposed that a covalent bond between two atoms
is maintained by a pair of electrons shared between them. Later, in
1927, Walter Heitler and Fritz London gave the full explanation of the
electron-pair formation and chemical bonding in terms of quantum mechanics. In 1919, the American chemist Irving
Langmuir elaborated on the Lewis' static model of the atom and suggested that all electrons were distributed in
successive "concentric (nearly) spherical shells, all of equal thickness". The shells were, in turn, divided by him in a
number of cells each containing one pair of electrons. With this model Langmuir was able to qualitatively explain
the chemical properties of all elements in the periodic table, which were known to largely repeat themselves
according to the periodic law.
In 1924, Austrian physicist Wolfgang Pauli observed that the shell-like structure of the atom could be explained by a
set of four parameters that defined every quantum energy state, as long as each state was inhabited by no more than a
single electron. (This prohibition against more than one electron occupying the same quantum energy state became
known as the Pauli exclusion principle.) The physical mechanism to explain the fourth parameter, which had two
distinct possible values, was provided by the Dutch physicists Samuel Goudsmit and George Uhlenbeck. In 1925,
Goudsmit and Uhlenbeck suggested that an electron, in addition to the angular momentum of its orbit, possesses an
intrinsic angular momentum and magnetic dipole moment. The intrinsic angular momentum became known as spin,
and explained the previously mysterious splitting of spectral lines observed with a high-resolution spectrograph; this
phenomenon is known as fine structure splitting.
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Quantum mechanics
In his 1924 dissertation Recherches sur la théorie des quanta (Research on Quantum Theory), French physicist
Louis de Broglie hypothesized that all matter possesses a de Broglie wave similar to light. That is, under the
appropriate conditions, electrons and other matter would show properties of either particles or waves. The
corpuscular properties of a particle are demonstrated when it is shown to have a localized position in space along its
trajectory at any given moment. Wave-like nature is observed, for example, when a beam of light is passed through
parallel slits and creates interference patterns. In 1927, the interference effect was found in a beam of electrons by
English physicist George Paget Thomson with a thin metal film and by American physicists Clinton Davisson and
Lester Germer using a crystal of nickel.
In quantum mechanics, the behavior of an
electron in an atom is described by an orbital,
which is a probability distribution rather than an
orbit. In the figure, the shading indicates the
relative probability to "find" the electron, having
the energy corresponding to the given quantum
numbers, at that point.
De Broglie's prediction of a wave nature for electrons led Erwin
Schrödinger to postulate a wave equation for electrons moving under
the influence of the nucleus in the atom. In 1926, this equation, the
Schrödinger equation, successfully described how electron waves
propagated. Rather than yielding a solution that determined the
location of an electron over time, this wave equation also could be used
to predict the probability of finding an electron near a position,
especially a position near where the electron was bound in space, for
which the electron wave equations did not change in time. This
approach led to a second formulation of quantum mechanics (the first
being by Heisenberg in 1925), and solutions of Schrödinger's equation,
like Heisenberg's, provided derivations of the energy states of an
electron in a hydrogen atom that were equivalent to those that had been
derived first by Bohr in 1913, and that were known to reproduce the
hydrogen spectrum. Once spin and the interaction between multiple
electrons were considered, quantum mechanics later allowed the
configuration of electrons in atoms with higher atomic numbers than
hydrogen to be successfully predicted.
In 1928, building on Wolfgang Pauli's work, Paul Dirac produced a
model of the electron – the Dirac equation, consistent with relativity
theory, by applying relativistic and symmetry considerations to the hamiltonian formulation of the quantum
mechanics of the electro-magnetic field. In order to resolve some problems within his relativistic equation, in 1930
Dirac developed a model of the vacuum as an infinite sea of particles having negative energy, which was dubbed the
Dirac sea. This led him to predict the existence of a positron, the antimatter counterpart of the electron. This particle
was discovered in 1932 by Carl Anderson, who proposed calling standard electrons negatrons, and using electron as
a generic term to describe both the positively and negatively charged variants.
In 1947 Willis Lamb, working in collaboration with graduate student Robert Retherford, found that certain quantum
states of hydrogen atom, which should have the same energy, were shifted in relation to each other, the difference
being the Lamb shift. About the same time, Polykarp Kusch, working with Henry M. Foley, discovered the magnetic
moment of the electron is slightly larger than predicted by Dirac's theory. This small difference was later called
anomalous magnetic dipole moment of the electron. This difference was later explained by the theory of quantum
electrodynamics, developed by Sin-Itiro Tomonaga, Julian Schwinger and Richard Feynman in the late 1940s.
Electron 6
Particle accelerators
With the development of the particle accelerator during the first half of the twentieth century, physicists began to
delve deeper into the properties of subatomic particles. The first successful attempt to accelerate electrons using
electromagnetic induction was made in 1942 by Donald Kerst. His initial betatron reached energies of 2.3 MeV,
while subsequent betatrons achieved 300 MeV. In 1947, synchrotron radiation was discovered with a 70 MeV
electron synchrotron at General Electric. This radiation was caused by the acceleration of electrons, moving near the
speed of light, through a magnetic field.
With a beam energy of 1.5 GeV, the first high-energy particle collider was ADONE, which began operations in
1968. This device accelerated electrons and positrons in opposite directions, effectively doubling the energy of their
collision when compared to striking a static target with an electron. The Large Electron–Positron Collider (LEP) at
CERN, which was operational from 1989 to 2000, achieved collision energies of 209 GeV and made important
measurements for the Standard Model of particle physics.
Confinement of individual electrons
Individual electrons can now be easily confined in ultra small (L=20 nm, W=20 nm) CMOS transistors operated at
cryogenic temperature over a range of −269 °C (4 K) to about −258 °C (15 K). The electron wavefunction spreads in
a semiconductor lattice and negligibly interacts with the valence band electrons, so it can be treated in the single
particle formalism, by replacing its mass with the effective mass tensor.
Characteristics
Classification
Standard Model of elementary particles. The electron is at lower left.
In the Standard Model of particle physics,
electrons belong to the group of subatomic
particles called leptons, which are believed
to be fundamental or elementary particles.
Electrons have the lowest mass of any
charged lepton (or electrically charged
particle of any type) and belong to the
first-generation of fundamental particles.
The second and third generation contain
charged leptons, the muon and the tau,
which are identical to the electron in charge,
spin and interactions, but are more massive.
Leptons differ from the other basic
constituent of matter, the quarks, by their
lack of strong interaction. All members of
the lepton group are fermions, because they
all have half-odd integer spin; the electron has spin 1⁄
2.
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Fundamental properties
The invariant mass of an electron is approximately 9.109×10−31 kilograms, or 5.489×10−4 atomic mass units. On the
basis of Einstein's principle of mass–energy equivalence, this mass corresponds to a rest energy of 0.511 MeV. The
ratio between the mass of a proton and that of an electron is about 1836. Astronomical measurements show that the
proton-to-electron mass ratio has held the same value for at least half the age of the universe, as is predicted by the
Standard Model.
Electrons have an electric charge of −1.602×10−19 coulomb,[] which is used as a standard unit of charge for
subatomic particles, and is also called the elementary charge. This elementary charge has a relative standard
uncertainty of 2.2×10−8.[] Within the limits of experimental accuracy, the electron charge is identical to the charge of
a proton, but with the opposite sign. As the symbol e is used for the elementary charge, the electron is commonly
symbolized by e−, where the minus sign indicates the negative charge. The positron is symbolized by e+ because it
has the same properties as the electron but with a positive rather than negative charge.
The electron has an intrinsic angular momentum or spin of 1⁄
2. This property is usually stated by referring to the
electron as a spin-1⁄
2 particle. For such particles the spin magnitude is √3⁄
2 ħ.[11] while the result of the measurement
of a projection of the spin on any axis can only be ±ħ⁄
2. In addition to spin, the electron has an intrinsic magnetic
moment along its spin axis. It is approximately equal to one Bohr magneton,[12] which is a physical constant equal to
9.27400915(23)×10−24 joules per tesla. The orientation of the spin with respect to the momentum of the electron
defines the property of elementary particles known as helicity.
The electron has no known substructure. Hence, it is defined or assumed to be a point particle with a point charge
and no spatial extent. Observation of a single electron in a Penning trap shows the upper limit of the particle's radius
is 10−22 meters. There is a physical constant called the "classical electron radius", with the much larger value of
2.8179×10−15 m. However, the terminology comes from a simplistic calculation that ignores the effects of quantum
mechanics; in reality, the so-called classical electron radius has little to do with the true fundamental structure of the
electron.[13]
There are elementary particles that spontaneously decay into less massive particles. An example is the muon, which
decays into an electron, a neutrino and an antineutrino, with a mean lifetime of 2.2×10−6 seconds. However, the
electron is thought to be stable on theoretical grounds: the electron is the least massive particle with non-zero electric
charge, so its decay would violate charge conservation. The experimental lower bound for the electron's mean
lifetime is 4.6×1026 years, at a 90% confidence level.
Quantum properties
As with all particles, electrons can act as waves. This is called the wave–particle duality and can be demonstrated
using the double-slit experiment. The wave-like nature of the electron allows it to pass through two parallel slits
simultaneously, rather than just one slit as would be the case for a classical particle. In quantum mechanics, the
wave-like property of one particle can be described mathematically as a complex-valued function, the wave function,
commonly denoted by the Greek letter psi (ψ). When the absolute value of this function is squared, it gives the
probability that a particle will be observed near a location—a probability density.:162-218
Electron 8
Example of an antisymmetric wave function for a
quantum state of two identical fermions in a
1-dimensional box. If the particles swap position,
the wave function inverts its sign.
Electrons are identical particles because they cannot be distinguished
from each other by their intrinsic physical properties. In quantum
mechanics, this means that a pair of interacting electrons must be able
to swap positions without an observable change to the state of the
system. The wave function of fermions, including electrons, is
antisymmetric, meaning that it changes sign when two electrons are
swapped; that is, ψ(r1, r2) = −ψ(r2, r1), where the variables r1 and r2
correspond to the first and second electrons, respectively. Since the
absolute value is not changed by a sign swap, this corresponds to equal
probabilities. Bosons, such as the photon, have symmetric wave
functions instead.:162-218
In the case of antisymmetry, solutions of the wave equation for
interacting electrons result in a zero probability that each pair will
occupy the same location or state. This is responsible for the Pauli exclusion principle, which precludes any two
electrons from occupying the same quantum state. This principle explains many of the properties of electrons. For
example, it causes groups of bound electrons to occupy different orbitals in an atom, rather than all overlapping each
other in the same orbit.:162-218
Virtual particles
Physicists believe that empty space may be continually creating pairs of virtual particles, such as a positron and
electron, which rapidly annihilate each other shortly thereafter. The combination of the energy variation needed to
create these particles, and the time during which they exist, fall under the threshold of detectability expressed by the
Heisenberg uncertainty relation, ΔE · Δt ≥ ħ. In effect, the energy needed to create these virtual particles, ΔE, can be
"borrowed" from the vacuum for a period of time, Δt, so that their product is no more than the reduced Planck
constant, ħ ≈ 6.6×10−16 eV·s. Thus, for a virtual electron, Δt is at most 1.3×10−21 s.
A schematic depiction of virtual
electron–positron pairs appearing at random near
an electron (at lower left)
While an electron–positron virtual pair is in existence, the coulomb
force from the ambient electric field surrounding an electron causes a
created positron to be attracted to the original electron, while a created
electron experiences a repulsion. This causes what is called vacuum
polarization. In effect, the vacuum behaves like a medium having a
dielectric permittivity more than unity. Thus the effective charge of an
electron is actually smaller than its true value, and the charge decreases
with increasing distance from the electron. This polarization was
confirmed experimentally in 1997 using the Japanese TRISTAN
particle accelerator. Virtual particles cause a comparable shielding
effect for the mass of the electron.[14]
The interaction with virtual particles also explains the small (about
0.1%) deviation of the intrinsic magnetic moment of the electron from the Bohr magneton (the anomalous magnetic
moment). The extraordinarily precise agreement of this predicted difference with the experimentally determined
value is viewed as one of the great achievements of quantum electrodynamics.
In classical physics, the angular momentum and magnetic moment of an object depend upon its physical dimensions.
Hence, the concept of a dimensionless electron possessing these properties might seem inconsistent. The apparent
paradox can be explained by the formation of virtual photons in the electric field generated by the electron. These
photons cause the electron to shift about in a jittery fashion (known as zitterbewegung), which results in a net
circular motion with precession. This motion produces both the spin and the magnetic moment of the electron. In
Electron 9
atoms, this creation of virtual photons explains the Lamb shift observed in spectral lines.
Interaction
An electron generates an electric field that exerts an attractive force on a particle with a positive charge, such as the
proton, and a repulsive force on a particle with a negative charge. The strength of this force is determined by
Coulomb's inverse square law. When an electron is in motion, it generates a magnetic field.:140 The
Ampère-Maxwell law relates the magnetic field to the mass motion of electrons (the current) with respect to an
observer. It is this property of induction which supplies the magnetic field that drives an electric motor. The
electromagnetic field of an arbitrary moving charged particle is expressed by the Liénard–Wiechert potentials, which
are valid even when the particle's speed is close to that of light (relativistic).
A particle with charge q (at left) is moving with
velocity v through a magnetic field B that is
oriented toward the viewer. For an electron, q is
negative so it follows a curved trajectory toward
the top.
When an electron is moving through a magnetic field, it is subject to
the Lorentz force that exerts an influence in a direction perpendicular
to the plane defined by the magnetic field and the electron velocity.
This centripetal force causes the electron to follow a helical trajectory
through the field at a radius called the gyroradius. The acceleration
from this curving motion induces the electron to radiate energy in the
form of synchrotron radiation.:160[15] The energy emission in turn
causes a recoil of the electron, known as the Abraham–Lorentz–Dirac
Force, which creates a friction that slows the electron. This force is
caused by a back-reaction of the electron's own field upon itself.
Photons mediate electromagnetic interactions between particles in
quantum electrodynamics. An isolated electron at a constant velocity
cannot emit or absorb a real photon; doing so would violate
conservation of energy and momentum. Instead, virtual photons can
transfer momentum between two charged particles. This exchange of virtual photons, for example, generates the
Coulomb force. Energy emission can occur when a moving electron is deflected by a charged particle, such as a
proton. The acceleration of the electron results in the emission of Bremsstrahlung radiation.
Here, Bremsstrahlung is produced by an electron
e deflected by the electric field of an atomic
nucleus. The energy change E2 − E1 determines
the frequency f of the emitted photon.
An inelastic collision between a photon (light) and a solitary (free)
electron is called Compton scattering. This collision results in a
transfer of momentum and energy between the particles, which
modifies the wavelength of the photon by an amount called the
Compton shift.[16] The maximum magnitude of this wavelength shift is
h/mec, which is known as the Compton wavelength. For an electron, it
has a value of 2.43×10−12 m. When the wavelength of the light is long
(for instance, the wavelength of the visible light is 0.4–0.7 μm) the
wavelength shift becomes negligible. Such interaction between the
light and free electrons is called Thomson scattering or Linear
Thomson scattering.
The relative strength of the electromagnetic interaction between two
charged particles, such as an electron and a proton, is given by the
fine-structure constant. This value is a dimensionless quantity formed
by the ratio of two energies: the electrostatic energy of attraction (or
repulsion) at a separation of one Compton wavelength, and the rest
energy of the charge. It is given by α ≈ 7.297353×10−3, which is
approximately equal to 1⁄
137.
Electron 10
When electrons and positrons collide, they annihilate each other, giving rise to two or more gamma ray photons. If
the electron and positron have negligible momentum, a positronium atom can form before annihilation results in two
or three gamma ray photons totalling 1.022 MeV. 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.
In the theory of electroweak interaction, the left-handed component of electron's wavefunction forms a weak isospin
doublet with the electron neutrino. This means that during weak interactions, electron neutrinos behave like
electrons. Either member of this doublet can undergo a charged current interaction by emitting or absorbing a W and
be converted into the other member. Charge is conserved during this reaction because the W boson also carries a
charge, canceling out any net change during the transmutation. Charged current interactions are responsible for the
phenomenon of beta decay in a radioactive atom. Both the electron and electron neutrino can undergo a neutral
current interaction via a Z0 exchange, and this is responsible for neutrino-electron elastic scattering.
Atoms and molecules
Probability densities for the first few hydrogen atom orbitals, seen in
cross-section. The energy level of a bound electron determines the orbital it
occupies, and the color reflects the probability to find the electron at a given
position.
An electron can be bound to the nucleus of an
atom by the attractive Coulomb force. A system
of one or more electrons bound to a nucleus is
called an atom. If the number of electrons is
different from the nucleus' electrical charge,
such an atom is called an ion. The wave-like
behavior of a bound electron is described by a
function called an atomic orbital. Each orbital
has its own set of quantum numbers such as
energy, angular momentum and projection of
angular momentum, and only a discrete set of
these orbitals exist around the nucleus.
According to the Pauli exclusion principle each
orbital can be occupied by up to two electrons,
which must differ in their spin quantum number.
Electrons can transfer between different orbitals
by the emission or absorption of photons with an
energy that matches the difference in potential.
Other methods of orbital transfer include
collisions with particles, such as electrons, and
the Auger effect. In order to escape the atom, the energy of the electron must be increased above its binding energy
to the atom. This occurs, for example, with the photoelectric effect, where an incident photon exceeding the atom's
ionization energy is absorbed by the electron.
The orbital angular momentum of electrons is quantized. Because the electron is charged, it produces an orbital
magnetic moment that is proportional to the angular momentum. The net magnetic moment of an atom is equal to the
vector sum of orbital and spin magnetic moments of all electrons and the nucleus. The magnetic moment of the
nucleus is negligible compared with that of the electrons. The magnetic moments of the electrons that occupy the
same orbital (so called, paired electrons) cancel each other out.
The chemical bond between atoms occurs as a result of electromagnetic interactions, as described by the laws of
quantum mechanics. The strongest bonds are formed by the sharing or transfer of electrons between atoms, allowing
the formation of molecules. Within a molecule, electrons move under the influence of several nuclei, and occupy
Electron 11
molecular orbitals; much as they can occupy atomic orbitals in isolated atoms. A fundamental factor in these
molecular structures is the existence of electron pairs. These are electrons with opposed spins, allowing them to
occupy the same molecular orbital without violating the Pauli exclusion principle (much like in atoms). Different
molecular orbitals have different spatial distribution of the electron density. For instance, in bonded pairs (i.e. in the
pairs that actually bind atoms together) electrons can be found with the maximal probability in a relatively small
volume between the nuclei. On the contrary, in non-bonded pairs electrons are distributed in a large volume around
nuclei.
Conductivity
A lightning discharge consists primarily of a flow
of electrons. The electric potential needed for
lightning may be generated by a triboelectric
effect.
If a body has more or fewer electrons than are required to balance the
positive charge of the nuclei, then that object has a net electric charge.
When there is an excess of electrons, the object is said to be negatively
charged. When there are fewer electrons than the number of protons in
nuclei, 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 triboelectric effect.
Independent electrons moving in vacuum are termed free electrons.
Electrons in metals also behave as if they were free. In reality the
particles that are commonly termed electrons in metals and other solids
are quasi-electrons—quasiparticles, which have the same electrical
charge, spin and magnetic moment as real electrons but may have a
different mass. When free electrons—both in vacuum and
metals—move, they produce a net flow of charge called an electric
current, which generates a magnetic field. Likewise a current can be created by a changing magnetic field. These
interactions are described mathematically by Maxwell's equations.
At a given temperature, each material has an electrical conductivity that determines the value of electric current
when an electric potential is applied. Examples of good conductors include metals such as copper and gold, whereas
glass and Teflon are poor conductors. In any dielectric material, the electrons remain bound to their respective atoms
and the material behaves as an insulator. Most semiconductors have a variable level of conductivity that lies between
the extremes of conduction and insulation. On the other hand, metals have an electronic band structure containing
partially filled electronic bands. The presence of such bands allows electrons in metals to behave as if they were free
or delocalized electrons. These electrons are not associated with specific atoms, so when an electric field is applied,
they are free to move like a gas (called Fermi gas) through the material much like free electrons.
Because of collisions between electrons and atoms, the drift velocity of electrons in a conductor is on the order of
millimeters per second. However, the speed at which a change of current at one point in the material causes changes
in currents in other parts of the material, the velocity of propagation, is typically about 75% of light speed. This
occurs because electrical signals propagate as a wave, with the velocity dependent on the dielectric constant of the
material.
Metals make relatively good conductors of heat, primarily because the delocalized electrons are free to transport
thermal energy between atoms. However, unlike electrical conductivity, the thermal conductivity of a metal is nearly
independent of temperature. This is expressed mathematically by the Wiedemann–Franz law, which states that the
ratio of thermal conductivity to the electrical conductivity is proportional to the temperature. The thermal disorder in
the metallic lattice increases the electrical resistivity of the material, producing a temperature dependence for
electrical current.
Electron 12
When cooled below a point called the critical temperature, materials can undergo a phase transition in which they
lose all resistivity to electrical current, in a process known as superconductivity. In BCS theory, this behavior is
modeled by pairs of electrons entering a quantum state known as a Bose–Einstein condensate. These Cooper pairs
have their motion coupled to nearby matter via lattice vibrations called phonons, thereby avoiding the collisions with
atoms that normally create electrical resistance. (Cooper pairs have a radius of roughly 100 nm, so they can overlap
each other.) However, the mechanism by which higher temperature superconductors operate remains uncertain.
Electrons inside conducting solids, which are quasi-particles themselves, when tightly confined at temperatures close
to absolute zero, behave as though they had split into three other quasiparticles: spinons, Orbitons and holons. The
former carries spin and magnetic moment, the next carries its orbital location while the latter electrical charge.
Motion and energy
According to Einstein's theory of special relativity, as an electron's speed approaches the speed of light, from an
observer's point of view its relativistic mass increases, thereby making it more and more difficult to accelerate it
from within the observer's frame of reference. The speed of an electron can approach, but never reach, the speed of
light in a vacuum, c. However, when relativistic electrons—that is, electrons moving at a speed close to c—are
injected into a dielectric medium such as water, where the local speed of light is significantly less than c, the
electrons temporarily travel faster than light in the medium. As they interact with the medium, they generate a faint
light called Cherenkov radiation.
Lorentz factor as a function of velocity. It starts at
value 1 and goes to infinity as v approaches c.
The effects of special relativity are based on a quantity known as the
Lorentz factor, defined as where v is the speed of the
particle. The kinetic energy Ke of an electron moving with velocity v is:
where me is the mass of electron. For example, the Stanford linear
accelerator can accelerate an electron to roughly 51 GeV. Since an
electron behaves as a wave, at a given velocity it has a characteristic de
Broglie wavelength. This is given by λe = h/p where h is the Planck
constant and p is the momentum. For the 51 GeV electron above, the
wavelength is about 2.4×10−17 m, small enough to explore structures
well below the size of an atomic nucleus.
Formation
Electron 13
Pair production caused by the collision of a
photon with an atomic nucleus
The Big Bang theory is the most widely accepted scientific theory to
explain the early stages in the evolution of the Universe. For the first
millisecond of the Big Bang, the temperatures were over
10 billion Kelvin and photons had mean energies over a million
electronvolts. These photons were sufficiently energetic that they could
react with each other to form pairs of electrons and positrons.
Likewise, positron-electron pairs annihilated each other and emitted
energetic photons:
γ + γ ↔ e+ + e−
An equilibrium between electrons, positrons and photons was
maintained during this phase of the evolution of the Universe. After 15
seconds had passed, however, the temperature of the universe dropped below the threshold where electron-positron
formation could occur. Most of the surviving electrons and positrons annihilated each other, releasing gamma
radiation that briefly reheated the universe.
For reasons that remain uncertain, during the process of leptogenesis there was an excess in the number of electrons
over positrons. Hence, about one electron in every billion survived the annihilation process. This excess matched the
excess of protons over antiprotons, in a condition known as baryon asymmetry, resulting in a net charge of zero for
the universe. The surviving protons and neutrons began to participate in reactions with each other—in the process
known as nucleosynthesis, forming isotopes of hydrogen and helium, with trace amounts of lithium. This process
peaked after about five minutes. Any leftover neutrons underwent negative beta decay with a half-life of about a
thousand seconds, releasing a proton and electron in the process,
n → p + e− + ν
e
For about the next 300000–400000 years, the excess electrons remained too energetic to bind with atomic nuclei.
What followed is a period known as recombination, when neutral atoms were formed and the expanding universe
became transparent to radiation.
Roughly one million years after the big bang, the first generation of stars began to form. Within a star, stellar
nucleosynthesis results in the production of positrons from the fusion of atomic nuclei. These antimatter particles
immediately annihilate with electrons, releasing gamma rays. The net result is a steady reduction in the number of
electrons, and a matching increase in the number of neutrons. However, the process of stellar evolution can result in
the synthesis of radioactive isotopes. Selected isotopes can subsequently undergo negative beta decay, emitting an
electron and antineutrino from the nucleus. An example is the cobalt-60 (60Co) isotope, which decays to form
nickel-60 (60Ni).
Electron 14
An extended air shower generated by an energetic cosmic ray striking the
Earth's atmosphere
At the end of its lifetime, a star with more than
about 20 solar masses can undergo gravitational
collapse to form a black hole. According to
classical physics, these massive stellar objects
exert a gravitational attraction that is strong
enough to prevent anything, even
electromagnetic radiation, from escaping past the
Schwarzschild radius. However, quantum
mechanical effects are believed to potentially
allow the emission of Hawking radiation at this
distance. Electrons (and positrons) are thought to
be created at the event horizon of these stellar
remnants.
When pairs of virtual particles (such as an
electron and positron) are created in the vicinity
of the event horizon, the random spatial
distribution of these particles may permit one of them to appear on the exterior; this process is called quantum
tunnelling. The gravitational potential of the black hole can then supply the energy that transforms this virtual
particle into a real particle, allowing it to radiate away into space. In exchange, the other member of the pair is given
negative energy, which results in a net loss of mass-energy by the black hole. The rate of Hawking radiation
increases with decreasing mass, eventually causing the black hole to evaporate away until, finally, it explodes.
Cosmic rays are particles traveling through space with high energies. Energy events as high as 3.0×1020 eV have
been recorded. When these particles collide with nucleons in the Earth's atmosphere, a shower of particles is
generated, including pions. More than half of the cosmic radiation observed from the Earth's surface consists of
muons. The particle called a muon is a lepton which is produced in the upper atmosphere by the decay of a pion.
π− → μ− + ν
μ
A muon, in turn, can decay to form an electron or positron.
μ− → e− + ν
e + ν
μ
Observation
Aurorae are mostly caused by energetic electrons
precipitating into the atmosphere.
Remote observation of electrons requires detection of their radiated
energy. For example, in high-energy environments such as the corona
of a star, free electrons form a plasma that radiates energy due to
Bremsstrahlung radiation. Electron gas can undergo plasma oscillation,
which is waves caused by synchronized variations in electron density,
and these produce energy emissions that can be detected by using radio
telescopes.
The frequency of a photon is proportional to its energy. As a bound
electron transitions between different energy levels of an atom, it will
Electron 15
absorb or emit photons at characteristic frequencies. For instance, when atoms are irradiated by a source with a broad
spectrum, distinct absorption lines will appear in the spectrum of transmitted radiation. Each element or molecule
displays a characteristic set of spectral lines, such as the hydrogen spectral series. Spectroscopic measurements of the
strength and width of these lines allow the composition and physical properties of a substance to be determined.
In laboratory conditions, the interactions of individual electrons can be observed by means of particle detectors,
which allow measurement of specific properties such as energy, spin and charge. The development of the Paul trap
and Penning trap allows charged particles to be contained within a small region for long durations. This enables
precise measurements of the particle properties. For example, in one instance a Penning trap was used to contain a
single electron for a period of 10 months. The magnetic moment of the electron was measured to a precision of
eleven digits, which, in 1980, was a greater accuracy than for any other physical constant.
The first video images of an electron's energy distribution were captured by a team at Lund University in Sweden,
February 2008. The scientists used extremely short flashes of light, called attosecond pulses, which allowed an
electron's motion to be observed for the first time.
The distribution of the electrons in solid materials can be visualized by angle-resolved photoemission spectroscopy
(ARPES). This technique employs the photoelectric effect to measure the reciprocal space—a mathematical
representation of periodic structures that is used to infer the original structure. ARPES can be used to determine the
direction, speed and scattering of electrons within the material.
Plasma applications
Particle beams
During a NASA wind tunnel test, a model of the
Space Shuttle is targeted by a beam of electrons,
simulating the effect of ionizing gases during
re-entry.
Electron beams are used in welding. They allow energy densities up to
107 W·cm−2 across a narrow focus diameter of 0.1–1.3 mm and usually
require no filler material. This welding technique must be performed in
a vacuum to prevent the electrons from interacting with the gas before
reaching their target, and it can be used to join conductive materials
that would otherwise be considered unsuitable for welding.
Electron-beam lithography (EBL) is a method of etching
semiconductors at resolutions smaller than a micrometer. This
technique is limited by high costs, slow performance, the need to
operate the beam in the vacuum and the tendency of the electrons to
scatter in solids. The last problem limits the resolution to about 10 nm.
For this reason, EBL is primarily used for the production of small
numbers of specialized integrated circuits.
Electron beam processing is used to irradiate materials in order to change their physical properties or sterilize
medical and food products. Electron beams fluidise or quasi-melt glasses without significant increase of temperature
on intensive irradiation: e.g. intensive electron radiation causes a many orders of magnitude decrease of viscosity
and stepwise decrease of its activation energy.[17]
Linear particle accelerators generate electron beams for treatment of superficial tumors in radiation therapy. Electron
therapy can treat such skin lesions as basal-cell carcinomas because an electron beam only penetrates to a limited
depth before being absorbed, typically up to 5 cm for electron energies in the range 5–20 MeV. An electron beam
can be used to supplement the treatment of areas that have been irradiated by X-rays.
Particle accelerators use electric fields to propel electrons and their antiparticles to high energies. These particles
emit synchrotron radiation as they pass through magnetic fields. The dependency of the intensity of this radiation
upon spin polarizes the electron beam—a process known as the Sokolov–Ternov effect.[18] Polarized electron beams
Electron 16
can be useful for various experiments. Synchrotron radiation can also cool the electron beams to reduce the
momentum spread of the particles. Electron and positron beams are collided upon the particles' accelerating to the
required energies; particle detectors observe the resulting energy emissions, which particle physics studies .
Imaging
Low-energy electron diffraction (LEED) is a method of bombarding a crystalline material with a collimated beam of
electrons and then observing the resulting diffraction patterns to determine the structure of the material. The required
energy of the electrons is typically in the range 20–200 eV. The reflection high-energy electron diffraction (RHEED)
technique uses the reflection of a beam of electrons fired at various low angles to characterize the surface of
crystalline materials. The beam energy is typically in the range 8–20 keV and the angle of incidence is 1–4°.
The electron microscope directs a focused beam of electrons at a specimen. Some electrons change their properties,
such as movement direction, angle, and relative phase and energy as the beam interacts with the material.
Microscopists can record these changes in the electron beam to produce atomically resolved images of the material.
In blue light, conventional optical microscopes have a diffraction-limited resolution of about 200 nm. By
comparison, electron microscopes are limited by the de Broglie wavelength of the electron. This wavelength, for
example, is equal to 0.0037 nm for electrons accelerated across a 100,000-volt potential. The Transmission Electron
Aberration-Corrected Microscope is capable of sub-0.05 nm resolution, which is more than enough to resolve
individual atoms. This capability makes the electron microscope a useful laboratory instrument for high resolution
imaging. However, electron microscopes are expensive instruments that are costly to maintain.
Two main types of electron microscopes exist: transmission and scanning. Transmission electron microscopes
function like overhead projectors, with a beam of electrons passing through a slice of material then being projected
by lenses on a photographic slide or a charge-coupled device. Scanning electron microscopes rasteri a finely focused
electron beam, as in a TV set, across the studied sample to produce the image. Magnifications range from 100× to
1,000,000× or higher for both microscope types. The scanning tunneling microscope uses quantum tunneling of
electrons from a sharp metal tip into the studied material and can produce atomically resolved images of its surface.
Other applications
In the free-electron laser (FEL), a relativistic electron beam passes through a pair of undulators that contain arrays of
dipole magnets whose fields point in alternating directions. The electrons emit synchrotron radiation that coherently
interacts with the same electrons to strongly amplify the radiation field at the resonance frequency. FEL can emit a
coherent high-brilliance electromagnetic radiation with a wide range of frequencies, from microwaves to soft X-rays.
These devices may find manufacturing, communication and various medical applications, such as soft tissue surgery.
Electrons are important in cathode ray tubes, which have been extensively used as display devices in laboratory
instruments, computer monitors and television sets. In a photomultiplier tube, every photon striking the photocathode
initiates an avalanche of electrons that produces a detectable current pulse. Vacuum tubes use the flow of electrons to
manipulate electrical signals, and they played a critical role in the development of electronics technology. However,
they have been largely supplanted by solid-state devices such as the transistor.
Electron 17
Notes
[1] The fractional version's denominator is the inverse of the decimal value (along with its relative standard uncertainty of ).
[2] The electron's charge is the negative of elementary charge, which has a positive value for the proton.
[3] Dahl (1997:122–185).
[4] "electron, n.2". OED Online. March 2013. Oxford University Press. Accessed 12 April 2013 (http:/ / www. oed. com/ view/ Entry/
60302?rskey=owKYbt& result=2)
[5] Dahl (1997:55–58).
[6] Dahl (1997:64–78).
[7] Dahl (1997:99).
[8] Frank Wilczek: " Happy Birthday, Electron (http:/ / www. scientificamerican. com/ article. cfm?id=happy-birthday-electron)" Scientific
American, June 2012.
[9] Buchwald and Warwick (2001:90–91).
[10] Original publication in Russian:
[11] This magnitude is obtained from the spin quantum number as
UNIQ-math-0-fd5935ead3792f65-QINU
for quantum number s = .
See:
[12] Bohr magneton:
UNIQ-math-1-fd5935ead3792f65-QINU
[13] The classical electron radius is derived as follows. Assume that the electron's charge is spread uniformly throughout a spherical volume.
Since one part of the sphere would repel the other parts, the sphere contains electrostatic potential energy. This energy is assumed to equal the
electron's rest energy, defined by special relativity (E = mc2).
From electrostatics theory, the potential energy of a sphere with radius r and charge e is given by:
UNIQ-math-2-fd5935ead3792f65-QINU
where ε0 is the vacuum permittivity. For an electron with rest mass m0, the rest energy is equal to:
UNIQ-math-3-fd5935ead3792f65-QINU
where c is the speed of light in a vacuum. Setting them equal and solving for r gives the classical electron radius.
See:
[14] —lists a 9% mass difference for an electron that is the size of the Planck distance.
[15] Radiation from non-relativistic electrons is sometimes termed cyclotron radiation.
[16] The change in wavelength, Δλ, depends on the angle of the recoil, θ, as follows,
UNIQ-math-4-fd5935ead3792f65-QINU
where c is the speed of light in a vacuum and me is the electron mass. See Zombeck (2007: 393, 396).
[17] Mobus G. et al. (2010). Journal of Nuclear Materials, v. 396, 264–271, doi:10.1016/j.jnucmat.2009.11.020
[18] The polarization of an electron beam means that the spins of all electrons point into one direction. In other words, the projections of the
spins of all electrons onto their momentum vector have the same sign.
References
External links
• "The Discovery of the Electron" (http:/ / www. aip. org/ history/ electron/ ). American Institute of Physics, Center
for History of Physics.
• "Particle Data Group" (http:/ / pdg. lbl. gov/ ). University of California.
• Bock, R.K.; Vasilescu, A. (1998). The Particle Detector BriefBook (http:/ / physics. web. cern. ch/ Physics/
ParticleDetector/ BriefBook/ ) (14th ed.). Springer. ISBN 3-540-64120-3.
• Copeland, Ed. "Spherical Electron" (http:/ / www. sixtysymbols. com/ videos/ electron_sphere. htm). Sixty
Symbols. Brady Haran for the University of Nottingham.
Article Sources and Contributors 18
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Image Sources, Licenses and Contributors
File:Crookes tube-in use-lateral view-standing cross prPNr°11.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Crookes_tube-in_use-lateral_view-standing_cross_prPNr°11.jpg
License: unknown Contributors: D-Kuru, GianniG46, RJHall
File:Cyclotron motion wider view.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Cyclotron_motion_wider_view.jpg License: Creative Commons Attribution-Share Alike
Contributors: Marcin Białek
File:Bohr atom model English.svg Source: http://en.wikipedia.org/w/index.php?title=File:Bohr_atom_model_English.svg License: Creative Commons Attribution-ShareAlike 3.0 Unported
Contributors: Brighterorange
File:Orbital s1.png Source: http://en.wikipedia.org/w/index.php?title=File:Orbital_s1.png License: Creative Commons Attribution-Sharealike 3.0 Contributors: RJHall
File:Standard Model of Elementary Particles.svg Source: http://en.wikipedia.org/w/index.php?title=File:Standard_Model_of_Elementary_Particles.svg License: Creative Commons
Attribution 3.0 Contributors: MissMJ
File:Asymmetricwave2.png Source: http://en.wikipedia.org/w/index.php?title=File:Asymmetricwave2.png License: Creative Commons Attribution 3.0 Contributors: TimothyRias
File:Virtual pairs near electron.png Source: http://en.wikipedia.org/w/index.php?title=File:Virtual_pairs_near_electron.png License: Creative Commons Attribution-Sharealike 3.0
Contributors: RJHall
File:Lorentz force.svg Source: http://en.wikipedia.org/w/index.php?title=File:Lorentz_force.svg License: GNU Free Documentation License Contributors: User:Jaro.p
File:Bremsstrahlung.svg Source: http://en.wikipedia.org/w/index.php?title=File:Bremsstrahlung.svg License: Public Domain Contributors: Journey234, Pieter Kuiper, RJHall, Trex2001
File:Hydrogen Density Plots.png Source: http://en.wikipedia.org/w/index.php?title=File:Hydrogen_Density_Plots.png License: Public domain Contributors: PoorLeno (talk)
File:Lightning over Oradea Romania cropped.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Lightning_over_Oradea_Romania_cropped.jpg License: Public Domain
Contributors: Mircea Madau (crop by Lucas)
File:Lorentz factor.svg Source: http://en.wikipedia.org/w/index.php?title=File:Lorentz_factor.svg License: Public Domain Contributors: egg, Graph created with KmPlot, edited with Inkscape
Trassiorf (talk) 21:54, 2 March 2010 (UTC)
File:Pairproduction.png Source: http://en.wikipedia.org/w/index.php?title=File:Pairproduction.png License: GNU Free Documentation License Contributors: Original uploader was
Davidhorman at en.wikipedia. Later version(s) were uploaded by Falcorian at en.wikipedia.
File:AirShower.svg Source: http://en.wikipedia.org/w/index.php?title=File:AirShower.svg License: GNU Free Documentation License Contributors: Mpfiz
File:Aurore australe - Aurora australis.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Aurore_australe_-_Aurora_australis.jpg License: unknown Contributors: Diti, Ehquionest
File:Nasa Shuttle Test Using Electron Beam full.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Nasa_Shuttle_Test_Using_Electron_Beam_full.jpg License: Public Domain
Contributors: NASA
License
License 19
Creative Commons Attribution-Share Alike 3.0
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