electron


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Electron (e - )

classification
Elementary particle
fermion
lepton
properties
electric charge −1  e
(−1.602 176 634 · 10 −19 C )
Dimensions 5.485 799 090 65 (16) · 10 −4   u
9.109 383 7015 (28) · 10 −31   kg
Resting energy 0.510 998 950 00 (15)  MeV
Compton wavelength 2.426 310 238 67 (73) · 10 −12   m
magnetic moment −9.284 764 7043 (28) · 10 −24   J  /  D
g factor −2.002 319 304 362 56 (35)
gyromagnetic
ratio
1.760 859 630 23 (53) 10 11  1 / ( s T )
Spin 1/2
average lifespan stable
Interactions weak
electromagnetic
gravitation

The electron [ ˈeːlɛktrɔn, eˈlɛk-, elɛkˈtroːn ] (from ancient Greek ἤλεκτρον élektron = ' amber ', on which electricity was first observed; coined in 1874 by Stoney and Helmholtz ) is a negatively charged elementary particle . Its symbol is e - . The alternative name Negatron (from nega tive charge and Elek tron ) is rarely used and is possibly in the beta spectroscopy common.

The electrons bound in an atom or ion form its electron shell . All chemistry is essentially based on the properties and interactions of these bound electrons. In metals , some of the electrons can move freely and cause the high electrical conductivity of metallic conductors . This is the basis of electrical engineering and electronics . In semiconductors , the number of mobile electrons and thus the electrical conductivity can be easily influenced, both through the manufacture of the material and later through external influences such as temperature, electrical voltage , incidence of light, etc. This is the basis of semiconductor electronics . Electrons can escape from any material when heated or when a strong electric field is applied ( glow emission , field emission ). As free electrons , they can then be formed into an electron beam in a vacuum through further acceleration and focusing . This has made the development of the oscilloscope , the television, and the computer monitor possible. Further applications of free electrons are e.g. B. the X-ray tube , the electron microscope , electron beam welding , basic physical research using particle accelerators and the generation of synchrotron radiation for research and technical purposes.

During the beta-minus decay of an atomic nucleus , a new electron is generated and emitted.

The experimental proof of the electron was first achieved by Emil Wiechert in 1897 and a little later by Joseph John Thomson .

History of the discovery of the electron

The concept of a smallest, indivisible amount of electrical charge was proposed on various occasions around the middle of the 19th century, among others by Richard Laming , Wilhelm Weber and Hermann von Helmholtz .

In 1874, George Johnstone Stoney suggested the existence of electrical charge carriers associated with atoms. Based on the electrolysis , he estimated the size of the electron charge, but received a value that was about 20 times too low. At the meeting of the British Association in Belfast, he suggested using the elementary charge as another fundamental natural constant along with the gravitational constant and the speed of light as the basis of physical systems of measurement. Together with Helmholtz, Stoney coined the name electron for the “atom of electricity”.

Emil Wiechert found in 1897 that cathode radiation consists of negatively charged particles that are much lighter than an atom, but then stopped his research on this. In the same year Joseph John Thomson determined the mass of the particles (he first referred to them as corpuscules ) and was able to prove that the same particles are always involved regardless of the cathode material and the residual gas in the cathode ray tube. During this time, the Zeeman effect was used to demonstrate that these particles also occur in the atom and cause light emission there. The electron was thus identified as an elementary particle.

The elementary charge was measured in 1909 by Robert Millikan .

properties

The electron is the lightest of the electrically charged elementary particles. If the conservation laws for charge and energy apply - which corresponds to all physical experience - electrons must therefore be stable. In fact, there is no experimental evidence of electron decay to date.

The electron belongs to the leptons and, like all leptons, has a spin (more precisely: spin quantum number) of 1/2. As a particle with half-integer spin, it belongs to the class of fermions , so it is particularly subject to the Pauli principle . Its antiparticle is the positron , symbol e + , with which it corresponds in all properties except for its electrical charge.

Some of the basic properties of the electron, listed in the table above, are linked by the magnetic moment of the electron spin :

.

Here is the magnetic moment of the electron spin, the mass of the electron, its charge and the spin . is called the Landé or g factor . The term vor , which describes the ratio of the magnetic moment to the spin, is called the gyromagnetic ratio of the electron. For the electron, according to the Dirac theory (relativistic quantum mechanics), this would be exactly the same as 2. Effects that are only explained by quantum electrodynamics , however, cause a measurable slight deviation of 2. This deviation is called the anomalous magnetic moment of the electron designated.

Classic radius and punctiformity

Shortly after the discovery of the electron, attempts were made to estimate its size, especially because of the classic idea of ​​small billiard balls that collide in scattering experiments . The argument came down to the fact that the concentration of the electron charge on a very small extent of the electron requires energy which, according to the principle of equivalence, must be contained in the mass of the electron. Under the assumption that the energy of an electron at rest is equal to twice the self-energy of the electron charge in its own electric field, one obtains the classic electron radius

: Elementary charge , : Kreiszahl , : Electric field constant , : electron mass : the speed of light , : fine structure constant , : Bohr radius .

The self-energy mentally separates the electric charge and the electric field of the electron. If one puts the charge −e in the potential , whereby one thinks, for example, of a second electron evenly distributed over a spherical surface with the radius , then energy is required for this, the self-energy of a single electron is half of this. However, there were definitely other derivations for a possible expansion of the electron, which came to other values.

Today the view of the expansion of the electron is different: In the experiments that have been possible so far, electrons show neither expansion nor internal structure and can therefore be assumed to be point-like. The experimental upper limit for the size of the electron is currently around 10 −19  m. Nevertheless, the classical electron radius appears in many formulas in which a quantity of the dimension length (or area etc.) is formed from the fixed properties of the electron in order to be able to explain experimental results. For example, the theoretical formulas for the cross sections of the Photo and Compton effects contain the square of .

Even the search for an electrical dipole moment of the electron has so far remained without positive results. A dipole moment would arise if, in the case of a non-point electron, the center of gravity of the mass was not at the same time the center of gravity of the charge. Such a thing is predicted by theories of supersymmetry that go beyond the standard model of elementary particles. A measurement in October 2013, which uses the strong electric field in a polar molecule, showed that a possible dipole moment with a confidence level of 90% is no greater than 8.7 · 10 −31   m. This clearly means that the electron's center of charge and mass cannot be more than about 10 −30  m apart. Theoretical approaches, according to which larger values ​​were predicted, are thus refuted.

Cross section

A distinction must be made between the (possible) expansion of the electron and its cross-section for interaction processes. When X-rays are scattered by electrons, z. B. a scattering cross-section of about what would correspond to the circular area with the classic electron radius described above . In the limiting case of large wavelengths, i. H. smaller photon energies, the scattering cross-section increases (see Thomson scattering and Compton effect ).

Interactions

Many physical phenomena such as electricity , electromagnetism and electromagnetic radiation are essentially based on interactions between electrons. Electrons in an electrical conductor are displaced by a changing magnetic field and an electrical voltage is induced. The electrons in a current-carrying conductor generate a magnetic field. An accelerated electron - of course also in the case of curvilinear motion - emits photons, the so-called bremsstrahlung ( Hertzian dipole , synchrotron radiation , free-electron laser ).

In a solid body , the electron experiences interactions with the crystal lattice . Its behavior can then be described by using the deviating effective mass instead of the electron mass , which is also dependent on the direction of movement of the electron.

Electrons that have detached from their atoms in polar solvents such as water or alcohols are known as solvated electrons . When alkali metals are dissolved in ammonia , they are responsible for the strong blue color.

An electron is a quantum object , that is, is it the by the Heisenberg uncertainty principle local described and momentum spread in the measurable range, so that, as for light both shafts - as well as particle properties can be observed, which as a wave-particle duality designated becomes. In an atom, the electron can be viewed as a standing wave of matter .

Experiments

The ratio e / m of the electron charge to the electron mass can be determined as a school experiment with the fine beam tube . The direct determination of the elementary charge was made possible by the Millikan experiment .

For electrons whose speed is not negligibly small compared to the speed of light , the non-linear contribution to the momentum must be taken into account according to the theory of relativity . Electrons with their low mass can be accelerated to such high speeds relatively easily; With a kinetic energy of 80 keV , an electron has half the speed of light. The impulse can be measured by deflecting it in a magnetic field. The deviation of the momentum from the value calculated according to classical mechanics was first demonstrated by Walter Kaufmann in 1901 and, after the discovery of the theory of relativity, initially described with the term “ relativistic mass increase ”, which is now regarded as obsolete.

Free electrons

Fluorescence from electrons in a shadow cross tube

In the cathode ray tube (Braun's tube), electrons emerge from a heated hot cathode and are accelerated in the vacuum by an electric field in the direction of the field (towards the positive anode ) . The electrons are deflected by magnetic fields perpendicular to the direction of the field and perpendicular to the current direction of flight ( Lorentz force ). It was these properties of the electrons that made the development of the oscilloscope , the television and the computer monitor possible.

Further applications of free electrons are e.g. B. the X-ray tube , the electron microscope , electron beam welding , basic physical research using particle accelerators and the generation of synchrotron radiation for research and technical purposes. See also electron beam technology .

Web links

Wiktionary: Elektron  - explanations of meanings, word origins, synonyms, translations
Commons : Electrons  - collection of images, videos and audio files

Individual evidence

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  3. CODATA Recommended Values. National Institute of Standards and Technology, accessed May 20, 2019 . Electron mass in kg . The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  4. CODATA Recommended Values. National Institute of Standards and Technology, accessed May 20, 2019 . Electron mass in MeV / c 2 . The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  5. CODATA Recommended Values. National Institute of Standards and Technology, accessed May 20, 2019 . Compton wavelength. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  6. CODATA Recommended Values. National Institute of Standards and Technology, accessed August 3, 2019 . Magnetic moment. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  7. CODATA Recommended Values. National Institute of Standards and Technology, accessed May 20, 2019 . g factor. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
  8. CODATA Recommended Values. National Institute of Standards and Technology, accessed May 20, 2019 . Gyromagnetic ratio. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
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