electron
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+ | ====== Electron ====== | ||
+ | |< 100% >| | ||
+ | | // | ||
+ | |||
+ | |||
+ | **Electron** (//**e**//) - a fundamental sub-atomic particle which has the intrinsic property of a **negative** elementary [[electric charge]].[(NIST> | ||
+ | |||
+ | An electron is a part of every [[atom]], with the number of electrons corresponding to the number of [[proton|protons]] ([[atomic number]]), so that their electrical charges balance out and an atom can be electrically neutral.[(Holbrow)] | ||
+ | |||
+ | <WRAP 25% left lo> | ||
+ | |< 100% >| | ||
+ | ^ Electron | ||
+ | | [[file/ | ||
+ | | [[Electric charge]]: | ||
+ | | [[Mass]]: | ||
+ | | [[Magnetic moment]]: | ||
+ | | [[Spin]]: ½ | | ||
+ | | [[Antiparticle]]: | ||
+ | </ | ||
+ | |||
+ | Electron' | ||
+ | |||
+ | <box 25% right #f0f0f0> | ||
+ | Prof. Frank Wilczek: | ||
+ | > //**So, what is an electron?** An electron is a particle and a wave; it is ideally simple and unimaginably complex; it is precisely understood and utterly mysterious; it is rigid and subject to creative disassembly. No single answer does justice to reality.// | ||
+ | </ | ||
+ | |||
+ | [[Electric properties|Electric]] and [[magnetic properties]] of electrons, as well as their electromagnetic interactions dictate many properties of matter, obviously [[electrical properties|electrical]], | ||
+ | |||
+ | The small size of electrons allows obtaining much finer resolution of an [[electron microscope]] than it is possible for an [[optical microscope]]. | ||
+ | |||
+ | The name " | ||
+ | |||
+ | {{page> | ||
+ | |||
+ | |||
+ | ===== Microscopic properties ===== | ||
+ | |||
+ | Microscopic properties of an electron have been extensively studied since its discovery. However, because of its very small size there are no experimental techniques which allow direct " | ||
+ | |||
+ | Many properties of electrons, such as electric charge or spin are detectable or measurable. It is possible to describe the rules by which they are bound, but it is not possible to explain the reason for their existence, and therefore they are assumed to be " | ||
+ | |||
+ | ==== Size ==== | ||
+ | |||
+ | <box 30% right #ffffff> | ||
+ | ^ Electron size ^^ | ||
+ | | Karim (2020)[([[https:// | ||
+ | | Mac Gregor (1992)[(MacGregor)] | ||
+ | | Dhobi et al. (2020)[(Dhobi)] | ||
+ | | Coey (2010)[(Coey)] | ||
+ | | Mac Gregor (1992)[(MacGregor)] | 5 × 10< | ||
+ | | Wilczek (2013)[(Wilczek)] | 2 × 10< | ||
+ | | Dhobi et al. (2020)[(Dhobi)] | ||
+ | </ | ||
+ | |||
+ | Size of a particle has meaning in [[classical physics]]. However, at very small scales the quantum effects begin to play a significant role and it difficult to define the meaning of " | ||
+ | |||
+ | > Wilczek (2013): | ||
+ | > //Attempts to pin down an electron' | ||
+ | |||
+ | Depending on the approach there can be several radius definitions for the electron: | ||
+ | * [[classical electron radius]] | ||
+ | * [[Compton radius]] | ||
+ | * quantum-mechanical Compton radius | ||
+ | * QED-corrected quantum-mechanical Compton radius | ||
+ | * electric charge radius | ||
+ | * observed QED charge distribution for a bound electron | ||
+ | * magnetic field radius | ||
+ | |||
+ | By using known physical constants and experimental data, the calculations based on these different approaches can give estimates which differ by several [[order of magnitude|orders of magnitude]].[(MacGregor)][(Dhobi> | ||
+ | |||
+ | Consequently, | ||
+ | |||
+ | Also, the internal structure of electrons is unknown. It is generally accepted that it is a point-like particle. However, there are many alternative theories, for example such that propose an electron to be composed of two massless particles orbiting each other at the speed of light.[(Giese> | ||
+ | |||
+ | |||
+ | ==== Electric charge ==== | ||
+ | |< 100% >| | ||
+ | | {{/ | ||
+ | |||
+ | <box 25% right #f0f0f0> | ||
+ | Schematic representation of [[electrostatic field]] of a stationary negative charge, by using [[electric field lines]] | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | Scientists can describe, but still cannot explain what exactly is [[electric charge]]. However, it is sufficient for such a basic property that it exists, it has some physical meaning and is measurable within the given [[system of units]].[([[http:// | ||
+ | |||
+ | An electron possesses an [[elementary charge|elementary amount of negative electric charge]] **//e//**. Its value is a physical constant, expressed in the [[SI system]] precisely (zero uncertainty) as: | ||
+ | |||
+ | Only such sub-atomic particles like [[quark|quarks]] are thought to have electric electric charge in non-integer quantities e.g. -1/3 //e// or +2/3 //e//, but they only exists in configurations which add up to integer values of charge. For example, proton comprises three quarks (//up, up, down//), which add up to +1 //e//. Therefore, in any macroscopic application the charge is always quantised by the elementary amount of 1 e.[(Purcell)][(Tong> | ||
+ | |||
+ | An electron in isolation is an [[electric monopole]] - it is a source of electric field. By convention, it is assumed that that imaginary [[electric field lines]] begin at positive charges and terminate at negative charges.[(Purcell> | ||
+ | |||
+ | Like charges repel, opposite charges attract, causing mechanical forces which act on the charged bodies. This can be referred to as the [[electrostatic force]], because it exists always, even if the charges remain stationary. This is different from [[magnetic force|magnetic or electromagnetic forces]], which arise when the electric charges are in motion. | ||
+ | |||
+ | A neutral body can become [[electrostatic polarisation|polarised]] in electric field, by means of [[electrostatic induction]], | ||
+ | |||
+ | <box 100% left #f0f0f0> | ||
+ | Opposite charges attract, like charges repel, neutral bodies generate no force (grey) but neutral bodies in the presence of other charges can become locally polarised due to [[electrostatic induction]] such that some force will occur | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | ==== Atom ==== | ||
+ | |||
+ | In an [[atom]], the nucleus comprises [[proton|protons]] and [[neutron|neutrons]], | ||
+ | |||
+ | <box 100% left #f0f0f0> | ||
+ | Diagram of electron structure in an [[atom]]: [[shell|shells]], | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | === Shells === | ||
+ | |||
+ | [[Subshell|Subshells]] are grouped in [[shell|shells]], | ||
+ | |||
+ | In some literature the names " | ||
+ | |||
+ | === Photons === | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | Electron is excited to a higher energy state (higher subshell or shell) by absorbing a photon, and photon is released when electron drops to a lower energy level | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | Electrons can transition to a higher energy level (higher subshell or shell) by absorbing a [[photon]] (a quantum of [[electromagnetic radiation]]). Conversely, if there is an empty position on a lower energy level, an electron can jump down, by emitting a photon.[(Holbrow)] | ||
+ | |||
+ | The lowest energy state (ground state) is when all the electrons are at the lowest possible orbitals. An atom will de-excite itself to the ground state if no energy is supplied to it, by emitting photons. | ||
+ | |||
+ | Heat represents energy, which excites atoms above the ground state. So any atom in a temperature higher than [[absolute zero]] (0 K) continuously gets excited and emits photos, producing photons of different wavelengths, | ||
+ | === Subshells === | ||
+ | |||
+ | Quantum restrictions dictate that there can be no two particles with the same set of quantum numbers in the same region of space ([[Pauli exclusion principle]]). | ||
+ | |||
+ | <box 30% left #f0f0f0> | ||
+ | Orbitals overlap and penetrate each other, forming a spherical shape | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | Therefore, each orbital can contain at most two electrons, because they can have different spin values (-1/2 and +1/ | ||
+ | |||
+ | The orbitals are organised in [[subshell|subshells]] denoted with letters: s, p, d, f, etc., such that a given subshell contains a full set of orbitals, as dictated by the given set of quantum numbers. Higher-order subshells can hold more electrons, such that: s = 2, p = 6, d = 10, f = 14, and so on. | ||
+ | |||
+ | The orbitals within a given subshell overlap and penetrate each other so that their probabilities add up to a spherical shape. | ||
+ | |||
+ | The binding energy is the strongest for the innermost subshells and shells, and it is said that these shells are filled with electrons first. | ||
+ | |||
+ | The higher subshells (and shells) are not filled in a linear order, because there are numerous interactions which take place: electrostatic repulsion, interaction of spins, [[spin-orbit coupling]], and so on. The interactions are very complex, and it is not possible to solve the Schrödinger equation analytically for a general case. Numerical methods are employed instead.[(Spaldin)] | ||
+ | |||
+ | For example, the 4s subshell is filled before the 3d shell, as dictated by the energy conditions (see also [[Hund rules|Hund' | ||
+ | |||
+ | |||
+ | === Orbitals === | ||
+ | |||
+ | Quantum mechanics is complex and the various quantum phenomena are usually introduced with illustration of analogies involving some classical physics, for the ease of understanding. The sequence of analogies often follows the way the understanding of the inside of the atom was developed over the years. | ||
+ | |||
+ | In a simple Bohr atom model, the negatively charged electrons are point-like particles which orbit positively charged nucleus, in a similar way as planets orbit around the Sun. However, circular orbits would require continuous acceleration of a particle requiring radiation of electromagnetic energy, and such orbiting electron would very quickly lose all the energy and collapse onto the nucleus. This is one of the reasons why the name //orbital// was introduced (to distinguish it from //orbit//). | ||
+ | |||
+ | Therefore, all such simplified illustrations should be used used only as an aid for explanation and do not represent what actually happens inside an atom. | ||
+ | |||
+ | The exact mechanics of how electrons move around the nucleus remains unknown. From experiments and calculations it is now understood that electron presence is spread over a volume of space called **[[orbital]]**. There is no well-defined movement involved, it is only said that at a given point in space there is certain probability of finding an electron.[(Jiles> | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | Atom of [[helium]]: blue - spherical [[orbital|orbitals]] of electrons (size around 100 pm), red - [[proton|protons]], | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | |||
+ | The probability distribution, | ||
+ | |||
+ | $$ \left[ -\frac{{\hbar}^2}{2m_e} \left( \frac{∂^2}{∂ r^2} + \frac{2}{r}\frac{∂}{∂r} - \frac{1}{{\hbar}^2 r^2} \boldsymbol{{\hat{l}}^2} \right) - \frac{Z e^2}{4π ε_0 r} \right] \psi_i = ε_i \psi_i $$ | ||
+ | |||
+ | (where the symbols are defined as in eq. (4.4) in Coey (2010)[(Coey)]; | ||
+ | |||
+ | The low-order orbitals are spherical, but higher quantum numbers produce increasingly complex three-dimensional shapes. If the data is plotted as calculated, then such probability distributions produce fuzzy images, which are difficult to visualise and interpret. The probability does not stop at a specific distance, but the function extends to infinity, reducing the probability in some non-linear way.[(Spaldin)] | ||
+ | |||
+ | For this reason, a number of simplifications is used in order to increase clarity of images. For example, planes, cones or spheres can be used to indicate locations of " | ||
+ | |||
+ | However, the simplest method appears to be to use "hard shape" with a specific limit. For example, the volume were probability is greater than 90% is plotted with full opacity, and all other is shown as completely transparent. Also, there could be some additional scaling factors which can make easier to indicate intricate details of a given shape.[(Mantley_software> | ||
+ | |||
+ | The red and blue colours in the images denote the positive and negative phase of the function. Any other colours can be used the represent the same information. | ||
+ | |||
+ | <box 100% left #f0f0f0> | ||
+ | An example of a 4d orbital. The probability distribution is smeared over space so a 2D image is fuzzy and difficult to interpret. Cones represent " | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | <box 100% left #f0f0f0> | ||
+ | Complexity of orbitals increases for higher order orbitals (just some typical examples are shown here for illustration)[(Mantley_software)][(Mantley_guide)] | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | <box 40% right #f0f0f0> | ||
+ | Demonstration of standing waves on vibrating circular plate, with radial nodes (circles) at lower frequencies, | ||
+ | [[file/ | ||
+ | //< | ||
+ | </ | ||
+ | |||
+ | The complex shape of orbitals can be explained with an analogy to vibrations of a body, with higher harmonics forming more complex shapes. An example is shown with a plate which can be made vibrating at different frequencies. | ||
+ | |||
+ | At lower frequency of vibration only concentric rings are present, representing standing waves, with clearly visible " | ||
+ | |||
+ | Similar vibration patterns can be expected for a spherical body, but with the standing waves extending over thee-dimensional space. | ||
+ | |||
+ | === Orbital magnetic moment === | ||
+ | |||
+ | [[Magnetic moment]] of an [[electric current]] flowing in a [[loop]] can be expressed as the product of the amplitude of the current and the [[area]] of the loop. | ||
+ | |||
+ | In an atom, an electron orbiting around the nucleus represents a moving electric charge which is equivalent to electric current, but it must be remembered that by convention, the direction of electric current (blue arrow of $I$ in the image) is opposite to the direction in which the electron moves (green arrow of $v$). For this reason the vector of magnetic dipole moment of an electron points in the opposite direction to the angular moment.[(Coey)] | ||
+ | |||
+ | The orbit would represent a circle with some area. Therefore, in from the classical physics viewpoint there would be a magnetic moment associated with the orbital motion of an electron, in a loop without [[resistance]], | ||
+ | |||
+ | <box 40% right #f0f0f0> | ||
+ | The analogy of orbital moment is an electron orbiting the nucleus on a circular orbit (left) and for spin the sphere spins around its own axis (right) | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | The orbital magnetic moment for the so-called first Bohr orbit can be calculated as: $μ_{orb} = \frac{e·h}{4·π·m}$ = 9.274 × 10< | ||
+ | |||
+ | Such orbital movement would have a " | ||
+ | |||
+ | The angular momentum due to orbital movement of an electron is $\boldsymbol{l} = m · \boldsymbol{r} × \boldsymbol{v}$, | ||
+ | |||
+ | There is a fixed proportionality between the electron' | ||
+ | |||
+ | The orbital angular momentum is quantised, in units of $\boldsymbol{l}$ (for orientation) or units of $\hbar$ (for value of component along the acting magnetic field).[(Spaldin)] | ||
+ | |||
+ | However, electrons do not follow circular orbits, and orbitals which can take quite complex 3D shapes especially for higher orders, as described above. | ||
+ | |||
+ | In [[chemical compound|chemical compounds]] there is electrostatic interaction between the [[ion|ions]] in [[molecule|molecules]], | ||
+ | |||
+ | === Spin magnetic moment === | ||
+ | |||
+ | <box 20% right #f0f0f0> | ||
+ | Rotating sphere as an analogy of electron spin[(Cullity)] | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | An electron possesses a fundamental property called **[[spin]]**, | ||
+ | |||
+ | Spin is a quantum property and does not have a direct equivalent in [[classical physics]]. However, because of the difficulty of explaining the concept, an analogy is typically used, in which an electron is portrayed as a sphere spinning around its own axis. Such spinning movement would also have a " | ||
+ | |||
+ | Spin magnetic moment is also explained conceptually by the analogy of a spinning sphere. If the surface of the sphere has [[electric charge]] distributed on its surface, then as the sphere is spinning the surface electric charges rotate with it. This is equivalent to charge moving in a circular pattern which is equivalent to electric current in a loop, and therefore there would be also [[magnetic dipole moment]] associated with such a structure.[(Cullity)][(Purcell)] However, such analogy should not be used for any quantitative calculations, | ||
+ | |||
+ | Electron' | ||
+ | |||
+ | Therefore, the angular momentum can be written as $L = \hbar/ | ||
+ | |||
+ | The spin magnetic moment $μ_{spin}$ is directly related to the angular momentum $m_s$ and Bohr magneton $μ_B$ such that $μ_{spin} = -g_e · μ_B · m_s $, where $g_e$ = 2.002319 (unitless constant). Therefore, $μ_{spin} \approx μ_B$.[(Spaldin)] | ||
+ | |||
+ | Calculation of particle momentum (and therefore spin magnetic moment) involves mass in the denominator. Therefore, the contribution of magnetic moments of protons and neutrons is mostly negligible, because of the mass being larger by more than 3 orders of magnitude that it is the case for electron.[(Coey)] | ||
+ | |||
+ | |||
+ | |||
+ | |||
+ | |||
+ | |||
+ | ==== Chemical properties ==== | ||
+ | Atoms can form multi-atom [[molecule|molecules]] of chemical compounds by forming bonds. All [[chemical bond|chemical bonds]] are electromagnetic in nature, and they arise because of the activity of the electrons on the outermost shells.[(Griffiths> | ||
+ | |||
+ | Atomic subshells have a preference to be fully occupied, and an atom with fully occupied outermost subshell is inert chemically ([[He]], [[Ne]], [[Ar]], etc.) On the other hand, if an atom has just a single electron in the outermost shell then it is very reactive chemically ([[H]], [[Li]], [[Na]], etc.) Some atoms are reactive enough that in the absence of other types of atoms they can form bonds between themselves. For example, in common air, both oxygen and nitrogen occur predominantly in diatomic configuration: | ||
+ | |||
+ | Depending on the exact details energetic conditions the bonds can be broadly classified as [[covalent bond|covalent]] or [[ionic bond|ionic]].[(Coey)] | ||
+ | ===== Antimatter ===== | ||
+ | Antimatter is a type of matter which has similar properties to normal matter. However, some of its property are exactly opposite, like for instance electric charge. Should a matter particle and its antimatter equivalent come in contact they will annihilate completely, producing a burst of electromagnetic radiation. | ||
+ | |||
+ | An antimatter equivalent of electron is called anti-electron or [[positron]]. It has the same mass and size, but positive electric charge, and therefore also the spin direction is reversed. | ||
+ | |||
+ | Positrons are generated in some radioactive processes. For example, unstable [[isotope|isotopes]] with shortage of neutrons decay with [[beta decay]] (β+), by emitting a positron, such that a proton becomes a neutron and the atomic number changes.[(Holbrow)] | ||
+ | |||
+ | This positron-electron annihilation process is used for example in [[positron emission tomography]] for medical diagnostic purposes. A suitable radioactive chemical (e.g. fluorine-18, | ||
+ | ===== Macroscopic phenomena ===== | ||
+ | Electrons are involved in microscopic (atom-level) phenomena which control a lot of macroscopic behaviour of materials, such as [[electric properties|electric]] and [[magnetic properties]]. | ||
+ | |||
+ | <box 100% left #f0f0f0> | ||
+ | Periodic table of elements, with magnetic properties[(Enghag> | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | ==== Electricity and electric current ==== | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | By convention, direction of [[electric current]] is opposite to the movement of [[electrons]] | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | Electricity is related to the presence ([[electrostatics]]) and movement ([[electric current]]) of electric charges. The properties of electricity have been harnessed for generation, storage, transmission, | ||
+ | |||
+ | [[Electric energy]] is based on the flow of [[electric charges]], which is equivalent to [[electric current]]. In ordinary metal conductors the electrons are free to move, and even small electric field applied across a conductor can results in a significant current flow.[(Griffiths)] | ||
+ | |||
+ | In [[liquid|liquids]] and [[gas|gasses]] the electric field can separate electrons from atoms, thus forming positively charged [[ion|ions]] ([[cation|cations]]), | ||
+ | |||
+ | Depending on the mobility of electrons and the associated [[resistivity]], | ||
+ | |||
+ | ^ Material type ^ Typical resistivity range (Ω·m)[(Griffiths)] | ||
+ | | Insulators | ||
+ | | Semiconductors | ||
+ | | Conductors | ||
+ | | Superconductors | ||
+ | |||
+ | There are many materials and substances which can have resistivity values in-between these ranges, for various reasons (temperature, | ||
+ | |||
+ | <box 100% left #f0f0f0> | ||
+ | Resistivity of materials at room temperature spans over more than 30 [[order of magnitude|orders of magnitude]] (superconductors have zero resistivity and cannot be represented on a [[logarithmic scale]], but they would lie to the left of conductors) | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | === Insulators === | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | Electric wire engineered to have copper conductor inside, and insulator outside | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | [[Electric insulator|Electric insulators]] are all materials, in which the electrons remain bound to the atoms. Therefore, there are few free electrons to sustain the current flow. Such materials are used to insulate a given electrical conductor, so the current flows in the intended path, not leaking away in an uncontrolled way. | ||
+ | |||
+ | However, there are no perfect insulators, because some electrons are always present and will move under the applied [[electric field]]. Also, when the [[voltage]] across an insulator is increased to very large values the electrons can be ripped away from the atoms ([[ionisation]]), | ||
+ | |||
+ | Electrical insulators typically degrade over time (their resistivity decreases by increasing mobility of electrons), especially if they remain energised. The flow of electricity through an insulator is very small but it can be measured by very sensitive devices. For example, the state of electrical insulation can be verified by using devices such us [[insulator resistance tester]]. A voltage is applied to an insulator, typically with a value equal or greater than the nominal operating voltage of the system. The small resulting current is measured and the resistance is calculated. Industrial testers can measure resistance from MΩ to tens of TΩ[(Megger_S1> | ||
+ | |||
+ | Higher energy of the system frees up more electrons, so resistivity decreases with increasing temperature. Also, higher temperature reduces insulating properties in terms of lifetime, and around a room temperature an increase by 10°C reduces the insulation resistance (and useful life of insulation) roughly by half.[(Stitch> | ||
+ | |||
+ | Once a breakdown of solid insulation happens, then typically an irreversible damage occurs, for example by creating carbonised path, which serves as a low-resistance path for the electrons. | ||
+ | |||
+ | == Vacuum == | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | [[Deflection coils]] in a [[cathode-ray tube]] display | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | Perfect [[vacuum]] itself does not conduct electricity, | ||
+ | |||
+ | Electrons travelling in vacuum were utilised extensively in [[cathode-ray tube]] displays (CRT) in TV sets and oscilloscopes popular in XX century, as well as other [[vacuum tube|vacuum tubes]]. Electrons were emitted from a heated cathode, and accelerated by [[electric field]] due to high voltage (typically between 10 kV and 35 kV) towards the display covered with a luminescent layer. | ||
+ | |||
+ | The position of the beam of electrons hitting the display was controlled by [[deflection coils]], whose magnetic field was rapidly modifying the trajectory of electrons, due to [[Lorentz force]]. The luminescent layer was required to convert the energy of electrons (invisible) to the spectrum of light visible to human eye. | ||
+ | === Semiconductors === | ||
+ | |||
+ | <box 20% right #f0f0f0> | ||
+ | Modern [[electronics]] relies on [[semiconductor|semiconductors]] | ||
+ | [[file/ | ||
+ | //< | ||
+ | </ | ||
+ | |||
+ | In [[semiconductor|semiconducting materials]] the electrons require less energy to leave atoms, and their mobility can be controlled by various means, like increasing temperature, | ||
+ | |||
+ | Technical semiconductors are sophisticated materials, whose performance is fine-tuned to specific application. For example, pure [[silicon]] is not conductive enough to be useful in its raw form. In order to obtain the required performance it is [[semiconductor doping|doped]] with other atoms, such as [[phosphorus]] (donating one extra electron, n-type) or [[boron]] (creating a shortage of one electron, called [[electron hole|hole]], | ||
+ | |||
+ | The difference in mobility and behaviour of these electrons or electron holes is the basis for the widely useful electronics technology. The word **// | ||
+ | |||
+ | In semiconductors typically mobility of electrons increases with increasing temperature and thus resistivity reduces accordingly. However, there are additional effects resulting from the mobility of electrons and holes, and the interaction between them, that highly non-linear effects can become more important. For example, a combination of [[p-type semiconductor|p-type]] and [[n-type semiconductor|n-type semiconductors]], | ||
+ | |||
+ | The changing mobility of electrons with temperature is used in variable-resistance devices such as [[thermistor|thermistors]]. Typically, they exhibit an exponential relationship: | ||
+ | |||
+ | === Magnetic semiconductors === | ||
+ | Physicist work on combining the magnetic and semiconducting effects, in which both the movement of electrons (electric current) and their magnetic spins are utilised. These phenomena give rise to new classes of materials, with names such as: [[magnetic semiconductor]], | ||
+ | |||
+ | |||
+ | === Conductors === | ||
+ | In [[metal|metals]], | ||
+ | |||
+ | Inside an unenergised conductor, the electrons move freely and randomly, with very large speed of around 10< | ||
+ | |||
+ | At increased temperature the atoms vibrate more vigorously and the movement of electrons is scattered more, so the resistivity of metals generally increases with increasing temperature. | ||
+ | |||
+ | The electrons can also hit the ends of the conductor and thus generate electrical noise ([[Johnson noise|Johnson' | ||
+ | |||
+ | == Faraday cage == | ||
+ | An isolated object can be charged electrostatically by depositing electric charges on its surface. | ||
+ | |||
+ | The surface of insulators can be charged by rubbing other insulating materials against them, which builds up the electrostatic charges due to [[triboelectric effect]]. | ||
+ | |||
+ | In conductors it is sufficient to touch the surface with other charged body and the charges will equalise between the two systems.[(Purcell)] But these surfaces charges (electrons) repel each other and they will tend to occupy the farthest possible distance from one another. On a hollow conductor all charges will remain only on the outer surface, leaving the inside of it with zero electric field. Similar applies even if the construction is made from a mesh, rather than solid surface. Such a conducting " | ||
+ | |||
+ | === Superconductors === | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | Niobium-titanium superconducting cable[(CERN_superconductor> | ||
+ | [[file/ | ||
+ | //< | ||
+ | </ | ||
+ | |||
+ | The resistivity of metals decreases with lowering temperature, | ||
+ | |||
+ | H.K. Onnes carried out experiments on [[mercury]] in 1911, and discovered that its resistivity was reducing at lower temperatures, | ||
+ | |||
+ | In superconductors the electrons can move without electrical resistance, and without the energy loss associated with it. Therefore, in a closed superconducting loop, once a [[DC current]] is induced it can flow indefinitely (there is no energy loss), producing DC magnetic field around itself. This behaviour is used in [[superconducting electromagnet|superconducting electromagnets]] which can operate in [[persistent mode]].[(Purcell)] | ||
+ | |||
+ | For the [[type I superconductor|type I superconductors]] (with sharp transition of [[critical field]]) the theoretical explanation is that electrons form pairs ([[Cooper pair|Cooper pairs]]) whose movement is mediated coherently by the lattice vibrations. Understanding of the electron mobility in type II and high-temperature superconductors is still incomplete.[(Spaldin)] | ||
+ | |||
+ | ==== Magnetism ==== | ||
+ | |||
+ | === Magnetic field === | ||
+ | Any moving charge creates [[magnetic field]] around itself ([[velocity field]]). The individual fields from each electron in a current-conducting wire overlap and create a macroscopic magnetic field around such wire. | ||
+ | |||
+ | The wires can be wound in [[coil|coils]] or windings to shape or direct the global field in the desired manner so that operation of many devices is possible: [[generator]], | ||
+ | |||
+ | <box 35% left #f0f0f0> | ||
+ | [[Magnetic field]] around a moving [[electron]] (because of the convention the electron moves in the opposite direction to electric current)[(Maxfield> | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | <box 30% left #f0f0f0> | ||
+ | Electric current //**I**// generates [[magnetic field strength]] //**H**// whose vector is always perpendicular to the direction of I, according to the [[right-hand rule]] | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | <box 25% left #f0f0f0> | ||
+ | [[Magnetic field lines]] of a [[solenoid]] (cross-section view) | ||
+ | [[file/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | === Magnetism === | ||
+ | All materials respond to magnetic field to some extend, including [[vacuum]] (which is a reference point for the [[magnetic constant]])[(BIPM_2019> | ||
+ | |||
+ | There are three main types of magnetic responses, or [[types of magnetism]]: | ||
+ | * [[Diamagnetism]] - all electrons are paired in all orbitals. As a result there is no net spin moment. Application of magnetic field to such materials introduces changes to the shape of orbitals, similar to a current induced in a loop, in the direction opposing the applied field. Thus diamagnets have [[permeability]] lower than vacuum and are repelled from magnetic field. However, this effect is so small that in everyday applications they are simply classified as [[non-magnetic material|non-magnetic materials]]. | ||
+ | * [[Paramagnetism]] - some atoms have at least one unpaired electron, and its spin can respond to the applied field. The more the spin can be oriented with the field the larger the permeability. Paramagnets are attracted to magnetic field, but the effect is also very weak (non-magnetic). | ||
+ | * [[Ordered magnetism]] - the atoms have unpaired electrons and they are positioned such that they can interact with each other, which leads to [[spontaneous magnetisation]], | ||
+ | * [[ferromagnetism]] | ||
+ | * [[ferrimagnetism]] | ||
+ | * [[antiferromagnetism]] (ordered but non-magnetic) | ||
+ | * and several others. | ||
+ | |||
+ | All magnetic materials (exhibiting ordered magnetism) become paramagnetic at sufficiently high temperatures (above [[Curie temperature]]), | ||
+ | |||
+ | ==== Electron microscopy ==== | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | [[Scanning electron microscope|Scanning electron micrograph]] of a single N. meningitidis cell, with resolutions far exceeding 200 nm available from the optical microscopes | ||
+ | [[file/ | ||
+ | //< | ||
+ | </ | ||
+ | |||
+ | The resolution of ordinary microscopes is limited by the wavelengths of [[visible light]], so objects smaller than around 200 nm cannot be resolved.[(Davidson> | ||
+ | |||
+ | However, using techniques such as [[scanning electron microscopy]] (SEM), the resolution can be improved by up to three orders of magnitude, so that features around 0.2 nm in size can be resolved.[(Rijal> | ||
+ | |||
+ | In SEM, a beam of electrons is generated from a heated cathode and accelerated with high voltage towards the sample. [[Magnetic lens|Magnetic lenses]] are used to focus and direct the electron beam (in some sense similar to [[CRT display|CRT displays]]). | ||
+ | |||
+ | The high-energy electrons impact the sample and cause secondary electrons to be scattered - these can be detected and translated into useful information, | ||
+ | |||
+ | However, there are some additional conditions which have to be met, for instance the sample must be conductive, or be prepared by applying some conductive coating (e.g. [[gold]] or [[palladium]]), | ||
+ | ===== See also ===== | ||
+ | *[[Proton]] | ||
+ | *[[Atom]] | ||
+ | *[[Electric charge]] | ||
+ | *[[Magnetic field]] | ||
+ | |||
+ | |||
+ | ===== References ===== | ||
+ | ~~REFNOTES~~ | ||
+ | |||
+ | {{tag> Electron Sub-atomic_particles Electric_charge Ion Counter}} |
electron.txt · Last modified: 2023/09/04 14:39 by stan_zurek