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A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction,
positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop Feedback occurs when outputs of a system are routed back as inputs as part of a chain Image:Kettenvergleich.jpg, Roller c ...
leads to a self-amplifying
chain of events ''Chain of Events'' is a 1958 British British may refer to: Peoples, culture, and language * British people The British people, or Britons, are the citizens of the United Kingdom of Great Britain and Northern Ireland, the British Over ...
. Chain reactions are one way that systems which are not in
thermodynamic equilibrium Thermodynamic equilibrium is an axiomatic An axiom, postulate or assumption is a statement that is taken to be true True most commonly refers to truth Truth is the property of being in accord with fact or reality.Merriam-Webster's Online ...
can release energy or increase entropy in order to reach a state of higher entropy. For example, a system may not be able to reach a lower energy state by releasing energy into the environment, because it is hindered or prevented in some way from taking the path that will result in the energy release. If a reaction results in a small energy release making way for more energy releases in an expanding chain, then the system will typically collapse explosively until much or all of the stored energy has been released. A macroscopic metaphor for chain reactions is thus a snowball causing a larger snowball until finally an avalanche results ("
snowball effect Metaphor A metaphor is a figure of speech A figure of speech or rhetorical figure is a word or phrase that entails an intentional deviation from ordinary language use in order to produce a rhetoric Rhetoric () is the Art (skill ...
"). This is a result of stored gravitational potential energy seeking a path of release over friction. Chemically, the equivalent to a snow avalanche is a spark causing a forest fire. In nuclear physics, a single stray neutron can result in a
prompt critical Prompt may refer to: Computing * Command prompt, characters indicating the computer is ready to accept input * Command Prompt, also known as cmd.exe or cmd, the command-line interpreter in some operating systems * Prompt (natural language) Prompt ...
event, which may finally be energetic enough for a nuclear reactor meltdown or (in a bomb) a nuclear explosion. Numerous chain reactions can be represented by a
mathematical model A mathematical model is a description of a system A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environm ...
based on
Markov chain A Markov chain is a stochastic model describing a sequence of possible events in which the probability of each event depends only on the state attained in the previous event. A countably infinite sequence, in which the chain moves state at discr ...

Markov chain
s.


Chemical chain reactions


History

In 1913, the German chemist
Max Bodenstein Max Ernst August Bodenstein (July 15, 1871 – September 3, 1942) was a German physical chemist known for his work in chemical kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned wi ...

Max Bodenstein
first put forth the idea of
chemical A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which ...

chemical
chain reactions. If two molecules react, not only molecules of the final reaction products are formed, but also some unstable molecules which can further react with the parent molecules with a far larger probability than the initial reactants. (In the new reaction, further unstable molecules are formed besides the stable products, and so on.) In 1918,
Walther Nernst Walther Hermann Nernst (25 June 1864 – 18 November 1941) was a German chemist known for his work in thermodynamics, physical chemistry, electrochemistry, and solid state physics. His formulation of the Nernst heat theorem helped pave the way f ...

Walther Nernst
proposed that the
photochemical Photochemistry is the branch of chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behav ...
reaction between
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
and
chlorine Chlorine is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemica ...

chlorine
is a chain reaction in order to explain what's known as the ''
quantum yieldThe quantum yield (Φ) of a radiation upThe international symbol for types and levels of ionizing radiation (radioactivity) that are unsafe for unshielded humans. Radiation, in general, exists throughout nature, such as in light and sound. I ...
'' phenomena. This means that one
photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

photon
of light is responsible for the formation of as many as 106 molecules of the product
HCl The hydrogen chloride has the and as such is a . At , it is a colourless , which forms white fumes of upon contact with atmospheric . Hydrogen chloride gas and hydrochloric acid are important in technology and industry. Hydrochloric acid, th ...
. Nernst suggested that the photon dissociates a Cl2 molecule into two Cl atoms which each initiate a long chain of reaction steps forming HCl.Laidler K.J., ''Chemical Kinetics'' (3rd ed., Harper & Row 1987) p.288-290 In 1923, Danish and Dutch scientists
Christian Christiansen Christian Christiansen (9 October 1843 in Lønborg, Denmark – 28 November 1917 Frederiksberg) was a Danish physicist. Christiansen first taught at the local polytechnical school. In 1886, he was appointed to a chair for physics at the Universi ...
and
Hendrik Anthony Kramers Hendrik Anthony "Hans" Kramers (2 February 1894 – 24 April 1952) was a Netherlands, Dutch physicist who worked with Niels Bohr to understand how electromagnetic waves interact with matter and made important contributions to quantum mechanics and s ...
, in an analysis of the formation of polymers, pointed out that such a chain reaction need not start with a molecule excited by light, but could also start with two molecules colliding violently due to thermal energy as previously proposed for initiation of chemical reactions by
van' t Hoff
van' t Hoff
. Christiansen and Kramers also noted that if, in one link of the reaction chain, two or more unstable
molecules A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecules
are produced, the reaction chain would branch and grow. The result is in fact an exponential growth, thus giving rise to explosive increases in reaction rates, and indeed to chemical explosions themselves. This was the first proposal for the mechanism of chemical explosions. A quantitative chain chemical reaction theory was created later on by Soviet physicist
Nikolay Semyonov Nikolay Nikolayevich Semyonov (or Semënov), (russian: Никола́й Никола́евич Семёнов; – 25 September 1986) was a Soviet physicist A physicist is a scientist A scientist is a person who conducts Scientific meth ...
in 1934. Semyonov shared the Nobel Prize in 1956 with Sir
Cyril Norman Hinshelwood Sir Cyril Norman Hinshelwood (19 June 1897 – 9 October 1967) was a British physical chemist Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be vi ...
, who independently developed many of the same quantitative concepts.http://nobelprize.org/nobel_prizes/chemistry/laureates/1956/press.html History of the chemical chain reaction from 1913 to the Nobel work recognized in 1956


Typical steps

The main types of steps in chain reaction are of the following types. *
Initiation Initiation is a rite of passage marking entrance or acceptance into a group or society. It could also be a formal admission to adulthood in a community or one of its formal components. In an extended sense it can also signify a transformation in ...
(formation of active particles or chain carriers, often
free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabo ...
s, in either a thermal or a photochemical step) * Propagation (may comprise several elementary steps in a cycle, where the active particle through reaction forms another active particle which continues the reaction chain by entering the next elementary step). In effect the active particle serves as a catalyst for the overall reaction of the propagation cycle. Particular cases are: :: * chain branching (a propagation step where one active particle enters the step and two or more are formed); :: *
chain transfer Chain transfer is a polymerization Chemical reaction, reaction by which the activity of a growing polymer chain is transferred to another molecule. :P• + XR' → PX + R'• Chain transfer reactions reduce the average molecular weight of the fin ...
(a propagation step in which the active particle is a growing
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
chain which reacts to form an inactive polymer whose growth is terminated and an active small particle (such as a radical), which may then react to form a new polymer chain). *
Termination Termination may refer to: Science *Termination (geomorphology), the period of time of relatively rapid change from cold, glacial conditions to warm interglacial condition *Termination factor, in genetics, part of the process of transcribing RNA ...
(elementary step in which the active particle loses its activity; e. g. by recombination of two free radicals). The ''chain length'' is defined as the average number of times the propagation cycle is repeated, and equals the overall reaction rate divided by the initiation rate. Some chain reactions have complex
rate equation The rate law or rate equation for a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical A chemical substance is a form of matter ha ...
s with fractional order or
mixed orderThe Mixed Order (french: Ordre Mixte) was a military tactics, tactical formation originally used by demi-brigade, ''demi-brigades'' of the French Revolutionary Army and then later by Napoleon I of France, Napoleon's Grande Armée to great effect. Fi ...
kinetics.


Detailed example: the hydrogen-bromine reaction

The reaction H2 + Br2 → 2 HBr proceeds by the following mechanism: Laidler K.J., ''Chemical Kinetics'' (3rd ed., Harper & Row 1987) p.291-4 P. Atkins and J. de Paula ''Physical Chemistry'' (8th ed., W.H. Freeman 2006), p.830-1 * Initiation : Br2 → 2 Br• (thermal) or Br2 + hν → 2 Br• (photochemical) : each Br atom is a free radical, indicated by the symbol « • » representing an unpaired electron. * Propagation (here a cycle of two steps) : Br• + H2 → HBr + H• : H• + Br2 → HBr + Br• : the sum of these two steps corresponds to the overall reaction H2 + Br2 → 2 HBr, with
catalysis Catalysis () is the process of increasing the rate of a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In cla ...

catalysis
by Br• which participates in the first step and is regenerated in the second step. * Retardation (inhibition) : H• + HBr → H2 + Br• : this step is specific to this example, and corresponds to the first propagation step in reverse. * Termination 2 Br• → Br2 : recombination of two radicals, corresponding in this example to initiation in reverse. As can be explained using the steady-state approximation, the thermal reaction has an initial rate of fractional order (3/2), and a complete rate equation with a two-term denominator ( mixed-order kinetics).


Further chemical examples

*The reaction 2 H2 + O2 → 2 H2O provides an example of chain branching. The propagation is a sequence of two steps whose net effect is to replace an H atom by another H atom plus two OH radicals. This leads to an explosion under certain conditions of temperature and pressure. **H + O2 → OH + O **O + H2 → OH + H *In
chain-growth polymerizationChain-growth polymerization (American spelling) or chain-growth polymerisation (British spelling) is a polymerization technique where Unsaturated compound, unsaturated monomer molecules add onto the active site on a growing polymer chain one at a tim ...
, the propagation step corresponds to the elongation of the growing
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
chain. Chain transfer corresponds to transfer of the activity from this growing chain, whose growth is terminated, to another molecule which may be a second growing polymer chain. For polymerization, the
kinetic chain lengthIn polymer chemistry the kinetic chain length of a polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling mic ...
defined above may differ from the
degree of polymerizationThe degree of polymerization, or DP, is the number of monomeric units in a macromolecule or polymer or oligomer molecule. For a homopolymer, there is only one type of monomeric unit and the ''number-average'' degree of polymerization is given by DP ...

degree of polymerization
of the product macromolecule. *
Polymerase chain reaction Polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete copies or partial copies) of a specific DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM. ...

Polymerase chain reaction
, a technique used in
molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, P ...
to amplify (make many copies of) a piece of
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
by ''
in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies Study or studies may refer to: General * Education **Higher education * Clinical trial * Experiment * Observational study * Research * Study skills, abilities and approaches applie ...

in vitro
''
enzymatic Enzymes () are protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme ...

enzymatic
using a
DNA polymerase A DNA polymerase is a member of a family of enzyme Enzymes () are s that act as s (biocatalysts). Catalysts accelerate . The molecules upon which enzymes may act are called , and the enzyme converts the substrates into different molecules ...

DNA polymerase
.


Acetaldehyde pyrolysis and rate equation

The
pyrolysis The pyrolysis (or devolatilization) process is the of materials at elevated temperatures in an inert atmosphere. It involves a change of . The word is coined from the -derived s ''pyro'' "fire" and ' "separating". Pyrolysis is most commonly us ...

pyrolysis
(thermal decomposition) of
acetaldehyde Ethanal (common name acetaldehyde) is an organic chemical compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds ...
, CH3CHO (g) → CH4 (g) + CO (g), proceeds via the Rice-Herzfeld mechanism: *Initiation (formation of
free radicals In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they un ...
): : CH3CHO (g) → •CH3 (g) + •CHO (g) k1 The methyl and CHO groups are
free radicals In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they un ...
. *Propagation (two steps): : •CH3 (g) + CH3CHO (g) → CH4 (g) + •CH3CO (g) k2 This reaction step provides
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
, which is one of the two main products. : •CH3CO (g) → CO (g) + •CH3 (g) k3 The product •CH3CO (g) of the previous step gives rise to
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom. It is the simplest molecule of the oxocarbon family. In ...

carbon monoxide
(CO), which is the second main product. The sum of the two propagation steps corresponds to the overall reaction CH3CHO (g) → CH4 (g) + CO (g),
catalyzed that utilizes a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide at room temperature. It can also remove formaldehyde from the air. Catalysis () is the process of increasing the reaction rate, rate of a ...

catalyzed
by a methyl radical •CH3. *Termination: : •CH3 (g) + •CH3 (g) → C2H6 (g) k4 This reaction is the only source of
ethane Ethane ( or ) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), organ Chemistry * Organic matter, matter that has come from a once-living ...
(minor product) and it is concluded to be the main chain ending step. Although this mechanism explains the principal products, there are others that are formed in a minor degree, such as
acetone Acetone, or propanone, is an organic compound , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, ...

acetone
(CH3COCH3) and (CH3CH2CHO). Applying the Steady State Approximation for the intermediate species CH3(g) and CH3CO(g), the rate law for the formation of methane and the order of reaction are found: The rate of formation of the product methane is (1)... \frac = k_2 ce CH_3 ce CH_3CHO/math> For the intermediates (2)... \frac = k_1 ce CH_3CHO- k_2 ce CH_3 ce CH_3CHO+ k_3 ce CH_3CO- 2 k_4 ce CH_32 = 0 and (3)... \frac = k_2 ce CH_3 ce CH_3CHO- k_3 ce CH_3CO= 0 Adding (2) and (3), we obtain k_1 ce CH_3CHO- 2 k_4 ce CH_32 = 0 so that (4)...= \frac H_3CHO Using (4) in (1) gives the rate law (5) \frac = \frac k_2 ce CH_3CHO, which is order 3/2 in the reactant CH3CHO.


Nuclear chain reactions

A ''nuclear'' chain reaction was proposed by
Leo Szilard Leo Szilard (; hu, Szilárd Leó ; born Leó Spitz; February 11, 1898 – May 30, 1964) was a Hungarian-American physicist and inventor. He conceived the nuclear chain reaction in 1933, patented the idea of a nuclear fission reactor i ...

Leo Szilard
in 1933, shortly after the neutron was discovered, yet more than five years before
nuclear fission Nuclear fission is a reaction Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic tr ...

nuclear fission
was first discovered. Szilárd knew of ''chemical'' chain reactions, and he had been reading about an energy-producing
nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two atomic nucleus, nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a t ...
involving high-energy protons bombarding lithium, demonstrated by
John Cockcroft Sir John Douglas Cockcroft, (27 May 1897 – 18 September 1967) was a British physicist who shared with Ernest Walton the Nobel Prize in Physics in 1951 for splitting the atomic nucleus The atomic nucleus is the small, dense region ...
and
Ernest Walton Ernest Thomas Sinton Walton (6 October 1903 – 25 June 1995) was an Ireland, Irish physicist and Nobel Prize for Physics, Nobel laureate for his work with John Cockcroft with "Cockcroft–Walton generator, atom-smashing" experiments done at Camb ...

Ernest Walton
, in 1932. Now, Szilárd proposed to use neutrons theoretically produced from certain nuclear reactions in lighter isotopes, to induce further reactions in light isotopes that produced more neutrons. This would in theory produce a chain reaction at the level of the nucleus. He did not envision fission as one of these neutron-producing reactions, since this reaction was not known at the time. Experiments he proposed using
beryllium Beryllium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science t ...

beryllium
and
indium Indium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that c ...

indium
failed. Later, after fission was discovered in 1938, Szilárd immediately realized the possibility of using neutron-induced fission as the particular nuclear reaction necessary to create a chain-reaction, so long as fission also produced neutrons. In 1939, with Enrico Fermi, Szilárd proved this neutron-multiplying reaction in uranium. In this reaction, a
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...

neutron
plus a fissionable
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of ato ...

atom
causes a fission resulting in a larger number of neutrons than the single one that was consumed in the initial reaction. Thus was born the practical
nuclear chain reaction 300px, A possible nuclear fission chain reaction: 1) A uranium-235 atom absorbs a neutron">uranium-235.html" ;"title="nuclear fission chain reaction: 1) A uranium-235">nuclear fission chain reaction: 1) A uranium-235 atom absorbs a neutron, ...
by the mechanism of neutron-induced nuclear fission. Specifically, if one or more of the produced neutrons themselves interact with other fissionable nuclei, and these also undergo fission, then there is a possibility that the macroscopic overall fission reaction will not stop, but continue throughout the reaction material. This is then a self-propagating and thus self-sustaining chain reaction. This is the principle for
nuclear reactor A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction 300px, A possible nuclear fission chain reaction: 1) A uranium-235 atom absorbs a neutron">uranium-235.html" ;"ti ...

nuclear reactor
s and
atomic bomb A nuclear weapon (also known as an atom bomb, atomic bomb, nuclear bomb or nuclear warhead, and colloquially as an A-bomb or nuke) is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission, fiss ...

atomic bomb
s. Demonstration of a self-sustaining nuclear chain reaction was accomplished by
Enrico Fermi Enrico Fermi (; 29 September 1901 - 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and ...

Enrico Fermi
and others, in the successful operation of
Chicago Pile-1 Chicago Pile-1 (CP-1) was the world's first artificial nuclear reactor A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reac ...
, the first artificial nuclear reactor, in late 1942.


Electron avalanche in gases

An
electron avalanche An electron avalanche is a process in which a number of free electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particl ...

electron avalanche
happens between two unconnected electrodes in a gas when an electric field exceeds a certain threshold. Random thermal collisions of gas atoms may result in a few free electrons and positively charged gas ions, in a process called
impact ionization Impact may refer to: * Impact (mechanics), a high force or shock (mechanics) over a short time period * Impact, Texas, a town in Taylor County, Texas, US Science and technology * Impact crater, a meteor crater caused by an impact event * Impac ...
. Acceleration of these free electrons in a strong
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically-charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' ' ...

electric field
causes them to gain energy, and when they impact other atoms, the energy causes release of new free electrons and ions (ionization), which fuels the same process. If this process happens faster than it is naturally quenched by ions recombining, the new ions multiply in successive cycles until the gas breaks down into a plasma and current flows freely in a discharge. Electron avalanches are essential to the
dielectric breakdown showing the ribbon-like plasma filaments from a Tesla coil. Electrical breakdown or dielectric breakdown is a process that occurs when an Insulator (electricity), electrical insulating material, subjected to a high enough voltage Voltage, ...
process within gases. The process can culminate in
corona discharge A corona discharge is an electrical discharge An electric discharge is the release and transmission of electricity in an applied electric field An electric field (sometimes E-field) is the physical field that surrounds each electric cha ...

corona discharge
s,
streamer Streamer or streamers may refer to: * Pennon A pennon or pennant is a flag A flag is a piece of textile, fabric (most often rectangular or quadrilateral) with a distinctive design and colours. It is used as a symbol, a signalling device, or ...
s,
leader Leadership, both as a research area and as a practical skill, encompasses the ability of an individual, group or organization An organization, or organisation (Commonwealth English The use of the English language English ...
s, or in a spark or continuous
electric arc An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma (ph ...

electric arc
that completely bridges the gap. The process may extend huge sparks — streamers in
lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden flow of electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan ...

lightning
discharges propagate by formation of electron avalanches created in the high potential gradient ahead of the streamers' advancing tips. Once begun, avalanches are often intensified by the creation of
photoelectron The photoelectric effect is the emission of electron The electron is a subatomic particle, symbol or , whose electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electrom ...
s as a result of ultraviolet radiation emitted by the excited medium's atoms in the aft-tip region. The extremely high temperature of the resulting plasma cracks the surrounding gas molecules and the free ions recombine to create new chemical compounds. The process can also be used to detect radiation that initiates the process, as the passage of a single particles can be amplified to large discharges. This is the mechanism of a
Geiger counter A Geiger counter (also known as a Geiger–Müller counter) is an electronic instrument used for detecting and measuring ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles o ...

Geiger counter
and also the visualization possible with a
spark chamber{{short description, Charged particle detector A spark chamber is a particle detector: a device used in particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐ ...

spark chamber
and other
wire chamber A wire chamber or multi-wire proportional chamber is a type of proportional counter that detects charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phý ...
s.


Avalanche breakdown in semiconductors

An
avalanche breakdown An avalanche (also called a snowslide) is a rapid flow of snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo fu ...
process can happen in semiconductors, which in some ways conduct electricity analogously to a mildly ionized gas. Semiconductors rely on free electrons knocked out of the crystal by thermal vibration for conduction. Thus, unlike metals, semiconductors become better conductors the higher the temperature. This sets up conditions for the same type of positive feedback—heat from current flow causes temperature to rise, which increases charge carriers, lowering resistance, and causing more current to flow. This can continue to the point of complete breakdown of normal resistance at a semiconductor junction, and failure of the device (this may be temporary or permanent depending on whether there is physical damage to the crystal). Certain devices, such as
avalanche diode In electronics, an avalanche diode is a diode (made from silicon Silicon is a chemical element with the Symbol (chemistry), symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a Tetr ...
s, deliberately make use of the effect.


See also

*
Cascading failure A cascading failure is a process in a system of interconnected parts in which the failure of one or few parts can trigger the failure of other parts and so on. Such a failure may happen in many types of systems, including power transmission, comp ...
* Multiple-vehicle collision *
Rube Goldberg machine A Rube Goldberg machine, named after American cartoonist Rube Goldberg, is a chain reaction-type machine or contraption intentionally designed to perform a simple task in an indirect and overly complicated way. Usually, these machines consist of ...


References


External links


IUPAC Gold Book - Chain reaction
{{DEFAULTSORT:Chain Reaction Chemical kinetics Metaphors referring to objects Causality