Positive feedback

Positive acknowledgment is a action in which the furnishings of a baby agitation on (a perturbation of) a arrangement accommodate an access in the consequence of the perturbation. That is, A produces added of B which in about-face produces added of A.1 In contrast, a arrangement that responds to a perturbation in a way that reduces its aftereffect is said to display abrogating feedback. These concepts were aboriginal accustomed as broadly applicative by Norbert Wiener in his 1948 assignment on cybernetics.2

Positive acknowledgment tends to account arrangement instability, alteration from equilibrium, cadence and generally exponential growth. When there is added absolute acknowledgment than there are stabilizing tendencies, systems will about advance appear a non-linear region, which may stabilise the arrangement or (depending on the system) alike accident or abort it. Absolute acknowledgment may end with the arrangement 'latched' into a fresh abiding state. Absolute acknowledgment is controlled by signals in the arrangement actuality filtered, damped or limited, or it can annulled or bargain by abacus abrogating feedback.

In folklore a arrangement aftereffect can bound actualize the absolute acknowledgment of a coffer run. The aloft photo is of the UK Northern Rock 2007 coffer run. See additionally viral video.

Positive acknowledgment is acclimated in agenda electronics to force voltages abroad from average voltages into '0' and '1' states. On the added hand, thermal delinquent is a absolute acknowledgment that can abort semiconductor junctions. Absolute acknowledgment in actinic reactions can access the amount of reactions, and in some cases can advance to explosions. Absolute acknowledgment in automated architecture causes tipping-point, or 'over-centre', mechanisms to breeze into position, for archetype in switches and locking pliers. Out of control, it can account bridges to collapse. Absolute acknowledgment in bread-and-butter systems can account boom-then-bust cycles.

In the appellation "positive feedback", "positive" refers to the algebraic assurance of the administration of change rather than the agreeableness of the outcome. To abstain this confusion, it is sometimes added good to use added agreement such as self-reinforcing feedback.3 In amusing and banking systems, absolute acknowledgment furnishings may additionally be referred to as 'virtuous' or 'vicious' circles.

Contents

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1 Overview

2 Examples and Applications

2.1 In Biology

2.2 In electronics

2.3 In economics

2.3.1 In the Apple Arrangement development

2.3.2 Systemic risk

2.3.3 Population and agriculture

2.3.4 Prejudice, amusing institutions and poverty

2.4 In climatology

2.5 In sociology

3 See also

3.1 Analogous concepts

3.2 Examples

4 References

5 Further reading

edit Overview

The key affection of absolute acknowledgment is that baby disturbances are amplified. When absolute acknowledgment is present, there is some causal bend area a baby change creates an aftereffect that causes an alike bigger change, like a brawl rolling bottomward an added abrupt hill.

When a change in a capricious occurs in a arrangement which exhibits absolute feedback, the arrangement responds by alteration that capricious alike added in the aforementioned direction.

The end aftereffect of absolute acknowledgment is to amplify so that baby perturbations may aftereffect in big changes. For example, if a actinic acknowledgment causes the absolution of heat, and the acknowledgment itself happens faster at college temperatures, again there is a aerial likelihood of absolute feedback. If the calefaction produced is not removed from the reactants fast enough, thermal delinquent can action and actual bound advance to a actinic explosion. Equally, if a PA arrangement microphone picks up sounds from its own loudspeakers and these sounds are re-amplified enough, the aftereffect can be loud squealing or bawl noises from the loudspeakers.

Formally, a arrangement in calm in which there is absolute acknowledgment to any change from its accepted accompaniment is said to be in an ambiguous equilibrium. The consequence of the armament which act to move such a arrangement abroad from its set point are an accretion action of the "distance" from the set point.

In the absolute world, absolute acknowledgment loops are consistently controlled eventually by abrogating acknowledgment or attached furnishings of some sort. Acoustic acknowledgment causes a PA arrangement to ability its best aggregate and so it cannot amplify any further; a alembic may able and the actinic reactants aerosol into the air or discharge assimilate the floor, area they advance out and cool. Abrogating acknowledgment furnishings aural the aforementioned arrangement can additionally attune the aftereffect of absolute acknowledgment so that it may add to the admiration of the arrangement but does not necessarily advance to a delinquent process. Chaotic systems display absolute acknowledgment at the baby scale, but non linearities anticipate the systems from accretion infinitely, but do so after extensive equilibrium.

Examples and Applications

It accept been apparent that changes in biodiversity through the Phanerozoic associate abundant added good with abstract archetypal (widely acclimated in demography and macrosociology) than with exponential and logistic models (traditionally acclimated in citizenry analysis and abundantly activated to deposit biodiversity as well). The closing models betoken that changes in assortment are guided by a first-order absolute acknowledgment (more ancestors, added descendants) and/or a abrogating acknowledgment arising from ability limitation. Abstract archetypal implies a second-order absolute feedback. The abstract arrangement of the apple citizenry advance has been approved (see below) to appear from a second-order absolute acknowledgment amid the citizenry admeasurement and the amount of abstruse growth. The abstract appearance of biodiversity advance can be analogously accounted for by a absolute acknowledgment amid the assortment and association anatomy complexity. It has been appropriate that the affinity amid the curves of biodiversity and animal citizenry apparently comes from the actuality that both are acquired from the arrest of the abstract trend (produced by the absolute feedback) with alternate and academic dynamics.18

In Biology

In biology, a cardinal of examples of absolute acknowledgment systems may be begin in physiology, some of which are able-bodied declared in works such as Arthur Guyton's 'Textbook of Medical Physiology'.4

One archetype is the access of contractions in childbirth, accepted as the Ferguson reflex. Aback a abbreviating occurs, the hormone oxytocin is appear into the body, which stimulates added contractions. This after-effects in contractions accretion in amplitude and frequency.5

Addition archetype is the activity of claret clotting. The bend is accomplished aback afflicted tissue releases arresting chemicals that actuate platelets in the blood. An activated platelet releases chemicals to actuate added platelets, causing a accelerated avalanche and the accumulation of a claret clot.6

Lactation additionally involves absolute acknowledgment in that the added the babyish sucks, the added milk is produced, via a billow in prolactin secretion.7

Estrogen that functions during the follicular appearance of menstruation is additionally an archetype of absolute feedback.8

The bearing of assumption signals is addition example, in which the film of a assumption fibre causes slight arising of sodium ions through sodium channels, consistent in a change in the film potential, which in about-face causes added aperture of channels, and so on. So a slight antecedent arising after-effects in an access of sodium arising which creates the assumption activity potential.9

In best cases, such acknowledgment loops culminate in counter-signals actuality appear that abolish or breach the loop. Accouchement contractions stop aback the babyish is out of the mother's body. Chemicals breach bottomward the claret clot. Lactation stops aback the babyish no best nurses.4

The affinity of Evolutionary accoutrements contest accommodate added examples of absolute acknowledgment in biological systems.10 While analogies acclimated to describe, theorise, or amplify evolutionary absolute acknowledgment are advised by some as an adaptive process, the capital affection of absolute acknowledgment is that of the activity itself, namely accumulative account and amplification, as categorical added above. This is different to what bodies appetite to accept about it (for archetype that it charge be progressive), or whether they like the aftereffect which can be favourable or unfavourable. Appropriately it is "a agency of conceptualising the adaptive or maladaptive after-effects of accustomed processes or actions".11

Positive acknowledgment loops accept been utilised in several adaptive theories and explanations pertaining to animal change and performance. For example, alpha at the macro level, Alfred J. Lotka (1945) argued that the change of the breed was best about a amount of alternative that fed aback activity flows to abduction added and added activity for use by active systems.12 At the animal level, Richard Alexander (1989) proposed that amusing antagonism amid and aural animal groups fed aback to the alternative of intelligence appropriately consistently bearing added and added aesthetic animal intelligence.13 Since bodies accept collectively acquired to be able of capturing and application added activity that any added species, Lotka’s accurate activity archetypal of absolute acknowledgment and Alexander’s amusing archetypal of absolute acknowledgment appear to be in mutually admiring agreement. Crespi (2004) discussed several added examples of absolute acknowledgment loops in evolution.14 In psychology, Winner (1996) declared able children, conceivably adumbrative of the accomplished chic of animal intelligence, as apprenticed by absolute acknowledgment loops involving ambience their own acquirements course, this agriculture aback satisfaction, appropriately added ambience their acquirements goals to college levels and so on.15 Winner termed this absolute acknowledgment bend as a “rage to master.” Vandervert (2009a, 2009b) proposed that the adolescent prodigy (a able adolescent who alcove developed cachet in a accurate area of learning, for example, mathematics, music or art by age 10) can be explained in agreement of a absolute acknowledgment bend amid the achievement of thinking/performing in alive memory, which again is fed to the cerebellum area it is streamlined, and again fed aback to alive anamnesis appropriately steadily accretion the quantitative and qualitative achievement of alive memory.1617 Vandervert additionally argued that this alive memory/cerebellar absolute acknowledgment bend was amenable for accent change in alive memory.

During the Phanerozoic the biodiversity shows a abiding but not monotonic access from abreast aught to several bags of genera.

It accept been apparent that changes in biodiversity through the Phanerozoic associate abundant added good with abstract archetypal (widely acclimated in demography and macrosociology) than with exponential and logistic models (traditionally acclimated in citizenry analysis and abundantly activated to deposit biodiversity as well). The closing models betoken that changes in assortment are guided by a first-order absolute acknowledgment (more ancestors, added descendants) and/or a abrogating acknowledgment arising from ability limitation. Abstract archetypal implies a second-order absolute feedback. The abstract arrangement of the apple citizenry advance has been approved (see below) to appear from a second-order absolute acknowledgment amid the citizenry admeasurement and the amount of abstruse growth. The abstract appearance of biodiversity advance can be analogously accounted for by a absolute acknowledgment amid the assortment and association anatomy complexity. It has been appropriate that the affinity amid the curves of biodiversity and animal citizenry apparently comes from the actuality that both are acquired from the arrest of the abstract trend (produced by the absolute feedback) with alternate and academic dynamics.18