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Encyclopedia > Corona discharge

In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor, which occurs when the potential gradient exceeds a certain value, in situations where sparking (also known as arcing) is not favoured. Lightning strikes during a night-time thunderstorm. ... An electrostatic discharge (ESD) is a sudden flow of electric current through a material that is normally an insulator. ... ... A subset of the phases of matter, fluids include liquids, gases, plasmas and, to some extent, plastic solids. ... In science and engineering, conductors are materials that contain movable charges of electricity. ... In electrostatics, the potential gradient is the difference of electric potential, per unit distance (vertical unless otherwise specified) between two points. ... Look up Spark in Wiktionary, the free dictionary A spark may be a small airborne ember from a fire. ... An electric arc can melt calcium oxide. ...

Contents


Introduction

A corona is a process by which a current, perhaps sustained, develops from an electrode with a high potential in a neutral fluid, usually air, by ionising that fluid so as to create a plasma around the electrode. The ions generated eventually pass charge to nearby areas of lower potential, or recombine to form neutral gas molecules. It has been suggested that this article or section be merged with Scalar potential. ... // An ion is an atom or group of atoms with a net electric charge. ... A Plasma lamp, illustrating some of the more complex phenomena of a plasma, including filamentation A solar coronal mass ejection blasts plasma throughout the solar system. ... Charge is a word with many different meanings. ...


When the potential gradient is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive. If a charged object has a sharp point, the air around that point will be at a much higher gradient than elsewhere, and can become conductive while other points in the air do not. When the air becomes conductive, it effectively increases the size of the conductor. If the new conductive region is less sharp, the ionization may not extend past this local region. Outside of this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized.


If the geometry and gradient are such that the ionized region continues to grow instead of stopping at a certain radius, a completely conductive path may be formed, resulting in a momentary spark or a continuous arc. Look up Spark in Wiktionary, the free dictionary The word spark has several meanings: Sparks produced by grinding In electricity, spark usually refers to a momentary electrostatic discharge across a spark gap. ... An electric arc can melt calcium oxide. ...


Corona discharge usually involves two asymmetric electrodes; one highly curved (such as the tip of a needle, or a narrow wire) and one of low curvature (such as a plate, or the ground). The high curvature ensures a high potential gradient around one electrode, for the generation of a plasma. In electrostatics, the potential gradient is the difference of electric potential, per unit distance (vertical unless otherwise specified) between two points. ... An electrode is a conductor used to make contact with a nonmetallic part of a circuit (e. ...


Coronas may be positive or negative. This is determined by the polarity of the voltage on the highly-curved electrode. If the curved electrode is positive with respect to the flat electrode we say we have a positive corona, if negative we say we have a negative corona. (See below for more details.) The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, with only the electron having the ability to undergo a significant degree of ionising inelastic collision at common temperatures and pressures. // An ion is an atom or group of atoms with a net electric charge. ... Properties The electron is a lightweight fundamental subatomic particle that carries a negative electric charge. ...


An important reason for considering coronas is the production of ozone around conductors undergoing corona processes. A negative corona generates much more ozone than the corresponding positive corona. For other uses, see Ozone (disambiguation). ...


Applications of corona discharge

Corona discharge has a number of commercial and industrial applications.

Coronas can be used to generate charged surfaces, which is an effect used in electrostatic copying (photocopying). They can also be used to remove particulate matter from air streams by first charging the air, and then passing the charged stream through a comb of alternating polarity, to deposit the charged particles on the oppositely charged plates. For other uses, see Ozone (disambiguation). ... Air-conditioning systems serve to maintain indoor air within the two-dimensional comfort zone defined by a temperature and relative humidity region. ... Layers of Atmosphere (NOAA) Earths atmosphere is a layer of gases surrounding the planet Earth and retained by the Earths gravity. ... Polymer is a generic term used to describe a very long molecule consisting of structural units and repeating units connected by covalent chemical bonds. ... A small, much-used Xerox copier in a high school library. ... An air ioniser is a device which ionizes, or electrically charges, the air it intakes to give it a positive charge. ... Kirlian photography refers to a form of contact print photography, theoretically associated with high-voltage. ... EHD Thruster stands for Electrohydrodynamic thruster. ... The Lifter is an electrokinetic, or electrohydrodynamic device. ... The Biefeld-Brown effect is an electrokinetic effect that was discovered by Thomas Townsend Brown (USA) and Dr. Paul Alfred Biefeld (CH). ... This article needs to be cleaned up to conform to a higher standard of quality. ... Polystyrene is a polymer made from the monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum. ... A small, much-used Xerox copier in a high school library. ...


The free-radicals and ions generated in corona reactions can be used to scrub the air of certain noxious products, through chemical reactions, and can be used to produce ozone. For other uses, see Ozone (disambiguation). ...


Problems caused by corona discharges

Coronas can generate audible and radio-frequency noise, particularly in electric power transmission lines. They also represent a power loss and can indicate equipment degradation. Their action on atmospheric particulates, and their ozone and NOx production can also be disadvantageous to human health where power lines run through built-up areas. Therefore, power transmission equipment is designed to minimise the formation of corona discharge. Corona discharge is generally undesirable in: Transmission lines in Lund, Sweden Electric power transmission is one process in the delivery of electricity to consumers. ... For other uses, see Ozone (disambiguation). ... There are several possible use of the 3 letter word/formula nox: Nox is a goddess of Night: see Nyx. ...

Transmission lines in Lund, Sweden Electric power transmission is one process in the delivery of electricity to consumers. ... Three-phase pole-mounted step-down transformer A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts. ... A capacitor is a device that stores energy in the electric field created between a pair of conductors on which equal but opposite electric charges have been placed. ... Electric motors of various sizes. ... Electrical generator Generator (Mathematics) ...

Mechanism of corona discharge

Corona discharge of both the positive and negative variety have certain mechanisms in common.

  1. A neutral atom or molecule of the medium, in a region of strong field (high potential gradient, near the curved electrode) is ionized by an exogenous environmental event (for example, as the result of a photon interaction), to create a positive ion and a free electron.
  2. The strong field then operates on these charged particles, separating them, and preventing their recombination, and also accelerating them, imparting each of them with kinetic energy.[citation needed]
  3. As a result of the energisation of the electrons (which have a much higher charge/mass ratio and so are accelerated to a higher velocity), further electron/positive-ion pairs may be created by collision with neutral atoms. These then undergo the same separating process creating an electron avalanche. Both positive and negative coronas rely on electron avalanches.
  4. In processes which differ between positive and negative coronas, the energy of these plasma processes is converted into further initial electron dissociations to seed further avalanches.
  5. An ion species created in this series of avalanches (which differs between positive and negative coronas) is attracted to the uncurved electrode, completing the circuit, and sustaining the current flow.

The onset voltage of corona or Corona Inception Voltage (CIV) can be found with Peek's law (1929), formulated from empirical observations. Later papers derived more accurate formulas. In electrostatics, the potential gradient is the difference of electric potential, per unit distance (vertical unless otherwise specified) between two points. ... In physics, the photon (from Greek φως, phōs, meaning light) is the quantum of the electromagnetic field; for instance, light. ... // An ion is an atom or group of atoms with a net electric charge. ... Properties The electron is a lightweight fundamental subatomic particle that carries a negative electric charge. ... The strong interaction or strong force is today understood to represent the interactions between quarks and gluons as detailed by the theory of quantum chromodynamics. ... An electron avalanche is a process in which a number of free electrons in a medium (usually a gas) are subjected to a strong electric field accelerate, ionizing the mediums atoms by collision (creating positive ions), forming new electrons to undergo the same process in successive cycles. ... Peeks law is a description of the conditions necessary for corona discharge between two wires: ev is the visual critical corona voltage or corona inception voltage (CIV), the voltage (in kilovolts) required to initiate a visible corona discharge between the wires. ...


Electrical properties

The current carried by the corona is determined by integrating the current density over the surface of the conductor. The power loss is determined by multiplying the current and the voltage. In electricity, current refers to electric current, which is the flow of electric charge. ... In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ... In physics, power (symbol: P) is the amount of work done per unit of time. ...


Positive coronas

Properties

A positive corona is manifested as a uniform plasma across the length of a conductor. It can often be seen glowing blue/white, though much of the emissions are in the ultraviolet. The uniformity of the plasma owes itself to the homogeneous source of secondary avalanche electrons described in the mechanism section, below. With the same geometry and voltages, it appears a little smaller than the corresponding negative corona, owing to the lack of a non-ionising plasma region between the inner and outer regions. There are many fewer free electrons in a positive corona, when compared to a negative corona, except very close to the curved electrode: perhaps a thousandth of the electron density, and a hundredth of the total number of electrons.


However, the electrons in a positive corona are concentrated close to the surface of the curved conductor, in a region of high-potential gradient (and therefore the electrons have a high energy), whereas in a negative corona many of the electrons are in the outer, lower-field areas. Therefore, if electrons are to be used in an application which requires a high activation energy, positive coronas may support a greater reaction constants than corresponding negative coronas; though the total number of electrons may be lower, the number of a very high energy electrons may be higher.


Coronas are efficient producers of ozone in air. A positive corona generates much less ozone than the corresponding negative corona, as the reactions which produce ozone are relatively low-energy. Therefore, the greater number of electrons of a negative corona leads to an increased production.


Beyond the plasma, in the unipolar region, the flow is of low-energy positive ions toward the flat electrode.


Mechanism

As with a negative corona, a positive corona is initiated by an exogenous ionisation event in a region of high potential gradient. The electrons resulting from the ionisation are attracted toward the curved electrode, and the positive ions repelled from it. By undergoing inelastic collisions closer and closer to the curved electrode, further molecules are ionized in an electron avalanche.


In a positive corona, secondary electrons, for further avalanches, are generated predominantly in the fluid itself, in the region outside the plasma or avalanche region. They are created by ionization caused by the photons emitted from that plasma in the various de-excitation processes occurring within the plasma after electron collisions, the thermal energy liberated in those collisions creating photons which are radiated into the gas. The electrons resulting from the ionisation of a neutral gas molecule are then electrically attracted back toward the curved electrode, attracted into the plasma, and so begins the process of creating further avalanches inside the plasma. A Plasma lamp, illustrating some of the more complex phenomena of a plasma, including filamentation A solar coronal mass ejection blasts plasma throughout the solar system. ...


As can be seen, the positive corona is divided into two regions, concentric around the sharp electrode. The inner region contains ionising electrons, and positive ions, acting as a plasma, the electrons avalanche in this region, creating many further ion/electron pairs. The outer region consists almost entirely of the slowly migrating massive positive ions, moving toward the uncurved electrode along with, close to the interface of this region, secondary electrons, liberated by photons leaving the plasma, being re-accelerated into the plasma. The inner region is known as the plasma region, the outer as the unipolar region.


Negative coronas

Properties

A negative corona is manifested in a non-uniform corona, varying according to the surface features and irregularities of the curved conductor. It often appears as tufts of corona at sharp edges, the number of tufts altering with the strength of the field. The form of negative coronas is a result of its source of secondary avalanche electrons (see below). It appears a little larger than the corresponding positive corona, as electrons are allowed to drift out of the ionising region, and so the plasma continues some distance beyond it. The total number of electrons, and electron density is much greater than in the corresponding positive corona. However, they are of a predominantly lower energy, owing to being in a region of lower potential-gradient. Therefore, whilst for many reactions the increased electron density will increase the reaction rate, the lower energy of the electrons will mean that reactions which require a higher electron energy may take place at a lower rate.


Mechanism

Negative coronas are more complex than positive coronas in construction. As with positive coronas, the establishing of a corona begins with an exogenous ionisation event generating a primary electron, followed by an electron avalanche.


Electrons ionised from the neutral gas are not useful in sustaining the negative corona process by generating secondary electrons for further avalanches, as the general movement of electrons in a negative corona is outward from the curved electrode. For negative corona, instead, the dominant process generating secondary electrons is the photoelectric effect, from the surface of the electrode itself. The work-function of the electrons (the energy required to liberate the electrons from the surface) is considerably lower than the ionisation energy of air at standard temperatures and pressures, making it a more liberal source of secondary electrons under these conditions. Again, the source of energy for the electron-liberation is a high-energy photon from an atom within the plasma body relaxing after excitation from an earlier collision. The use of ionised neutral gas as a source of ionisation is further diminished in a negative corona by the high-concentration of positive ions clustering around the curved electrode. The photoelectric effect is the emission of electrons from matter upon the absorption of electromagnetic radiation, such as ultraviolet radiation or x-rays. ...


Under other conditions, the collision of the positive species with the curved electrode can also cause electron liberation.


The difference, then, between positive and negative coronas, in the matter of the generation of secondary electron avalanches, is that in a positive corona they are generated by the gas surrounding the plasma region, the new secondary electrons travelling inward, whereas in a negative corona they are generated by the curved electrode itself, the new secondary electrons travelling outward.


A further feature of the structure of negative coronas is that as the electrons drift outwards, they encounter neutral molecules and, with electronegative molecules (such as oxygen and water vapour), combine to produce negative ions. These negative ions are then attracted to the positive uncurved electrode, completing the 'circuit'. It has been suggested that this article or section be merged with electronegativity. ... In science, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ... Boundaries: Phase, Pressure, Temperature Evaporation/Sublimation Whenever a water molecule leaves a surface, it is said to have evaporated. ...


A negative corona can be divided into three radial areas, around the sharp electrode. In the inner area, high-energy electrons inelastically collide with neutral atoms and cause avalanches, whilst outer electrons (usually of a lower energy) combine with neutral atoms to produce negative ions. In the intermediate region, electrons combine to form negative ions, but typically have insufficient energy to cause avalanche ionisation, but remain part of a plasma owing to the different polarities of the species present, and the ability to partake in characteristic plasma reactions. In the outer region, only a flow of negative ions and, to a lesser and radially-decreasing extent, free electrons toward the positive electrode takes place. The inner two regions are known as the corona plasma. The inner region is an ionising plasma, the middle a non-ionising plasma. The outer region is known as the unipolar region.


Negative coronas can only be sustained in fluids with electronegative molecules, to capture free electrons. Without the electronegative molecules capturing the free electrons, a simple path of electron flow of ionised gas exists between the two electrodes and an arc, or spark, develops.


References

  • Junhong Chen, "Direct-Current Corona Enhanced Chemical Reactions" (PDF), Ph.D. Thesis, University of Minnesota, USA. August 2002.
  • F.W. Peek (1929). Dielectric Phenomena in High Voltage Engineering, McGraw-Hill. ISBN 0972659668.
  • Leonard Loeb (1965). Electrical Coronas Their Basic Physical Mechanisms, University of California Press.
  • James D. Cobine (1941, reprints in 1958, 1970)). Gaseous Conductors; Theory and Engineering Applications, McGraw-Hill or Dover reprints.

PDF is an abbreviation with several meanings: Portable Document Format Post-doctoral fellowship Probability density function There also is an electronic design automation company named PDF Solutions. ...

External links


  Results from FactBites:
 
Corona discharge - definition of Corona discharge in Encyclopedia (2142 words)
In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor, which occurs when the potential gradient exceeds a certain value, in situations where sparking is not favoured.
Corona discharge usually involves two asymmetric electrodes, one highly curved (such as the tip of a needle, or a narrow wire) and one of low curvature (such as a plate, or the ground).
However, the electrons in a positive corona are concentrated close to the surface of the curved conductor, in a region of high-potential gradient (and therefore the electrons have a high energy), whereas in a negative corona many of the electrons are in the outer, lower-field areas.
  More results at FactBites »

 
 

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