electrostatics

Dictionary: e·lec·tro·stat·ics (ĭ-lĕk'trō-stăt'ĭks) pronunciation

n. (used with a sing. verb)
The physics of electrostatic phenomena.

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The class of phenomena recognized by the presence of electrical charges, either stationary or moving, and the interaction of these charges, this interaction being solely by reason of the charges and their positions and not by reason of their motion. See also Electric charge.

At least 90% of the topics that are normally classified as electrostatics are concerned with the manipulation of charged particles by electric fields. When a particle becomes charged by rubbing or other means, it has either a surplus or a deficit of electrons. A body with a surplus of electrons is said to be negatively charged; a body with a deficiency, positively charged. The amount or quantity of charge on a body is expressed in coulombs (positive or negative). A coulomb is an enormous amount of charge, and in most electrostatic situations charge levels of a small fraction of a coulomb give rise to significant effects. Electrostatic forces always exist between charged bodies. Bodies with like charge experience repulsive forces, while oppositely charged bodies experience attraction.

Principles

If two bodies are charged to Q₁ and Q₂ coulombs and are separated in vacuum by a distance of r meters, the force F in newtons between them is given by Coulomb's law, Eq. (1).
1. $F = {Q_{1}Q_{2}\over 4\pi\epsilon_{0}r^{2}}$

In electrical science, ε₀ is an important constant known as the permittivity or dielectric constant of free space, and is also sometimes called the primary electric constant. It has the value ε₀ = 8.85416 × 10⁻¹² farad per meter. See also Coulomb's law; Permittivity.

Coulomb's law shows that a body charged to Q₁ experiences a force due to the presence of another body charged to Q₂. Q₂ may be considered to influence the whole of space surrounding it, because if Q₁ were to be positioned anywhere it would experience a force due to the presence of Q₂. The property of a charge to influence the whole of space can be modeled by a three-dimensional force field permeating the whole of the space surrounding the charge Q₂. This field is called the electric field. When there are many charged bodies present in an environment, the force that would be exerted on a charged particle at any location can be found by calculating the field at the location due to the presence of each charged body separately, and the net field is obtained by adding up the individual components. See also Electric field.

A system of charged particles or bodies is unstable unless the particles are prevented from moving, since the like-charged particles will repel each other until they are infinitely far apart, and oppositely charged bodies will attract one another and come together. The system has potential energy. The potential energy of two charged particles separated by a distance r can be shown to be given by Eq. (2).
2. ${\rm Potential\ energy} = {Q_{1}Q_{2}\over 4\pi\epsilon_{0}r} \quad {\rm joules}$
See also Energy; Potentials.

Charging methods

The three principal methods of applying electric charge to objects are corona charging, induction charging, and tribocharging. See also Energy.

The corona-charging method relies upon the impact of charged atoms or molecules (ions) on charged bodies. Copious quantities of ions may be generated by a corona discharge, which is a region in which an intense electric field acts upon air molecules and ionizes them so that free ions are produced. A sharply pointed electrode maintained at a high positive or negative potential induces a stream of positive or negative ions which may be used for charging surfaces. The stream of ions from a corona point is usually so intense that neutral air molecules become entrained in the flow to produce a corona wind which can deflect a candle flame. Ions from a corona discharge may be used to charge isolated bodies, insulating surfaces or particles by simply directing a corona wind onto the surface to be charged. In the case of particles, it is normally sufficient for them to pass through a corona discharge region to receive a significant charge from ion-particle collisions. See also Corona discharge.

Surfaces may be charged by exposure to an electrostatic field. If the surface is a liquid and it is disrupted into droplets, they will be electrically charged. Induction charging of equipment and personnel may occur when they are exposed to an electric field. Personnel charged in this way may generate electrostatic discharges when approaching grounded surfaces. Sensitive microelectronic devices can be damaged and computer data can be corrupted by such discharges.

Applications

Electrostatics is put to good use in a wide variety of applications. For examples, the electrostatic precipitator enables smoke emissions from power-station chimneys, smelting plants, and other industrial plants to be reduced to relatively low, acceptable levels. On a smaller scale, efficient filters exist for removing dust from the air in offices, public places, and the home. In some filters, dust particles undergo corona charging as they are sucked by a fan through a duct, and are then collected on grounded electrodes; in others, permanently electrified filter material is used, made from thin plastic sheets which have been treated by surface bombardment from a corona ion source. See also Air filter; Electrostatic precipitator.

In several applications which utilize electrostatics, solid or liquid particles are charged and sprayed onto grounded objects. Dry powder coating is used in many industries in preference to the wet-paint-spraying process. Crop spraying is another important application in which electrostatic forces help to efficiently apply herbicides or insecticides. Research into electrospraying, sometimes called electrohydrodynamic atomization, is leading to new applications for the deposition of ceramic, glass, and polymer films and for powder particle production of special materials. The electrospraying of materials is also used for analysis by means of mass spectrometry, as the electrospray process is gentle and does not disrupt delicate complex molecules.

In electrophotography an optical system is used to project the image to be copied onto a light-sensitive semiconducting surface precharged by a corona source. Exposure of the surface to light reduces the electrical conductivity of the material and allows surface charge to leak away to a back plate in proportion to the intensity of the light, so that bright parts of the image are regions that have lost most of the original charge while dark zones remain fully charged. A mixture of very fine black toner particles and coarser carrier particles is then brought into contact with the charged surface. Transfer of only charged toner particles onto the latent charged surface occurs. A sheet of paper is then laid over the toner-covered surface, and transfer of toner to paper occurs so that an image remains on the paper when it is peeled off the surface. Some ink-jet printers also utilize electrostatic principles; by ensuring that ink drops are formed in the presence of an electrostatic field, they become charged and may be deflected electrostatically to a printing surface. See also Photocopying processes; Printing.

Another development being commercially exploited is the production of metallic ion or droplet beams using electrostatic forces acting upon a liquid-metal surface. Considerable success has been achieved with many molten metals including gold and silver. Either ion or charged droplet beams may be formed depending on the operating conditions of the source. The beams so formed may be very well defined and directed with great accuracy onto targets where they can be used for ion implantation or for the formation of conducting tracks in the fabrication of microelectronic circuits. See also Integrated circuits; Ion implantation.

Electrostatic treators using electric fields have been used to separate water droplets from crude oil as well as move and deposit inorganic particles of sand, mud, and clay and organic particles.

Ion engines which produce thrust by electrostatically accelerating mercury or cesium ions have been successfully operated in space. Colloid thrusters, operating on exactly the same principles as electrostatic paint or crop sprayers, have also been developed. In these a propellant such as glycerol is atomized and accelerated from a nozzle by an electrostatic field. See also Ion propulsion.

Columbia Encyclopedia: electrostatics

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electrostatics, study of phenomena associated with charged bodies at rest (see charge; electricity). A charged body has an excess of positive or negative charges, a condition usually brought about by the transfer of electrons to or from the body. Such bodies exert forces on each other, as described by Coulomb's law, and their behavior can be analyzed in terms of the concept of an electric field surrounding any charged body such that another charged body located at any point in the field is subject to a force proportional to the magnitude of its charge and its attraction or repulsion, depending on the polarity of the charge. The combined electric field in a given region depends on the location, magnitude, and polarity of the charges in that region. Electric fields need not be constant with time. Time-varying electric fields are used in some devices that accelerate charged atomic particles. Electrostatics has many other applications, ranging from the analysis of phenomena such as thunderstorms to the study of the behavior of electron tubes.

WordNet: electrostatics

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Note: click on a word meaning below to see its connections and related words.

The noun has one meaning:

Meaning #1: the branch of physics that deals with static electricity

Wikipedia: Electrostatics

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For a less technical introduction, see Static electricity.

Electromagnetism

Electricity · Magnetism

Electrostatics
Electric charge · Coulomb's law · Electric field · Electric flux · Gauss's law · Electric potential · Electrostatic induction · Electric dipole moment · Polarization density ·

Magnetostatics
Ampère’s law · Electric current · Magnetic field · Magnetization · Magnetic flux · Biot–Savart law · Magnetic dipole moment · Gauss's law for magnetism ·

Electrodynamics
Free space · Lorentz force law · emf · Electromagnetic induction · Faraday’s law · Lenz's law · Displacement current · Maxwell's equations · EM field · Electromagnetic radiation · Liénard-Wiechert Potential · Maxwell tensor · Eddy current ·

Electrical Network
Electrical conduction · Electrical resistance · Capacitance · Inductance · Impedance · Resonant cavities · Waveguides ·

Covariant formulation
Electromagnetic tensor · EM Stress-energy tensor · Four-current · Electromagnetic four-potential ·

Scientists
Ampère · Coulomb · Faraday · Gauss · Heaviside · Henry · Hertz · Lorentz · Maxwell · Tesla · Weber · Ørsted ·

Paper shavings attracted by a charged CD

Electrostatics is the branch of science that deals with the phenomena arising from stationary or slow-moving electric charges.

Since classical antiquity it was known that some materials such as amber attract light particles after rubbing. The Greek word for amber, ήλεκτρον (electron), was the source of the word 'electricity'. Electrostatic phenomena arise from the forces that electric charges exert on each other. Such forces are described by Coulomb's law. Even though electrostatically induced forces seem to be rather weak, the electrostatic force between e.g. an electron and a proton, that together make up a hydrogen atom, is about 40 orders of magnitude stronger than the gravitational force acting between them.

Electrostatic phenomena include many examples as simple as the attraction of the plastic wrap to your hand after you remove it from a package, to the apparently spontaneous explosion of grain silos, to damage of electronic components during manufacturing, to the operation of photocopiers. Electrostatics involves the buildup of charge on the surface of objects due to contact with other surfaces. Although charge exchange happens whenever any two surfaces contact and separate, the effects of charge exchange are usually only noticed when at least one of the surfaces has a high resistance to electrical flow. This is because the charges that transfer to or from the highly resistive surface are more or less trapped there for a long enough time for their effects to be observed. These charges then remain on the object until they either bleed off to ground or are quickly neutralized by a discharge: e.g., the familiar phenomenon of a static 'shock' is caused by the neutralization of charge built up in the body from contact with nonconductive surfaces.

1 Fundamental concepts
2 The electrostatic approximation
- 2.1 Electrostatic potential
3 Triboelectric series
4 Electrostatic generators
5 Charge neutralization
6 Charge induction
7 'Static' electricity
- 7.1 Static electricity and chemical industry
8 Electrostatic induction in commercial applications
9 See also
10 References
11 Further reading
12 External links

Fundamental concepts

Coulomb's law

The fundamental equation of electrostatics is Coulomb's law, which describes the force between two point charges. The magnitude of the electrostatic force between two point electric charges is directly proportional to the product of the magnitudes of each charge and inversely proportional to the square of the distance between the charges. $Q 1$ and $Q 2$ :

$F = \frac{Q_1Q_2}{4\pi\varepsilon_0 r^2}\ ,$

where ε₀ is the electric constant, a defined value:

$\varepsilon_0 \ \stackrel{\mathrm{def}}{=}\ \frac {1}{\mu_0 {c_0}^2} = 8.854\ 187\ 817\ \times 10^{-12}$ in A²s⁴ kg^-1m⁻³ or C²N⁻¹m⁻² or F m⁻¹.

The electric field

The electric field (in units of volts per meter) at a point is defined as the force (in newtons) per unit charge (in coulombs) on a charge at that point:

$\vec{F} = q\vec{E}\,$

From this definition and Coulomb's law, it follows that the magnitude of the electric field E created by a single point charge Q is:

$E = \frac{Q}{4\pi\varepsilon_0 r^2}.$

Gauss's law

Gauss' law states that "the total electric flux through a closed surface is proportional to the total electric charge enclosed within the surface". The constant of proportionality is the permittivity of free space.

Mathematically, Gauss's law takes the form of an integral equation:

$\oint_S\varepsilon_0\vec{E} \cdot\mathrm{d}\vec{A} = \int_V\rho\cdot\mathrm{d}V.$

Alternatively, in differential form, the equation becomes

$\vec{\nabla}\cdot\varepsilon_0\vec{E} = \rho.$

Poisson's equation

The definition of electrostatic potential, combined with the differential form of Gauss's law (above), provides a relationship between the potential φ and the charge density ρ:

${\nabla}^2 \phi = - {\rho\over\varepsilon_0}.$

This relationship is a form of Poisson's equation. Where ${\varepsilon_0}$ is Vacuum permittivity.

Laplace's equation

In the absence of unpaired electric charge, the equation becomes

${\nabla}^2 \phi = 0,$

which is Laplace's equation.

The electrostatic approximation

The validity of the electrostatic approximation rests on the assumption that the electric field is irrotational:

$\vec{\nabla}\times\vec{E} = 0.$

From Faraday's law, this assumption implies the absence or near-absence of time-varying magnetic fields:

${\partial\vec{B}\over\partial t} = 0.$

In other words, electrostatics does not require the absence of magnetic fields or electric currents. Rather, if magnetic fields or electric currents do exist, they must not change with time, or in the worst-case, they must change with time only very slowly. In some problems, both electrostatics and magnetostatics may be required for accurate predictions, but the coupling between the two can still be ignored.

Electrostatic potential

Because the electric field is irrotational, it is possible to express the electric field as the gradient of a scalar function, called the electrostatic potential (also known as the voltage). An electric field, $E$ , points from regions of high potential, φ, to regions of low potential, expressed mathematically as

$\vec{E} = -\vec{\nabla}\phi.$

The electrostatic potential at a point can be defined as the amount of work per unit charge required to move a charge from infinity to the given point.

Triboelectric series

Main article: Triboelectric effect

The triboelectric effect is a type of contact electrification in which certain materials become electrically charged when they are brought into contact with a different material and then separated. One of the materials acquires a positive charge, and the other acquires an equal negative charge. The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties. Amber, for example, can acquire an electric charge by friction with a material like wool. This property, first recorded by Thales of Miletus, was the first electrical phenomenon investigated by man. Other examples of materials that can acquire a significant charge when rubbed together include glass rubbed with silk, and hard rubber rubbed with fur.

Electrostatic generators

Main article: Electrostatic generator

The presence of surface charge imbalance means that the objects will exhibit attractive or repulsive forces. This surface charge imbalance, which yields static electricity, can be generated by touching two differing surfaces together and then separating them due to the phenomena of contact electrification and the triboelectric effect. Rubbing two nonconductive objects generates a great amount of static electricity. This is not just the result of friction; two nonconductive surfaces can become charged by just being placed one on top of the other. Since most surfaces have a rough texture, it takes longer to achieve charging through contact than through rubbing. Rubbing objects together increases amount of adhesive contact between the two surfaces. Usually insulators, e.g., substances that do not conduct electricity, are good at both generating, and holding, a surface charge. Some examples of these substances are rubber, plastic, glass, and pith. Conductive objects only rarely generate charge imbalance except, for example, when a metal surface is impacted by solid or liquid nonconductors. The charge that is transferred during contact electrification is stored on the surface of each object. Static electric generators, devices which produce very high voltage at very low current and used for classroom physics demonstrations, rely on this effect.

Note that the presence of electric current does not detract from the electrostatic forces nor from the sparking, from the corona discharge, or other phenomena. Both phenomena can exist simultaneously in the same system.

See also: Friction machines, Wimshurst machines, and Van de Graaf generators.

Charge neutralization

Natural electrostatic phenomena are most familiar as an occasional annoyance in seasons of low humidity, but can be destructive and harmful in some situations (e.g. electronics manufacturing). When working in direct contact with integrated circuit electronics (especially delicate MOSFETs), or in the presence of flammable gas, care must be taken to avoid accumulating and suddenly discharging a static charge (see electrostatic discharge).

Charge induction

Main article: Electrostatic induction

Charge induction occurs when a negatively charged object repels electrons from the surface of a second object. This creates a region in the second object that is more positively charged. An attractive force is then exerted between the objects. For example, when a balloon is rubbed, the balloon will stick to the wall as an attractive force is exerted by two oppositely charged surfaces (the surface of the wall gains an electric charge due to charge induction, as the free electrons at the surface of the wall are repelled by the negative balloon, creating a positive wall surface, which is subsequently attracted to the surface of the balloon). You can explore the effect with a simulation of the balloon and static electricity.

'Static' electricity

Main article: Static electricity

Lightning over Oradea in Romania

Before the year 1832, when Michael Faraday published the results of his experiment on the identity of electricities, physicists thought "static electricity" was somehow different from other electrical charges. Michael Faraday proved that the electricity induced from the magnet, voltaic electricity produced by a battery, and static electricity are all the same.

Static electricity is usually caused when certain materials are rubbed against each other, like wool on plastic or the soles of shoes on carpet. The process causes electrons to be pulled from the surface of one material and relocated on the surface of the other material.

A static shock occurs when the surface of the second material, negatively charged with electrons, touches a positively-charged conductor, or vice-versa.

Static electricity is commonly used in xerography, air filters, and some automotive paints. Static electricity is a build up of electric charges on two objects that have become separated from each other. Small electrical components can easily be damaged by static electricity. Component manufacturers use a number of antistatic devices to avoid this.

Static electricity and chemical industry

When different materials are brought together and then separated, an accumulation of electric charge can occur which leaves one material positively charged while the other becomes negatively charged. The mild shock that you receive when touching a grounded object after walking on carpet is an example of excess electrical charge accumulating in your body from frictional charging between your shoes and the carpet. The resulting charge build-up upon your body can generate a strong electrical discharge. Although experimenting with static electricity may be fun, similar sparks create severe hazards in those industries dealing with flammable substances, where a small electrical spark may ignite explosive mixtures with devastating consequences.

A similar charging mechanism can occur within low conductivity fluids flowing through pipelines - a process called flow electrification. Fluids which have low electrical conductivity (below 50 pico siemens/m, where pico siemens/m is a measure of electrical conductivity), are called accumulators. Fluids having conductivities above 50 pico siemens/m are called non-accumulators. In non-accumulators, charges recombine as fast as they are separated and hence electrostatic charge generation is not significant. In the petrochemical industry, 50 pico siemens/m is the recommended minimum value of electrical conductivity for adequate removal of charge from a fluid.

An important concept for insulating fluids is the static relaxation time. This is similar to the time constant (tau) within an RC circuit. For insulating materials, it is the ratio of the static dielectric constant divided by the electrical conductivity of the material. For hydrocarbon fluids, this is sometimes approximated by dividing the number 18 by the electrical conductivity of the fluid. Thus a fluid that has an electrical conductivity of 1 pico siemens /cm will have an estimated relaxation time of about 18 seconds. The excess charge within a fluid will be almost completely dissipated after 4 to 5 times the relaxation time, or 90 seconds for the fluid in the above example.

Charge generation increases at higher fluid velocities and larger pipe diameters, becoming quite significant in pipes 8 inches (200 mm) or larger. Static charge generation in these systems is best controlled by limiting fluid velocity. The British standard BS PD CLC/TR 50404:2003 (formerly BS-5958-Part 2) Code of Practice for Control of Undesirable Static Electricity prescribes velocity limits. Because of its large impact on dielectric constant, the recommended velocity for hydrocarbon fluids containing water should be limited to 1 m/s.

Bonding and earthing are the usual ways by which charge buildup can be prevented. For fluids with electrical conductivity below 10 pico siemens/m, bonding and earthing are not adequate for charge dissipation, and anti-static additives may be required.

Applicable Standards

1.BS PD CLC/TR 50404:2003 Code of Practice for Control of Undesirable Static Electricity

2.NFPA 77 (2007) Recommended Practice on Static Electricity

3.API RP 2003 (1998) Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents

Electrostatic induction in commercial applications

The principle of electrostatic induction has been harnessed to beneficial effect in industry for many years, beginning with the introduction of electrostatic industrial painting systems for the economical and even application of enamel and polyurethane paints to consumer goods, including automobiles, bicycles, and other products.

References

Faraday, Michael (1839). Experimental Researches in Electricity. London: Royal Inst. e-book at Project Gutenberg
Halliday, David; Robert Resnick; Kenneth S. Krane (1992). Physics. New York: John Wiley & Sons. ISBN 0-471-80457-6.
Griffiths, David J. (1999). Introduction to Electrodynamics. Upper Saddle River, NJ: Prentice Hall. ISBN 0-13-805326-X.
Hermann A. Haus and James R. Melcher (1989). Electromagnetic Fields and Energy. Englewood Cliffs, NJ: Prentice-Hall. ISBN 0-13-249020-X.

External links

Look up electrostatics in Wiktionary, the free dictionary.

"Man's static jacket sparks alert". BBC News, 16 September 2005.
Static Electricity and Plastics^{[dead link]}
"Can shocks from static electricity damage your health?". Wolfson Electrostatics News pages.
Invisible wall of static:
The inverse cube law The inverse cube law for dipoles (PDF file) by Eng. Xavier Borg

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