Electrostatics

Electromagnetism
Electricity · Magnetism
Electrostatics Electric charge Coulomb's law Electric field Gauss's law Electric potential Electric dipole moment
Magnetostatics Ampère's circuital law Magnetic field Magnetic flux Biot-Savart law Magnetic dipole moment
Electrodynamics Electric current Lorentz force law Electromotive force (EM) Electromagnetic induction Faraday-Lenz law Displacement current Maxwell's equations (EMF) Electromagnetic field (EM) Electromagnetic radiation
Electrical Network Conduction Resistance Capacitance Inductance Impedance Resonant cavities Waveguides
Tensors in Relativity Electromagnetic tensor Electromagnetic stress-energy tensor

Electrostatics is the branch of physics that deals with the phenomena arising from what seem to be stationary electric charges. This includes phenomena as simple as the attraction of plastic wrap to your hand after you remove it from a package to 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.

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

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

Fundamental concepts

Coulomb's law

The fundamental equation of electrostatics is Coulomb's law, which describes the force between two point charges $Q_{1}$ and $Q_{2}$:

${\vec {F}}={\frac {Q_{1}Q_{2}}{4\pi \varepsilon _{0}r^{2}}}{\hat {r}}.$

The electric field

The electric field (in units of volts per meter) is defined as the force (in newtons) per unit charge (in coulombs). 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

${\vec {E}}={\frac {Q}{4\pi \varepsilon _{0}r^{2}}}{\hat {r}}.$

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.

Laplace's equation

In the absence of unpaired electric charge, the equation becomes

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

which is Laplace's equation.

Triboelectric series

The triboelectric effect is a type of contact electrification in which certain materials become electrically charged when coming into contact with another, different, material, and are then separated. The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties. It is therefore not very predictable, and only broad generalizations can be made. Amber, for example, can acquire an electric charge by friction with a material like wool. This property, first recorded by Thales of Miletus, suggested the word "electricity," from the Greek word for amber, èlectròn. 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

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

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

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 was 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.

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.

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 within 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/cm, where pico siemens/cm is a measure of electrical conductivity), are called accumulators. Fluids having conductivities above 50 pico siemens/cm 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/cm 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. Thu a flood that has an electrical conduct of 1 picho 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 or laneration in these systems is best controlled by limiting fluid vity. The British standtr BS PD CLC/TR 50404:2003 (formerly BS-5958-Part 2) Code of Practice for Control of Undesirable Static Electricity prescribes city limits. Because of its large impact on electric constant, the recommended velocity for hydrocarbon fluids containing water should be limited to 1 m/sec.

Bonding and earthing are the usual ways by which charge buildup can be prevented. For fluids with electrical conductivity bpico siemens/cm, 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 (2009) Recommended Practice on Static Electricity

3.API RP 2003 (1998) Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents iy gets eaten by a dinosaurus

See also

General

• Electromagnetism
• Electrostatic force
• Ionic bond
• Electronegativity

References

ISBN links support NWE through referral fees

• Faraday, Michael. Experimental Researches in Electricity. London: Royal Inst. e-book, available for free via Project Gutenberg, 1839.
• Halliday, David; Robert Resnick; Kenneth S. Krane. Physics. New York: John Wiley & Sons, 1992. ISBN 0471804576. 
• Griffiths, David J.. Introduction to Electrodynamics. Upper Saddle River, NJ: Prentice Hall, 1999. ISBN 013805326X. 
• Haus, Hermann A. and James R. Melcher. Electromagnetic Fields and Energy. Englewood Cliffs, NJ: Prentice-Hall, 1989. ISBN 013249020X. 
• Dampier, William Cecil, "The theory of experimental electricity." Cambridge [Eng.]:University Press, 1905.

External links and further reading

All links retrieved November 19, 2007

General

• RMCybernetics: High Voltage Physics. RMCybernetics.com, 2006.
• "Man's static jacket sparks alert." BBC News, news.bbc.co.uk., September 16, 2005.
• "Can shocks from static electricity damage your health?". Wolfsonelectrostatics.com, 2007.
• Aspden, H. "Can Gravity be an Electrostatic Force?" A Quantum theory of Gravitation, Aspden.org, 2005.
• Beaty, Bill. [1] SPECIAL SESSION, 17th Annual EOS/ESD Symposium, 1995.
• Beaty, William J. "Humans and sparks; The Cause, Stopping the Pain, and 'Electric People." Amasci.com, 1997.

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