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Electrostatics

For a less technical introduction, see Static electricity.

Electromagnetism
Electricity · Magnetism
[hide]Electrostatics
 · Electric charge · Coulomb’s law · Electric field · Electric flux · Gauss’ law · Electric potential · Electrostatic induction · Electric dipole moment ·
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Paper shavings attracted by a charged CD
Paper shavings attracted by a charged CD

Electrostatics is the branch of science that deals with the phenomena arising from what seems to be stationary 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 examples as simple as the attraction of 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.

Contents

[hide]

[edit] The electrostatic approximation

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

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

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.

[edit] 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

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.

[edit] Fundamental concepts

[edit] 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.Q1 and Q2:

where ε0 is the electric constant, a defined value:

  in A2s4 kg-1m−3 or C2N−1m−2 or F m−1.

[edit] 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:

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:

[edit] 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:

Alternatively, in differential form, the equation becomes

[edit] 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:

This relationship is a form of Poisson's equation. Where is Vacuum permittivity.

[edit] Laplace's equation

In the absence of unpaired electric charge, the equation becomes

which is Laplace's equation.

[edit] 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 another, 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.

[edit] 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.

[edit] 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).

[edit] 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.

[edit] 'Static' electricity

Main article: Static electricity
Lightning over Oradea in Romania
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 manufactures use a number of antistatic devices to avoid this.

[edit] 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/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. 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/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 (2007) Recommended Practice on Static Electricity

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

[edit] See also

General

[edit] 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. 

[edit] External links and further reading

General
Essays
Books
Retrieved from "http://en.wikipedia.org/wiki/Electrostatics"
طاها یکشنبه 28 مهر‌ماه سال 1387 ساعت 07:44 ب.ظ
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نیروی هسته ای قوی

نیروی هسته‌ای قوی، یکی از چهار نیروی پایه در فیزیک است، که نقش آن پایدار و با‌هم‌نگه‌داشتن، کوارک‌ها و ذرات تشکیل شده از آن‌ها (مانند نوترون‌ها و پروتون‌ها) در هسته اتم‌ها است. به این معنی که نیروی هسته‌ای نیز نام‌گذاری می‌شود. این نیرو به همین خاطر، از نیروی الکترومغناطیسی بسیار قوی‌تر است و می‌تواند هسته اتم‌ها را، با وجود نیروی دافعه بین پروتون‌های آن (با بار الکتریکی مثبت) پایدار نگه دارد.

همانند نیروی الکترومغناطیسی و نیروی هسته‌ای ضعیف، این نیرو نیز توسط تبادل بوزون‌ها انجام می‌گیرد (یا توجیه می‌شود) که در اینجا، ذّره تبادل شده گلوون نام دارد. گلوون‌ها از ۸ نوع مختلف هستند که دارای بار رنگی می‌باشند و آن را بین کوارک‌ها انتقال می‌دهند.

برهم‌کنش یا نیروی قوی با ارایه بر‌هم‌کنش میان کوارک‌ها و گلوان‌ها درک شده است و جزییات با نظریه کرومودینامیک کوانتومی(QCD)توصیف می‌شود. این نیروی بنیادی عامل اتحاد ذرات در هسته اتم‌هاست، واسطه انتقال این نیرو گلوان‌ها (نوعی بوزون) هستند که با عمل بروی کوارک‌ها، پادکوارک‌ها و بین گلوان‌هاانجام می‌گیرد.

نیروی قوی فقط روی ذرات بنیادی اثر می‌کند، با این حال اثر بین هادرون‌ها به نیروی هسته‌ای مشاهده می‌شود (بهترین مثال برای فهم نیروی که بین گلوان‌ها اثر می‌کند هسته‌ها است ) در این‌جا نیروی هسته‌ای قوی بطور غیر مسقیم عمل می‌کند، در انتقال گلوان‌های که قسمتی از آن با piهای.

طاها یکشنبه 28 مهر‌ماه سال 1387 ساعت 07:40 ب.ظ
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تاریخچه الکترومعناطیس و لینکهای مربوطه

History of optics, History of electromagnetism, and Magnetism

While preparing for an evening lecture on 21 April 1820, Hans Christian Ørsted developed an experiment which provided evidence that surprised him. As he was setting up his materials, he noticed a compass needle deflected from magnetic north when the electric current from the battery he was using was switched on and off. This deflection convinced him that magnetic fields radiate from all sides of a wire carrying an electric current, just as light and heat do, and that it confirmed a direct relationship between electricity and magnetism.

At the time of discovery, Ørsted did not suggest any satisfactory explanation of the phenomenon, nor did he try to represent the phenomenon in a mathematical framework. However, three months later he began more intensive investigations. Soon thereafter he published his findings, proving that an electric current produces a magnetic field as it flows through a wire. The CGS unit of magnetic induction (oersted) is named in honor of his contributions to the field of electromagnetism.

His findings resulted in intensive research throughout the scientific community in electrodynamics. They influenced French physicist André-Marie Ampère's developments of a single mathematical form to represent the magnetic forces between current-carrying conductors. Ørsted's discovery also represented a major step toward a unified concept of energy.

Ørsted was not the first person to examine the relation between electricity and magnetism. In 1802 Gian Domenico Romagnosi, an Italian legal scholar, deflected a magnetic needle by electrostatic charges. He interpreted his observations as The Relation between electricity and magnetism. Actually, no galvanic current existed in the setup and hence no electromagnetism was present. An account of the discovery was published in 1802 in an Italian newspaper, but it was largely overlooked by the contemporary scientific community.

This unification, which was observed by Michael Faraday, extended by James Clerk Maxwell, and partially reformulated by Oliver Heaviside and Heinrich Hertz, is one of the accomplishments of 19th century mathematical physics. It had far-reaching consequences, one of which was the understanding of the nature of light. As it turns out, what is thought of as "light" is actually a propagating oscillatory disturbance in the electromagnetic field, i.e., an electromagnetic wave. Different frequencies of oscillation give rise to the different forms of electromagnetic radiation, from radio waves at the lowest frequencies, to visible light at intermediate frequencies, to gamma rays at the highest frequencies.

[edit] The electromagnetic force

Main article: Electromagnetic force

The force that the electromagnetic field exerts on electrically charged particles, called the electromagnetic force, is one of the fundamental forces, and is responsible for most of the forces we experience in our daily lives. The other fundamental forces are the strong nuclear force (which holds atomic nuclei together), the weak nuclear force and the gravitational force. All other forces are ultimately derived from these fundamental forces.

The electromagnetic force is the one responsible for practically all the phenomena encountered in daily life, with the exception of gravity. All the forces involved in interactions between atoms can be traced to the electromagnetic force acting on the electrically charged protons and electrons inside the atoms. This includes the forces we experience in "pushing" or "pulling" ordinary material objects, which come from the intermolecular forces between the individual molecules in our bodies and those in the objects. It also includes all forms of chemical phenomena, which arise from interactions between electron orbitals.

[edit] Classical electrodynamics

Main article: Classical electrodynamics

The scientist William Gilbert proposed, in his De Magnete (1600), that electricity and magnetism, while both capable of causing attraction and repulsion of objects, were distinct effects. Mariners had noticed that lightning strikes had the ability to disturb a compass needle, but the link between lightning and electricity was not confirmed until Benjamin Franklin's proposed experiments in 1752. One of the first to discover and publish a link between man-made electric current and magnetism was Romagnosi, who in 1802 noticed that connecting a wire across a voltaic pile deflected a nearby compass needle. However, the effect did not become widely known until 1820, when Ørsted performed a similar experiment. Ørsted's work influenced Ampère to produce a theory of electromagnetism that set the subject on a mathematical foundation.

An accurate theory of electromagnetism, known as classical electromagnetism, was developed by various physicists over the course of the 19th century, culminating in the work of James Clerk Maxwell, who unified the preceding developments into a single theory and discovered the electromagnetic nature of light. In classical electromagnetism, the electromagnetic field obeys a set of equations known as Maxwell's equations, and the electromagnetic force is given by the Lorentz force law.

One of the peculiarities of classical electromagnetism is that it is difficult to reconcile with classical mechanics, but it is compatible with special relativity. According to Maxwell's equations, the speed of light in a vacuum is a universal constant, dependent only on the electrical permittivity and magnetic permeability of free space. This violates Galilean invariance, a long-standing cornerstone of classical mechanics. One way to reconcile the two theories is to assume the existence of a luminiferous aether through which the light propagates. However, subsequent experimental efforts failed to detect the presence of the aether. After important contributions of Hendrik Lorentz and Henri Poincaré, in 1905, Albert Einstein solved the problem with the introduction of special relativity, which replaces classical kinematics with a new theory of kinematics that is compatible with classical electromagnetism. (For more information, see History of special relativity.)

In addition, relativity theory shows that in moving frames of reference a magnetic field transforms to a field with a nonzero electric component and vice versa; thus firmly showing that they are two sides of the same coin, and thus the term "electromagnetism". (For more information, see Classical electromagnetism and special relativity.)

[edit] The photoelectric effect

Main article: Photoelectric effect

In another paper published in that same year, Albert Einstein undermined the very foundations of classical electromagnetism. His theory of the photoelectric effect (for which he won the Nobel prize for physics) posited that light could exist in discrete particle-like quantities, which later came to be known as photons. Einstein's theory of the photoelectric effect extended the insights that appeared in the solution of the ultraviolet catastrophe presented by Max Planck in 1900. In his work, Planck showed that hot objects emit electromagnetic radiation in discrete packets, which leads to a finite total energy emitted as black body radiation. Both of these results were in direct contradiction with the classical view of light as a continuous wave. Planck's and Einstein's theories were progenitors of quantum mechanics, which, when formulated in 1925, necessitated the invention of a quantum theory of electromagnetism. This theory, completed in the 1940s, is known as quantum electrodynamics (or "QED"), and is one of the most accurate theories known to physics.

[edit] Definition

The term electrodynamics is sometimes used to refer to the combination of electromagnetism with mechanics, and deals with the effects of the electromagnetic field on the dynamic behavior of electrically charged particles.

[edit] Units

Electromagnetic units are part of a system of electrical units based primarily upon the magnetic properties of electric currents, the fundamental cgs unit being the ampere. The units are:

In the electromagnetic cgs system, electrical current is a fundamental quantity defined via Ampère's law and takes the permeability as a dimensionless quantity (relative permeability) whose value in a vacuum is unity. As a consequence, the square of the speed of light appears explicitly in some of the equations interrelating quantities in this system.

SI electromagnetism units
v • d • e
Symbol[1]Name of QuantityDerived UnitsUnitBase Units
IElectric currentampere (SI base unit)AA (= W/V = C/s)
QElectric chargecoulombCA·s
U, ΔV, Δφ; EPotential difference; Electromotive forcevoltVJ/C = kg·m2·s−3·A−1
R; Z; XElectric resistance; Impedance; ReactanceohmΩV/A = kg·m2·s−3·A−2
ρResistivityohm metreΩ·mkg·m3·s−3·A−2
PElectric powerwattWV·A = kg·m2·s−3
CCapacitancefaradFC/V = kg−1·m−2·A2·s4
EElectric field strengthvolt per metreV/mN/C = kg·m·A−1·s−3
DElectric displacement fieldcoulomb per square metreC/m2A·s·m−2
εPermittivityfarad per metreF/mkg−1·m−3·A2·s4
χeElectric susceptibility(dimensionless)--
G; Y; BConductance; Admittance; SusceptancesiemensSΩ−1 = kg−1·m−2·s3·A2
κ, γ, σConductivitysiemens per metreS/mkg−1·m−3·s3·A2
BMagnetic flux density, Magnetic inductionteslaTWb/m2 = kg·s−2·A−1 = N·A−1·m−1
ΦMagnetic fluxweberWbV·s = kg·m2·s−2·A−1
HMagnetic field strengthampere per metreA/mA·m−1
L, MInductancehenryHWb/A = V·s/A = kg·m2·s−2·A−2
μPermeabilityhenry per metreH/mkg·m·s−2·A−2
χMagnetic susceptibility(dimensionless)--

[edit] Electromagnetic phenomena

This article may require cleanup to meet Wikipedia's quality standards.
Please improve this article if you can. (September 2008)

In the theory, electromagnetism is the basis for optical phenomena, as discovered by James Clerk Maxwell while he studied electromagnetic waves.[2] Light, being an electromagnetic wave, has properties that can be explained through Maxwell's equations, such as reflection, refraction, diffraction, interference and others. Relativity is born on the electromagnetic fields, as shown by Albert Einstein when he tried to make the electromagnetic theory compatible with Planck's radiation formula.[3]

[edit] See also

Electromagnetism portal

[edit] References

This article or section includes a list of references or external links, but its sources remain unclear because it lacks inline citations.
You can improve this article by introducing more precise citations where appropriate. (February 2008)

  1. ^ International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry (2nd Edn). Oxford: Blackwell Science. ISBN 0-632-03583-8. pp. 14–15. Electronic version.
  2. ^ The Light Cone: Maxwell and the Electrodynamic Theory of Light
  3. ^ On Einstein's distrust of the electromagnetic theory: The origin of the light-velocity postulate

Web

Books

  • Durney, Carl H. and Johnson, Curtis C. (1969). Introduction to modern electromagnetics. McGraw-Hill. ISBN 0-07-018388-0. 
  • Rao, Nannapaneni N. (1994). Elements of engineering electromagnetics (4th ed.). Prentice Hall. ISBN 0-13-948746-8. 
  • Tipler, Paul (1998). Physics for Scientists and Engineers: Vol. 2: Light, Electricity and Magnetism, 4th ed., W. H. Freeman. ISBN 1-57259-492-6. 
  • Griffiths, David J. (1998). Introduction to Electrodynamics, 3rd ed., Prentice Hall. ISBN 0-13-805326-X. 
  • Jackson, John D. (1998). Classical Electrodynamics, 3rd ed., Wiley. ISBN 0-471-30932-X. 
  • Rothwell, Edward J.; Cloud, Michael J. (2001). Electromagnetics. CRC Press. ISBN 0-8493-1397-X. 
  • Wangsness, Roald K.; Cloud, Michael J. (1986). Electromagnetic Fields (2nd Edition). Wiley. ISBN 0-471-81186-6. 
  • Dibner, Bern (1961). Oersted and the discovery of electromagnetism. Blaisdell Publishing Company. ISSN 99-0317066-1 ; 18. 

[edit] External links

طاها یکشنبه 28 مهر‌ماه سال 1387 ساعت 07:31 ب.ظ
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