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Electric charge

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ARTICLE ORIGINS Electric charge
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{{pp-move-indef|small=yes}}{{short description|Physical property that quantifies an object's interaction with electric fields}}

factoids
| symbols = Q| baseunits = C = A s| dimension = T I| extensive = yes| conserved = yes| derivations =}}{{Electromagnetism|cTopic=Electrostatics}}Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charge: positive and negative (commonly carried by protons and electrons respectively). Like charges repel and unlike attract. An object with an absence of net charge is referred to as {{Visible anchor|neutral|neutral}}. Early knowledge of how charged substances interact is now called classical electrodynamics, and is still accurate for problems that do not require consideration of quantum effects.Electric charge is a conserved property; the net charge of an isolated system, the amount of positive charge minus the amount of negative charge, cannot change. Electric charge is carried by subatomic particles. In ordinary matter, negative charge is carried by electrons, and positive charge is carried by the protons in the nuclei of atoms. If there are more electrons than protons in a piece of matter, it will have a negative charge, if there are fewer it will have a positive charge, and if there are equal numbers it will be neutral. Charge is quantized; it comes in integer multiples of individual small units called the elementary charge, e, about {{val|1.602|e=-19|u=coulombs}},{{physconst|e|ref=only}} which is the smallest charge which can exist freely (particles called quarks have smaller charges, multiples of {{sfrac|1|3}}e, but they are only found in combination, and always combine to form particles with integer charge). The proton has a charge of +e, and the electron has a charge of âˆ’e.An electric charge has an electric field, and if the charge is moving it also generates a magnetic field. The combination of the electric and magnetic field is called the electromagnetic field, and its interaction with charges is the source of the electromagnetic force, which is one of the four fundamental forces in physics. The study of photon-mediated interactions among charged particles is called quantum electrodynamics.The SI derived unit of electric charge is the coulomb (C) named after French physicist Charles-Augustin de Coulomb. In electrical engineering, it is also common to use the ampere-hour (Ah); in physics and chemistry, it is common to use the elementary charge (e as a unit). Chemistry also uses the Faraday constant as the charge on a mole of electrons. The symbol Q often denotes charge.

Overview

File:Electric field point lines equipotentials.svg|thumb|right|250px|Diagram showing field lines and equipotentials around an electron, a negatively charged particle. In an electrically neutral atomatomCharge is the fundamental property of forms of matter that exhibit electrostatic attraction or repulsion in the presence of other matter. Electric charge is a characteristic property of many subatomic particles. The charges of free-standing particles are integer multiples of the elementary charge e; we say that electric charge is quantized. Michael Faraday, in his electrolysis experiments, was the first to note the discrete nature of electric charge. Robert Millikan's oil drop experiment demonstrated this fact directly, and measured the elementary charge. It has been discovered that one type of particle, quarks, have fractional charges of either âˆ’{{sfrac|1|3}} or +{{sfrac|2|3}}, but it is believed they always occur in multiples of integral charge; free-standing quarks have never been observed.By convention, the charge of an electron is negative, âˆ’e, while that of a proton is positive, +e. Charged particles whose charges have the same sign repel one another, and particles whose charges have different signs attract. Coulomb's law quantifies the electrostatic force between two particles by asserting that the force is proportional to the product of their charges, and inversely proportional to the square of the distance between them. The charge of an antiparticle equals that of the corresponding particle, but with opposite sign.The electric charge of a macroscopic object is the sum of the electric charges of the particles that make it up. This charge is often small, because matter is made of atoms, and atoms typically have equal numbers of protons and electrons, in which case their charges cancel out, yielding a net charge of zero, thus making the atom neutral.An ion is an atom (or group of atoms) that has lost one or more electrons, giving it a net positive charge (cation), or that has gained one or more electrons, giving it a net negative charge (anion). Monatomic ions are formed from single atoms, while polyatomic ions are formed from two or more atoms that have been bonded together, in each case yielding an ion with a positive or negative net charge.{{multiple image| align = right| image1 = VFPt plus thumb.svg| alt1 = Electric field induced by a positive electric charge| width1 = 200| image2 = VFPt minus thumb.svg| alt2 = Electric field induced by a negative electric charge| width2 = 200| footer = Electric field induced by a positive electric charge (left) and a field induced by a negative electric charge (right).}}During formation of macroscopic objects, constituent atoms and ions usually combine to form structures composed of neutral ionic compounds electrically bound to neutral atoms. Thus macroscopic objects tend toward being neutral overall, but macroscopic objects are rarely perfectly net neutral.Sometimes macroscopic objects contain ions distributed throughout the material, rigidly bound in place, giving an overall net positive or negative charge to the object. Also, macroscopic objects made of conductive elements, can more or less easily (depending on the element) take on or give off electrons, and then maintain a net negative or positive charge indefinitely. When the net electric charge of an object is non-zero and motionless, the phenomenon is known as static electricity. This can easily be produced by rubbing two dissimilar materials together, such as rubbing amber with fur or glass with silk. In this way non-conductive materials can be charged to a significant degree, either positively or negatively. Charge taken from one material is moved to the other material, leaving an opposite charge of the same magnitude behind. The law of conservation of charge always applies, giving the object from which a negative charge is taken a positive charge of the same magnitude, and vice versa.Even when an object's net charge is zero, charge can be distributed non-uniformly in the object (e.g., due to an external electromagnetic field, or bound polar molecules). In such cases the object is said to be polarized. The charge due to polarization is known as bound charge, while charge on an object produced by electrons gained or lost from outside the object is called free charge. The motion of electrons in conductive metals in a specific direction is known as electric current.

Units

The SI derived unit of quantity of electric charge is the coulomb (symbol: C). The coulomb is defined as the quantity of charge that passes through the cross section of an electrical conductor carrying one ampere for one second.WEB,weblink BIPM, CIPM, 1946: Resolution 2, This unit was proposed in 1946 and ratified in 1948. In modern practice, the phrase "amount of charge" is used instead of "quantity of charge".{{SIbrochure8th}}, p. 150 The amount of charge in 1 electron (elementary charge) is approximately {{val|1.6|e=-19|u=C}}, and 1 coulomb corresponds to the amount of charge for about {{val|6.24|e=18|u=electrons}}. The symbol Q is often used to denote a quantity of electricity or charge. The quantity of electric charge can be directly measured with an electrometer, or indirectly measured with a ballistic galvanometer.After finding the quantized character of charge, in 1891 George Stoney proposed the unit 'electron' for this fundamental unit of electrical charge. This was before the discovery of the particle by J. J. Thomson in 1897. The unit is today treated as nameless, referred to as {{em|elementary charge}}, {{em|fundamental unit of charge}}, or simply as {{em|e}}. A measure of charge should be a multiple of the elementary charge e, even if at large scales charge seems to behave as a real quantity. In some contexts it is meaningful to speak of fractions of a charge; for example in the charging of a capacitor, or in the fractional quantum Hall effect.The unit faraday is sometimes used in electrochemistry. One faraday of charge is the magnitude of the charge of one mole of electrons,BOOK, Gambhir, RS, Banerjee, D, Durgapal, MC, Foundations of Physics, Vol. 2, 1993, Wiley Eastern Limited, New Dehli, 51,weblink 10 October 2018, 9788122405231, i.e. 96485.33289(59) C.In systems of units other than SI such as cgs, electric charge is expressed as combination of only three fundamental quantities (length, mass, and time), and not four, as in SI, where electric charge is a combination of length, mass, time, and electric current.ARXIV, Babel of units: The evolution of units systems in classical electromagnetism, Carron, Neal J., 5, 21 May 2015class = physics.hist-ph, BOOK, Purcell, Edward M., David J. Morin, Electricity and Magnetism, Cambridge University Press, 2013, 3rd, 766,weblink 9781107014022,

The role of charge in static electricity

Static electricity refers to the electric charge of an object and the related electrostatic discharge when two objects are brought together that are not at equilibrium. An electrostatic discharge creates a change in the charge of each of the two objects.

Electrification by friction

{{Further|Triboelectric effect}}{{Close paraphrasing|section|source=https://archive.org/details/ATreatiseOnElectricityMagnetism-Volume1|free=yes|date=November 2014}}When a piece of glass and a piece of resinâ€”neither of which exhibit any electrical propertiesâ€”are rubbed together and left with the rubbed surfaces in contact, they still exhibit no electrical properties. When separated, they attract each other.A second piece of glass rubbed with a second piece of resin, then separated and suspended near the former pieces of glass and resin causes these phenomena:
• The two pieces of glass repel each other.
• Each piece of glass attracts each piece of resin.
• The two pieces of resin repel each other.
This attraction and repulsion is an electrical phenomenon, and the bodies that exhibit them are said to be electrified, or electrically charged. Bodies may be electrified in many other ways, as well as by friction. The electrical properties of the two pieces of glass are similar to each other but opposite to those of the two pieces of resin: The glass attracts what the resin repels and repels what the resin attracts.If a body electrified in any manner whatsoever behaves as the glass does, that is, if it repels the glass and attracts the resin, the body is said to be vitreously electrified, and if it attracts the glass and repels the resin it is said to be resinously electrified. All electrified bodies are either vitreously or resinously electrified.An established convention in the scientific community defines vitreous electrification as positive, and resinous electrification as negative. The exactly opposite properties of the two kinds of electrification justify our indicating them by opposite signs, but the application of the positive sign to one rather than to the other kind must be considered as a matter of arbitrary conventionâ€”just as it is a matter of convention in mathematical diagram to reckon positive distances towards the right hand.No force, either of attraction or of repulsion, can be observed between an electrified body and a body not electrified.James Clerk Maxwell (1891) A Treatise on Electricity and Magnetism, pp. 32â€“33, Dover Publications

The role of charge in electric current

Electric current is the flow of electric charge through an object, which produces no net loss or gain of electric charge. The most common charge carriers are the positively charged proton and the negatively charged electron. The movement of any of these charged particles constitutes an electric current. In many situations, it suffices to speak of the conventional current without regard to whether it is carried by positive charges moving in the direction of the conventional current or by negative charges moving in the opposite direction. This macroscopic viewpoint is an approximation that simplifies electromagnetic concepts and calculations.At the opposite extreme, if one looks at the microscopic situation, one sees there are many ways of carrying an electric current, including: a flow of electrons; a flow of electron holes that act like positive particles; and both negative and positive particles (ions or other charged particles) flowing in opposite directions in an electrolytic solution or a plasma.Beware that, in the common and important case of metallic wires, the direction of the conventional current is opposite to the drift velocity of the actual charge carriers; i.e., the electrons. This is a source of confusion for beginners.{{Flavour quantum numbers}}

Conservation of electric charge

The total electric charge of an isolated system remains constant regardless of changes within the system itself. This law is inherent to all processes known to physics and can be derived in a local form from gauge invariance of the wave function. The conservation of charge results in the charge-current continuity equation. More generally, the rate of change in charge density Ï within a volume of integration V is equal to the area integral over the current density J through the closed surface S = âˆ‚V, which is in turn equal to the net current I:
{{oiint|preintegral=- frac{d}{dt} int_V rho , mathrm{d}V = |intsubscpt=scriptstyle partial V|integrand=mathbf J cdotmathrm{d}mathbf S = int J mathrm{d}S costheta = I.}}
Thus, the conservation of electric charge, as expressed by the continuity equation, gives the result:
I = -frac{mathrm{d}Q}{mathrm{d}t}.
The charge transferred between times t_mathrm{i} and t_mathrm{f} is obtained by integrating both sides:
Q = int_{t_{mathrm{i}}}^{t_{mathrm{f}}} I, mathrm{d}t
where I is the net outward current through a closed surface and Q is the electric charge contained within the volume defined by the surface.

Relativistic invariance

Aside from the properties described in articles about electromagnetism, charge is a relativistic invariant. This means that any particle that has charge Q, no matter how fast it goes, always has charge Q. This property has been experimentally verified by showing that the charge of one helium nucleus (two protons and two neutrons bound together in a nucleus and moving around at high speeds) is the same as two deuterium nuclei (one proton and one neutron bound together, but moving much more slowly than they would if they were in a helium nucleus).JOURNAL,weblink Relativistic invariance of electric charge, Jefimenko, O.D., 1999, Zeitschrift fÃ¼r Naturforschung A, 54, 10â€“11, 637â€“644, 10.1515/zna-1999-10-1113, 11 April 2018, 1999ZNatA..54..637J, WEB,weblink How can we prove charge invariance under Lorentz Transformation?, physics.stackexchange.com, 2018-03-27, JOURNAL, 1992, Singal, A.K., On the charge invariance and relativistic electric fields from a steady conduction current, Physics Letters A, en, 162, 2, 91â€“95, 10.1016/0375-9601(92)90982-R, 0375-9601, 1992PhLA..162...91S,

References

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