4.1 Electrification by friction
5 Conservation of electric charge 6 Relativistic invariance 7 See also 8 References 9 External links
Diagram showing field lines and equipotentials around an electron, a negatively charged particle. In an electrically neutral atom, the number of electrons is equal to the number of protons (which are positively charged), resulting in a net zero overall charge
Charge is the fundamental property of forms of matter that exhibit
electrostatic attraction or repulsion in the presence of other matter.
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 unit of quantity of electric charge is the coulomb, which is equivalent to about 7018624200000000000♠6.242×1018 e (e is the charge of a proton). Hence, the charge of an electron is approximately 3018839800000000000♠−1.602×10−19 C. The coulomb is defined as the quantity of charge that has passed through the cross section of an electrical conductor carrying one ampere within one second. 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 "elementary charge", "fundamental unit of charge", or simply as "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, 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. History
Coulomb's torsion balance
As reported by the ancient Greek mathematician Thales of Miletus
around 600 BC, charge (or electricity) could be accumulated by rubbing
fur on various substances, such as amber. The Greeks observed that the
charged amber buttons could attract light objects such as hair. They
also found that if they rubbed the amber for long enough, they could
even get an electric spark to jump. This property derives from the
In 1600, the English scientist William Gilbert returned to the subject
in De Magnete, and coined the
This section contains close paraphrasing of an external source, https://archive.org/details/ATreatiseOnElectricityMagnetism-Volume1. Relevant discussion may be found on the talk page. Ideas in this article should be expressed in an original manner. (November 2014) (Learn how and when to remove this template message)
When a piece of glass and a piece of resin—neither of which exhibit any electrical properties—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 phenomena, 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
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 convention—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.
Actually, all bodies are electrified, but may appear not electrified
because of the relatively similar charge of neighboring objects in the
environment. An object further electrified + or – creates an
equivalent or opposite charge by default in neighboring objects, until
those charges can equalize. The effects of attraction can be observed
in high-voltage experiments, while lower voltage effects are merely
weaker and therefore less obvious. The attraction and repulsion forces
are codified by
Flavour in particle physics
Flavour quantum numbers
Isospin: I or I3 Charm: C Strangeness: S Topness: T Bottomness: B′
Related quantum numbers
Baryon number: B Lepton number: L Weak isospin: T or T3 Electric charge: Q X-charge: X
Y = (B + S + C + B′ + T) Y = 2 (Q − I3)
Weak hypercharge: YW
YW = 2 (Q − T3) X + 2YW = 5 (B − L)
CKM matrix PMNS matrix Flavour complementarity
v t e
Conservation of electric charge Main article: Charge conservation 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 net change in charge density ρ within a volume of integration V is equal to the area integral over the current density J through the closed surface S = ∂V, which is in turn equal to the net current I:
displaystyle - frac d dt int _ V rho ,mathrm d V=
displaystyle scriptstyle partial V
= ∫ J
S cos θ = I .
displaystyle mathbf J cdot mathrm d mathbf S =int Jmathrm d Scos theta =I.
Thus, the conservation of electric charge, as expressed by the continuity equation, gives the result:
I = −
displaystyle I=- frac mathrm d Q mathrm d t .
The charge transferred between times
displaystyle t_ mathrm i
displaystyle t_ mathrm f
is obtained by integrating both sides:
displaystyle 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). See also
Quantity of electricity SI electromagnetism units Color charge Partial charge
^ Carron, Neal J. (21 May 2015). "Babel of units: The evolution of
units systems in classical electromagnetism" (PDF). p. 5.
Retrieved 31 March 2018.
^ Purcell, Edward M.; David J. Morin (2013).
How fast does a charge decay? History of the electrical units.