Electric charge


 * See also: Electricity

Introduction
Electric charge, an isolatable form of charge, a broad term that includes forms other than electric charge. Electric charge underlies the phenomena of electricity and electromagnetism, and manifests itself as multiples of the elementary charge often denoted by e and with a value in SI units of 1.602 176 565 x 10-19 C. Moving electric charges constitute an electric current, and electric charges and currents are the sources of the electromagnetic field that determines the forces acting upon charges. In an approach based upon quantum field theory, these forces are mediated by photons, charge neutral, massless particles exchanged by interacting charges and currents.

In the atoms of matter, electric charge is a property of protons and electrons, assigned the charge-names, positive and negative, respectively, the two types of charged particles, though spatially separate, exhibiting mutual attraction, the particles within types exhibiting mutual repulsion. Electrically positively charged tangible (macroscopic) matter possesses an excess of protons over electrons, and electrically negatively charged tangible matter possesses an excess of electrons over protons.

Scientists have not determined how electric charge emerges in nature.

Classically, two types of electromagnetic charge are known, magnetic and electric. The distinguishing property of electric charge is that electric charges can be isolated, while while an isolated magnetic charge or magnetic monopole never has been observed. Electric charges interact with magnetic charges only when in relative motion one to the other.

Whatever constitutes electric charge, it exists with two separate qualities, or polarities, assigned the names 'positive' and 'negative', or 'plus' and 'minus'. The attractive force between electrically charged entities arises between oppositely-charged entities&mdash;positive-negative&mdash;whereas the repulsive force arises between like-charged entities&mdash;positive-positive, or negative-negative.

Given that the terms 'positive' and 'negative' serve only as labels to distinguish the two polarities observed in the electric charge of matter, 'positivity' and 'negativity' do not themselves imply anything about the fundamental nature of electric charge. Other labels connoting bi-polarity, such as yin/yang, black/white, or bitter/sweet, could serve for labeling.

The atoms that comprise the chemical elements of the periodic table, while consisting in part of the electrically charged particles, protons and electrons, do not themselves manifest an electric charge, because protons in the nuclei and the surrounding electrons are equal in number and quantity of charge, that balance ensuring that the atoms as a whole manifest no net electric charge&mdash;a state referred to as electrical neutrality.

Classically, electric charge occurs in discrete quantities, multiples of the charge of the electron. At the level of the elementary particle, quarks have fractional charges, multiples of 1/3 of an electron charge.

Electric charge is conserved, that is the total amount of electric charge does not change, although it can be partitioned differently among electrically charged objects.

Discovery and naming of electric charge
The ancient Greeks as far back as the beginning of the 6th century BCE, beginning with Thales of Miletus, had observed some of the simple phenomenology related to electric charge, Thales demonstrating it using the fossilized tree resin, amber, rubbed with cloth:

In 600 B.C. Thales, erudite philosopher and astronomer in the thriving Ionian port of Miletus, observed the special qualities of the rare yellow orange amber, jewel-like in its hardness and transparency. If rubbed briskly with a cloth, Thales showed, amber seemed to come alive, causing light objects—like feathers, straw, or leaves—to fly toward it, cling, and then gently detach and float away. Amber was similar to a magnet in its qualities, yet it was not a lodestone. As a youth, Thales of Miletus had studied in the sacred Egyptian cities of Memphis and Thebes. Perhaps it was there, under the burning sun, that this earliest of Greek philosophers first learned from the priests about the prized amber, with its seeming possession of a soul.

Thales, it appears, believed amber an animate thing, something with soul.

The Greek word for amber, elektron, ultimately through Latin, electrum, gave rise to the English words, electrical and electric &mdash; words used to refer to the amber phenomenon before the publication of William Gilbert's landmark work, De magnete, in 1600, describing the results of the first systematic experimental studies of magnetic and electrical phenomena in Western science.

The word, charge, used in its electrical sense, was first used by Benjamin Franklin, in 1747, as a verb, and subsequently by him as adjective and noun: Our spheres are fixed on iron axes, which is passed through them. At one and of the axis there is a small handle, with which you turn the sphere like a common grindstone. This we find very commodious, as the machine takes up little room, is portable, and may be enclosed in a tight box, when not in use. 'Tis true, the sphere does not turn so swift as when the great wheel is used, but swiftness we think of little importance, since a few turns will charge the phial, etc., sufficiently. [italics added]

Presumably, Franklin, who, in his many writings, frequently used the word, charge, and its variant forms (charging, charged, etc.), in its non-electrical sense, had in mind the word's sense of 'loading' or 'filling' something:

charge - ORIGIN: Middle English (in the general senses ‘to load’ and ‘a load’): from Old French charger (verb), charge (noun), from late Latin carricare, carcare ‘to load,’ from Latin carrus ‘wheeled vehicle.’...Examples: load or fill (a container, gun, etc.) to the full or proper extent: will you see to it that your glasses are charged? | fill or pervade (something) with a quality or emotion: the air was charged with menace.

William Gilbert, founder of electrical science
“Although the precise beginnings of electrical science are contestable, no one doubts that William Gilbert (1540–1603), an Englishman, carried out the first sustained and influential research on electrical phenomena.”

Relation to forces

 * Note: The SI units are used below.

The force upon a stationary point body with electrical charge q1 due to another such body with electrical charge q2 is governed by Coulomb's law:
 * $$ \mathbf{F_{12}} = -\frac{1}{4\pi \varepsilon_0}\frac{q_1 q_2 }{r_{12}^2}\mathbf{\hat u _{12}} \, $$

where r12 is their separation and û12 is a unit vector pointing from charge one to charge two. The minus sign indicates that the force F is repulsive when both charges have the same sign. The quantity &epsilon;0 is the electric constant, also called the permittivity of free space, and it is assumed that both charges are in classical vacuum.

For distributions of charge, rather than point charges, the force at any position in space can be found using Poisson's equation:
 * $$\nabla^2 \varphi = -\frac{\rho(\mathbf r)}{\varepsilon_0} \, $$

one of the Maxwell equations. Quantity &nabla;2 is the Laplacian operator of vector calculus. Here &rho;(r) is the charge density per unit volume located at position r, and &phi;(r) is the electric potential at position r. This equation is deceptively simple in appearance, and its solution involves careful consideration of the the materials in the space and their geometries. Having found the potential by solving this equation, the force upon a test charge of magnitude q (a point charge considered too small to affect the force in itself) at position r is determined by:
 * $$\mathbf F(\mathbf r) = -q\nabla \varphi \, $$

where &nabla; is the gradient operator of vector calculus. The quantity
 * $$\mathbf E (\mathbf r) = -\nabla \varphi (\mathbf r) \, $$

is called the electric field at point r.

When charges are moving relative to each other, the forces between them are much more complex. Moving charges constitute an electric current, generating magnetic fields and magnetic forces. See the article Liénard–Wiechert potentials.