Electrostatics

In physicselectrostatics is a branch of electromagnetism that studies electric charges “at rest”, that is those charges stationary in time, which in turn generate an electrostatic field. Phenomena of moving charges, which produce currents, are instead studied by electrodynamics.

Electrostatic phenomena derive from the forces that electric charges exert on each other. These forces are described by Coulomb’s law. Although electrostatically induced forces appear to be quite weak, some electrostatic forces such as that between an electron and a proton, which together form a hydrogen atom, are about 36 orders of magnitude stronger than the gravitational force acting between them.

In other words, electrostatics studies electromagnetic phenomena that occur when there are no moving charges, that is, after a static equilibrium has been established. Mathematical methods applied to electrostatics allow the calculation of electric field and electric potential distributions from a known configuration of charges.

Electrostatics involves the accumulation of charge on the surface of objects due to contact with other surfaces. Although charge exchange occurs whenever any two surfaces come into contact and then separate, the effects of charge exchange are usually only noticed when at least one of the surfaces has a high resistance to electric flow. This is because the transferred charges remain trapped long enough to be observed. These charges then remain on the object until they discharge to the ground or are rapidly neutralized by a discharge: for example, the familiar phenomenon of a static “shock” is caused by neutralization of the charge accumulated in the body by contact with isolated surfaces. Electrization of a body can generally be achieved by three different methods:

  • by rubbing;
  • by contact (permanent electrization)
  • by induction (temporary electrization).

The ability of some materials such as amber (natural resin), or glass, when rubbed on wool, to attract small pieces of paper was known since ancient times. Plato, in the fourth century BC considered the origin of these effects similar to that of magnetic phenomena. Plutarch, in the 1st century AD, observed that the nature of these effects must be dissimilar to that of magnetic phenomena since, while magnetite seemed to attract only iron, rubbed amber attracts different objects as long as they are light.

In the sixteenth century, W. Gilbert discovered that other substances when rubbed acquire the same properties, while others do not. He introduced the adjective electric to indicate this class of phenomena, from the Greek name of amber, ηλε xτρον. He formulated a theory that justified this phenomenon, called electrization by rubbing, assuming that, due to the heating of the bodies by rubbing, was emitted from the body a fluid that had the ability to attract light objects placed nearby. In the first half of the 18th century, the French scientist C. Du Fay began a methodical research activity around electrical phenomenology and verified that:

all materials, except metals, could be electrified by rubbing deducing that electricity is a property of matter;
rubbed objects did not always attract small bodies but, in some cases repelled them;
there had to exist two types of electricity that he called resinous electricity and glassy electricity and he proposed a theory that non-electrified bodies have the two types of electricity in equal measure.

Electrostatic phenomena

Are called electrostatic phenomena all those electrical phenomena that are produced in the space (and in the bodies that are immersed in it) by free electric charges, positive or negative, that are in static equilibrium on the electrified bodies. In 1887, J. Thomson discovered electron and identified atom as the fundamental component of matter. It was then possible to provide an explanation of the phenomenology of electrization by rubbing: the state of electric equilibrium of atoms of rubbed bodies has been changed, because the energy developed in the rubbing operation has literally “torn” some electrons. In correspondence to the rubbing of the glass rod with a wool cloth, some electrons from the rod are torn off by the abrasive action and are transferred to the cloth. Therefore the glass rod acquires a non-zero net charge: it becomes electrified.

There are many examples of electrostatic phenomena, such as when you rub a glass rod with a woolen cloth, some electrons free to move, leave the rod and transfer to the woolen cloth. The glass rod loses electrons and has more protons than electrons, so it is said to be positively charged; the cloth, instead, gains electrons and has more electrons than protons, so it is said to be negatively charged. If instead of a glass wand is used an amber wand, electrons pass from the wool cloth to the wand. In this case the amber wand, which gains electrons, is negatively charged and the cloth, which loses electrons, is positively charged. So when two bodies are rubbed, one is positively electrified and one negatively.

They were called insulators, material bodies that are charged by rubbing, that do not easily carry charges (for example rubber in the coating of electric wires). They were called conductors, material bodies in which electrical charges move freely. They were called semiconductors, the material bodies that have intermediate behavior between conductors and insulators (such as silicon and germanium used in integrated circuits of electronic computers). They were called superconductors, the material bodies perfectly conducting, that allow charges to move inside them without any obstacle (absence of electrical resistance).

If we remember that, according to Coulomb’s law, electric charges act mutually on each other with mutual attractions and repulsions, which are exerted in all directions radiating from each of them, we understand that electric actions are not manifested only within the bodies in which they are contained, but extend and invest the entire surrounding space: experience proves that all electric actions are exerted at a distance even through empty space without the intervention of any material continuity that must transmit them.

A point electric charge, positive or negative, acts radially in all directions on all other charges of opposite sign. We express this fact by saying that every positive or negative charge, is always subject to a force that is the resultant of attractions and repulsions that it feels from the surrounding elementary charges. This fact can be expressed by saying that every electric charge undergoes the action of the electric field resulting from the action of the proper fields of all the remaining electric charges.

An alternative procedure to electrization by rubbing, is called electrization by induction. Approaching a charged body, to a neutral isolated conducting sphere, the region of the sphere closest to the charged body is charged with the opposite sign, while the farthest one is charged with the same sign (in fact the electrons of the neutral sphere move, leaving uncovered the positive charge). If the sphere, instead of being isolated, is connected to the ground, some electrons flow towards the ground (the sphere and the ground are a single conductor, the electrons move away). Interrupting the connection the sphere remains positively charged. By subsequently moving the charged body away, the charge of the sphere is distributed uniformly due to the mutual repulsion of the equal charges. Also in this case, the principle of conservation of charge continues to apply.

Historical notes

The first documented experiments and researches on electrostatics date back to ancient Greece of 600 BC with Thales of Miletus and Theophrastus, discovered that rubbing of amber (which in ancient greek was called ἤλεκτρον, élektron) with a woolen cloth, allowed to attract towards itself straws, feathers, wires and the like. The different forms of this device were later called in 1600 “electrical phenomena” thanks to a second scientific contribution regarding the study of these phenomena, by William Gilbert, who distinguished these phenomena from magnetic ones, as he had observed that electrical phenomena had a finite energy (which was called electric fluid) and the force of attraction lasted as long as there was enough energy, coining also the term electric force.

The third contribution was made by Otto Von Guericke in the middle of XVII century, both for the realization of the first electrostatic generator (Guericke’s electrostatic rubbing machine), with which he observed the electric discharges generated during its loading and the relative luminescence and crackling (this phenomenon was called “electric fire”), but he also demonstrated how the electric force generated by a charged body could be transported, applying to this charged body a wire, which has the same properties of the charged body. Guericke made another important discovery: studying amber he noticed that objects that were initially attracted by it, once in contact with the charged amber, were repelled by it, thus demonstrating that the electric force can be both attractive and repulsive.

Subsequently Charles François de Cisternay du Fay, determined the existence of a positive electric charge and a negative one, which were generated by different substances, which were called “resinous” (amber, hard rubber, wax, resinous substances) and “glassy” (glass and similar), discovering also that bodies charged in the same way and therefore with the same electrical charges repelled each other (amber-amber or glass-glass), while bodies charged with different charges attracted each other (amber-glass), at the same time assumed that neutral bodies contained equal amounts of the two electrical fluids, while charged bodies had an excess of one fluid compared to the other.

Later it was defined with more precision how the electrization occurs by electron transfer (so positively charging) or by electron acquisition (negatively charging), because the electron movement requires very small energies compared to protons.

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