Electrophorus

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Electrophorus from the 1800s. Electrophorus device.png
Electrophorus from the 1800s.

An electrophorus or electrophore is a simple manual capacitive electrostatic generator used to produce electrostatic charge via the process of electrostatic induction. A first version of it was invented in 1762 by Swedish professor Johan Carl Wilcke, [1] [2] [3] [4] but Italian scientist Alessandro Volta improved and popularized the device in 1775, [5] and is sometimes erroneously credited with its invention. [6] [7] The word electrophorus was coined by Volta from the Greek ήλεκτρον ('elektron'), and ϕέρω ('phero'), meaning 'electricity bearer'. [8]

Capacitor Passive two-terminal electronic component that stores electrical energy in an electric field

A capacitor is a device that stores electrical energy in an electric field. It is a passive electronic component with two terminals.

Electrostatic generator

An electrostatic generator, or electrostatic machine, is an electromechanical generator that produces static electricity, or electricity at high voltage and low continuous current. The knowledge of static electricity dates back to the earliest civilizations, but for millennia it remained merely an interesting and mystifying phenomenon, without a theory to explain its behavior and often confused with magnetism. By the end of the 17th century, researchers had developed practical means of generating electricity by friction, but the development of electrostatic machines did not begin in earnest until the 18th century, when they became fundamental instruments in the studies about the new science of electricity. Electrostatic generators operate by using manual power to transform mechanical work into electric energy. Electrostatic generators develop electrostatic charges of opposite signs rendered to two conductors, using only electric forces, and work by using moving plates, drums, or belts to carry electric charge to a high potential electrode. The charge is generated by one of two methods: either the triboelectric effect (friction) or electrostatic induction.

Electrostatic induction Redistribution of electric charge due to presence of other charges

Electrostatic induction, also known as "electrostatic influence" or simply "influence" in Europe and Latin America, is a redistribution of electric charge in an object, caused by the influence of nearby charges. In the presence of a charged body, an insulated conductor develops a positive charge on one end and a negative charge on the other end. Induction was discovered by British scientist John Canton in 1753 and Swedish professor Johan Carl Wilcke in 1762. Electrostatic generators, such as the Wimshurst machine, the Van de Graaff generator and the electrophorus, use this principle. Due to induction, the electrostatic potential (voltage) is constant at any point throughout a conductor. Electrostatic Induction is also responsible for the attraction of light nonconductive objects, such as balloons, paper or styrofoam scraps, to static electric charges. Electrostatic induction laws apply in dynamic situations as far as the quasistatic approximation is valid. Electrostatic induction should not be confused with Electromagnetic induction.

Contents

Description and operation

The electrophorus consists of a dielectric plate (originally a 'cake' of resinous material such as pitch or wax, but in modern versions plastic is used) and a metal plate with an insulating handle. [9] The dielectric plate is first charged through the triboelectric effect by rubbing it with fur or cloth. For this discussion, imagine the dielectric gains negative charge by rubbing, as in the illustration below. The metal plate is then placed onto the dielectric plate. The dielectric does not transfer a significant fraction of its surface charge to the metal because the microscopic contact is poor. Instead the electrostatic field of the charged dielectric causes the charges in the metal plate to separate. It develops two regions of charge—the positive charges in the plate are attracted to the side facing down toward the dielectric, charging it positively, while the negative charges are repelled to the side facing up, charging it negatively, with the plate remaining electrically neutral as a whole. Then, the side facing up is momentarily grounded (which can be done by touching it with a finger), draining off the negative charge. Finally, the metal plate, now carrying only one sign of charge (positive in our example), is lifted. [4]

Dielectric electrically poorly conducting or non-conducting, non-metallic substance of which charge carriers are generally not free to move

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced in the direction of the field and negative charges shift in the opposite direction. This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axes align to the field.

Triboelectric effect

The triboelectric effect is a type of contact electrification on which certain materials become electrically charged after they are separated from a different material with which they were in contact. Rubbing the two materials each with the other increases the contact between their surfaces, and hence the triboelectric effect. Rubbing glass with fur for example, or a plastic comb through the hair, can build up triboelectricity. Most everyday static electricity is triboelectric. The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties.

Electrophorus cutaway.png
Showing induced charge on plate before grounding.
Electrophorus.png
Grounding electrophorus by touching it.

Since the charge on the dielectric is not depleted in this process, the charge on the metal plate can be used for experiments, for example by touching it to metal conductors allowing the charge to drain away, and the uncharged metal plate can be placed back on the dielectric and the process repeated to get another charge. This can be repeated as often as desired, so in principle an unlimited amount of induced charge can be obtained from a single charge on the dielectric. For this reason Volta called it elettroforo perpetuo (the perpetual electricity bearer). [10] In actual use the charge on the dielectric will eventually (within a few days at most) leak off through the surface of the cake or the atmosphere to recombine with opposite charges around to restore neutrality.

One of the largest examples of an electrophorus was built in 1777 by German scientist Georg Christoph Lichtenberg. [8] It was 6 feet (2 m) in diameter, with the metal plate raised and lowered using a pulley system. It could reportedly produce 15 inch (38 cm) sparks. Lichtenberg used its discharges to create the strange treelike marks known as Lichtenberg figures.

Georg Christoph Lichtenberg German scientist, satirist

Georg Christoph Lichtenberg was a German physicist, satirist, and Anglophile. As a scientist, he was the first to hold a professorship explicitly dedicated to experimental physics in Germany. He is remembered for his posthumously published notebooks, which he himself called Sudelbücher, a description modelled on the English bookkeeping term "scrapbooks", and for his discovery of tree-like electrical discharge patterns now called Lichtenberg figures.

Lichtenberg figure

Lichtenberg figures, or "Lichtenberg dust figures", are branching electric discharges that sometimes appear on the surface or in the interior of insulating materials. Lichtenberg figures are often associated with the progressive deterioration of high voltage components and equipment. The study of planar Lichtenberg figures along insulating surfaces and 3D electrical trees within insulating materials often provides engineers with valuable insights for improving the long-term reliability of high voltage equipment. Lichtenberg figures are now known to occur on or within solids, liquids, and gases during electrical breakdown.

The source of the charge

Charge in the universe is conserved. The electrophorus simply separates positive and negative charges. A positive or negative charge ends up on the metal plate (or other storage conductor), and the opposite charge is stored in another object after grounding (in the earth or the person touching the metal plate). This separation takes work since the lowest energy state implies uncharged objects. Work is done by raising the charged metal plate away from the oppositely charged resinous plate. This additional energy put into the system is converted to potential energy in the form of charge separation (opposite charges that were originally on the plate), so raising the metal plate actually increases its voltage relative to the dielectric plate.

Electric potential energy potential energy that results from conservative Coulomb forces

Electric potential energy, or electrostatic potential energy, is a potential energy that results from conservative Coulomb forces and is associated with the configuration of a particular set of point charges within a defined system. An object may have electric potential energy by virtue of two key elements: its own electric charge and its relative position to other electrically charged objects.

The electrophorus is thus actually a manually operated electrostatic generator, using the same principle of electrostatic induction as electrostatic machines such as the Wimshurst machine and the Van de Graaff generator. [4]

Electric generator device that converts other energy to electrical energy

In electricity generation, a generator is a device that converts motive power into electrical power for use in an external circuit. Sources of mechanical energy include steam turbines, gas turbines, water turbines, internal combustion engines, wind turbines and even hand cranks. The first electromagnetic generator, the Faraday disk, was invented in 1831 by British scientist Michael Faraday. Generators provide nearly all of the power for electric power grids.

Wimshurst machine

The Wimshurst influence machine is an electrostatic generator, a machine for generating high voltages developed between 1880 and 1883 by British inventor James Wimshurst (1832–1903).

Van de Graaff generator Electrostatic particle accelerator driven by the triboelectricity effect

A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate electric charge on a hollow metal globe on the top of an insulated column, creating very high electric potentials. It produces very high voltage direct current (DC) electricity at low current levels. It was invented by American physicist Robert J. Van de Graaff in 1929. The potential difference achieved by modern Van de Graaff generators can be as much as 5 megavolts. A tabletop version can produce on the order of 100,000 volts and can store enough energy to produce a visible spark. Small Van de Graaff machines are produced for entertainment, and for physics education to teach electrostatics; larger ones are displayed in some science museums.

Notes

  1. For information about Wilcke's research on the electrophorus (or "dissectible condenser"), see Wilcke, John Carl (1762) "Ytterligare rön och försök om contraira electriciteterne vid laddningen och därtil hörande delar" ("Additional findings and experiments on the opposing electric charges [that are created] during charging, and parts related thereto") in Kongliga Svenska Vetenskaps Academiens Handlingar (Proceedings of the Royal Swedish Science Academy), vol. 23 , pp. 206-229, 245-266. Reprinted in German as: John Carl Wilcke (1765) "Fernere Untersuchung von den entgegengesetzten Elecktricitäten bei der Ladung und den dazu gehörenden Theilen" (Further investigation of the opposing electric charges [that are created] during charging and the parts belonging thereto), Der Königliche schwedischen Akademie der Wissenschaften, Abhandlungen aus der Naturlehre, … , vol. 24, pp. 213-235, 253-274.
  2. Heilbron, J.L. Electricity in the 17th and 18th centuries: A study of early modern physics (Berkeley, California: University of California Press, 1979), pp. 418-419
  3. Pancaldi, Giuliano (2003). Volta, Science and Culture in the Age of Enlightenment. Princeton University Press. ISBN   0-691-12226-1. p. 73
  4. 1 2 3 Jones, Thomas B. (July 2007). "Electrophorus and accessories". Thomas B. Jones website. University of Rochester. Retrieved 27 December 2007.
  5. Pancaldi 2003, pp. 75-105
  6. Lewis, Nancy D. "Alesandro Volta, The Perpetual Electrophorus". Electricity:A Summary of Scientists and their Discoveries. Retrieved 27 December 2007.
  7. "Alessandro Volta". World Of Biography. Retrieved 27 December 2007.Cite web requires |website= (help)
  8. 1 2 Harris, William Snow (1867). A Treatise on Frictional Electricity in Theory and Practice. London: Virtue & Co. p. 86.
  9. "Electrophorus". Encyclopædia Britannica, 11th Ed. 9. The Encyclopædia Britannica Publishing Co. 1910. Retrieved 27 December 2007. p. 237
  10. Schiffer, Michael Brian (2003). Draw the Lightning Down:Benjamin Franklin and electrical technology in the Age of Enlightenment. University of California Press. ISBN   0-520-23802-8. pp. 55-57

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