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Contact electrification phenomenon allowed the construction of so-called 'frictional' electrostatic generators such as Ramsden's or Winter's machines, but it also led directly to the development of most modern electrical technology such as Batteries , Fuel Cell s, Electroplating , Thermocouple s, and semiconductor junction devices including Radio Detector Diodes , Photocell s, LED s, and Thermoelectric Cell s. HISTORY The theory held that static electricity was generated by means of contact between dissimilar materials, and was in close agreement with the principles of Static Electricity as then understood. It was eventually replaced by the current theory of Electrochemistry , namely, that electricity is generated by the action of Chemistry and the exchange of Electron s between Atom s making up the battery. An important fact leading to the rejection of the theory of contact tension was the observation that Corrosion , that is, the chemical degradation of the battery, seemed unavoidable with its use, and that the more electricity was drawn from the battery, the faster the corrosion proceeded. In fact, the Volta effect does correspond to a weak Electric Potential developed by the contact of different metals. This effect was first discovered by Alessandro Volta , and can be measured using a Capacitance Electroscope comprised of different metals. However, the actual effect is not sufficiently strong to account for the action of electric batteries. A number of High Voltage Dry Pile s were invented between the early 1800s and the 1830s in an attempt to determine the answer to this question, and specifically to support Volta’s hypothesis of contact tension. The Oxford Electric Bell is one example. TRIBOELECTRIC CONTACT ''Main article: Triboelectric Effect '' If two different Insulators are touched together, such as when a piece of rubber is touched against a piece of glass, then the surface of the rubber will acquire an excess negative charge, and the glass will acquire an equal positive charge. If the surfaces are then pulled apart, a very High Voltage is produced. This so-called "tribo" or "rubbing" effect is not well understood. It may be caused by electron-stealing via Quantum Tunneling , or by transfer of surface ions. Friction is not required, although in many situations it greatly increases the phenomenon. Certain phenomena related to frictionally generated electrostatic charges have been known since antiquity, though of course the modern theory of electricity was developed after the Scientific Revolution . ELECTROLYTIC-METALLIC CONTACT If a piece of metal is touched against an '' METALLIC CONTACT If two metals having differing Work Function s are touched together, one steals electrons from the other, and the opposite net charges grow larger and larger; this is the Volta Effect . The process is halted when the voltage between the two metals reaches a particular value (the difference in work function values; usually less than one volt.) If part of the junction between the metals is heated, and another part cooled, the voltage across the different parts of the junction will not be the same, and an electric current will appear. In this way contact electrification leads to the invention of the Thermocouple . This is the Peltier-Seebeck Effect . SEMICONDUCTOR CONTACT If metal touches a semiconductive material, or if two different semiconductors are placed into contact, one becomes charged slightly positive and the other slightly negative. It is found that if this junction between semiconductors is connected to a power supply, and if the power supply is set to a voltage slightly higher than the natural voltage appearing because of contact electrification, then for one polarity of voltage there will be a current between the two semiconductor parts, but if the polarity is reversed, the current stops. Thus contact electrification leads to the invention of the Diode or Rectifier and triggers the revolution in Semiconductor Electronics and Physics . If bright light is aimed at one part of the contact area between the two semiconductors, the voltage at that spot will rise, and an electric current will appear. When light meets contact electrification, the light energy is changed directly into electrical energy, allowing creation of Solar Cell s. Later it was found that the same process can be reversed, and if a current is forced backwards across the contact region between the semiconductors, sometimes light will be emitted, allowing creation of the LED . REFERENCES |
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