Contact electrification

From Academic Kids

In the late-18th century, scientists developed sensitive instruments for detecting 'electrification', otherwise known as electrostatic charge imbalance. They quickly discovered that when two objects are touched together, sometimes the objects became spontaneously charged. One object developed a net negative charge, while the other developed an equal and opposite positive charge. The simple phenomenon of Electrification by Contact 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 cells, electroplating, thermocouples, and semiconductor junction devices including radio detector diodes, photocells, LEDs, and thermoelectric cells.


Triboelectric contact

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.

Electrolytic-metallic contact

If a piece of metal is touched against an electrolyte material, the metal will spontaneously become charged, while the electrolyte will acquire an equal and opposite charge. Upon first contact, a chemical reaction called a 'half-cell reaction' occurs on the metal surface. As metal ions are transferred to or from the electrolyte, and as the metal and electrolyte become oppositely charged, the increasing voltage at the thin insulating layer between metal and electrolyte will oppose the motion of the flowing ions, causing the chemical reaction to come to a stop. If a second piece of a different type of metal is placed in the same electrolyte bath, it will charge up and rise to a different voltage. If the first metal piece is touched against the second, the voltage on each metal piece will be forced to a different level, and the chemical reactions will run constantly. In this way the 'contact electrification' becomes continuous. At the same time, an electric current will appear, with the path forming a closed loop which lead from one metal part to the other, and leading out through the chemical reactions on the metal surface, through the electrolyte, then back into the chemical reactions on the second metal surface. In this way, contact electrification leads to the invention of the Galvanic cell or battery.

Metallic contact

If two metals having differing work functions are touched together, one steals electrons from the other, and the opposite net charges grow larger and larger. 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 and the thermoelectric cooler.

Semiconductor Contact

If metal touches a semiconductive material, or if two different semiconductors are placed into contact, one becomes charged slightly postive 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 cells. 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.


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