14-11-2012, 02:37 PM
Electrostatic generator
Electrostatic generator - Wikipedia, the free encyclopedia.pdf (Size: 701.71 KB / Downloads: 74)
Description
Electrostatic machines are typically used in science classrooms to safely demonstrate electrical forces and high
voltage phenomena. The elevated potential differences achieved have been also used for a variety of practical
applications, such as operating X-ray tubes, medical applications, sterilization of food, and nuclear physics
experiments. Electrostatic generators such as the Van de Graaff generator, and variations as the Pelletron, also find
use in physics research.
Friction machines
History
The first electrostatic generators are called friction machines because of
the friction in the generation process. A primitive form of frictional
machine was invented around 1663 by Otto von Guericke, using a
sulphur globe that could be rotated and rubbed by hand. It may not
actually have been rotated during use and was not intended to produce
electricity (rather cosmic virtues),[1] but inspired many later machines that
used rotating globes. Isaac Newton suggested the use of a glass globe
instead of a sulphur one.[2] Francis Hauksbee improved the basic
design,[3] his frictional electrical machine enabling a glass sphere to be
rotated rapidly against a woollen cloth.[4]
Generators were further advanced when Prof. Georg Matthias Bose of
Wittenberg added a collecting conductor (an insulated tube or cylinder
supported on silk strings). Boze was the first to employ the "prime
conductor" in such machines, this consisting of an iron rod held in the
hand of a person whose body was insulated by standing on a block of
resin.
In 1746, Watson's machine had a large wheel turning several glass
globes with a sword and a gun barrel suspended from silk cords for its
prime conductors. J. H. Winkler, professor of physics at Leipzig,
substituted a leather cushion for the hand. Andreas Gordon of Erfurt, a
Scottish Benedictine monk, used a glass cylinder in place of a sphere.
Jesse Ramsden, in 1768, constructed a widely used version of a plate
electrical generator. By 1784, the van Marum machine could produce
voltage with either polarity. Martin van Marum constructed a large
electrostatic machine of high quality for his experiments (currently on
display at the Teylers Museum in the Netherlands).
Friction operation
The presence of surface charge imbalance means that the objects will exhibit attractive or repulsive forces. This
surface charge imbalance, which leads to static electricity, can be generated by touching two differing surfaces
together and then separating them due to the phenomena of contact electrification and the triboelectric effect.
Rubbing two non-conductive objects generates a great amount of static electricity. This is not just the result of
friction; two non-conductive surfaces can become charged by just being placed one on top of the other. Since most
surfaces have a rough texture, it takes longer to achieve charging through contact than through rubbing. Rubbing
objects together increases amount of adhesive contact between the two surfaces. Usually insulators, e.g.,
substances that do not conduct electricity, are good at both generating, and holding, a surface charge. Some
examples of these substances are rubber, plastic, glass, and pith. Conductive objects in contact generate charge
imbalance too, but retain the charges only if insulated. The charge that is transferred during contact electrification is
stored on the surface of each object. Note that the presence of electric current does not detract from the
electrostatic forces nor from the sparking, from the corona discharge, or other phenomena. Both phenomena can
exist simultaneously in the same system.
Modern electrostatic generators
Electrostatic generators had a fundamental role in the investigations about
the structure of matter, starting at the end of the 19th century. By the
1920s, it was evident that machines able to produce greater voltage were
needed. The Van de Graaff generator was developed, starting in 1929,
at MIT. The first model was demonstrated in October 1929. The basic
idea was to use an insulating belt to transport electric charge to the
interior of an insulated hollow terminal, where it could be discharged
regardless of the potential already present on the terminal, that does not
produce any electric field in its interior. The idea was not new, but the
implementation using an electric power supply to charge the belt was a
fundamental innovation that made the old machines obsolete. The first
machine used a silk ribbon bought at a five and dime store as the charge
transport belt. In 1931 a version able to produce 1,000,000 volts was
described in a patent disclosure. Nikola Tesla wrote a Scientific
American article, "Possibilities of Electro-Static Generators" in 1934
concerning the Van de Graaff generator (pp. 132–134 and 163-165). Tesla stated, "I believe that when new
types [of Van de Graaff generators] are developed and sufficiently improved a great future will be assured to
them". High-power machines were soon developed, working on pressurized containers to allow greater charge
concentration on the surfaces without ionization. Variations of the Van de Graaff generator were also developed for
Physics research, as the Pelletron, that uses a chain with alternating insulating and conducting links for charge
transport. Simplified Van de Graaff generators are commonly seen in demonstrations about static electricity, due to
its high-voltage capability, producing the curious effect of making the hair of people touching the terminal, standing
over an insulating support, stand up.