Static electricity

Oh, what a spectacular and exciting experience! How admiringly the women screamed and paled, when the eminent scientists demonstrators took long bluish-violet sparks out of the discharged cavaliers, when they simply ignited alcohol and a handful of gunpowder when a few dozen cavaliers holding hands received a stunning blow, costing only the two extreme Touch an outwardly harmless glass jar ...

All these amazing effects were provoked to ridiculous by simple means: a glass rod, rubbed with dry fur, rotating glass balls and cylinders rubbing against the palm of a man isolated from the floor.

The general interest in electricity from friction in the second half of the eighteenth century can only be compared with enthusiasm, a hundred years earlier, caused by the discovery of atmospheric pressure. Even the most sober scientists succumbed to general intoxication. As they once tried to reduce everything to the effect of atmospheric pressure, now the manifestation of electricity has been contrived to see both the rotation of the planets around the Sun, and the appearance of an earthquake, and many diseases. It is no coincidence that the 1750s and 1780s entered the history of physics as a "period of electricity from friction".

The end of this period was "the creation of a device that, in its actions, is similar to a Leiden jar ... but which, however, operates continuously, that is, its charge after each discharge is restored by itself." So in 1799, A. Volta described his electric battery - a great invention, dramatically changed the whole course of electrical research.

Volta's pillar, which made it possible to obtain comparatively large currents at low voltages, focused the attention of scientists on the magnetic, mechanical and thermal actions of electric current, which by the end of the XIX century already formed the basis of all electrical engineering. But it was only in the twentieth century that interest in the once-neglected "electricity from friction" began to revive. And the cause of this revival was an important invention, made at the turn of the century, - the crown rank ...

Truly "crown" category

The industrial experience of the last century testified mainly to the negative actions of "electricity from friction". Without knowing it, the engineers built electrostatic generators of enormous size and, alas, quite high efficiency. We say: "alas," because their effectiveness was confirmed by the strongest explosions in powder plants, flour mills and sugar factories.

It turns out that it is impossible to transport sugar, flour and, in general, any dry powder through pipes or conveyors without accumulating electric charge. Leather and rubberized belts on rotating pulleys are also electrified to very high voltages. Paper, fabrics, rubber cords and tapes - and they are highly electrified during processing. And if in the air hangs a small combustible dust - say, flour or powdered sugar - then a spark from the electrified body can cause an explosion.

XX century incredibly expanded the scope of the harmful manifestation of electrostatic electricity. Numerous plastics, synthetic and synthetic fibers, varnishes and paints, oil, oil products and other electrifying liquids are not a complete list.

The aircraft are electrified during the flight. The oil is electrified during pumping through pipelines, even steam is electrified in the process of evaporation and movement through the pipes. Therefore, in our century, the attention of specialists was primarily aimed primarily at reducing the efficiency of accidental electrostatic generators, getting rid of electrification and its unpleasant consequences. And in addition to moistening processed materials and there are methods of ionization of air - radioactive isotopes and corona discharge ...

If a voltage exceeding 30 thousand volts is applied to two plates separated by a centimeter gap, a break occurs - a spark jumps, air ceases to be an insulator and becomes a conductor. And what happens if a negative voltage of 100 thousand volts is applied to the wire passing in the center of a grounded cylinder with a radius of 10 cm?

At first glance, nothing should happen: after all, for every centimeter of space separating the wire and the cylinder, there are not 30,000 in, necessary for breakdown, but only 10,000.

So it would be if it were a question of parallel plates creating a uniform electric field in the gap. A thin wire in the cylinder creates an inhomogeneous field, near the cylinder walls it is weaker, and in the zone adjacent to the wire, a voltage of more than 30,000 volts can occur per cm.

Electrons that escape from the wire surface are introduced into oxygen molecules and turn them into negatively charged ions, rushing to the walls of the cylinder under the action of an electric field. At this moment around the wire, and there is a greenish glow - the corona discharge. Making the air electrically conductive, this discharge removes the charge from the electrified substances.

For this, he was applied first. But then it turned out that in such a phenomenon there was a key to a wide industrial application of static electricity.

In 1905, the English inventor F. Cottrell began to pass through a corona-stripped pipe a gas contaminated with soot and ash particles. The ions in the discharge "clung" to the solid particles and reported to them a large negative charge, after which such particles were quickly rejected by an electric field on the walls of a grounded pipe, from which the purified gas exited.

The corona discharge, which made it possible to give charges to dielectric bodies many times greater than those that could be reported to them due to friction, gave industrial importance to static electricity. Experiments, previously served for entertainment, formed the basis of important technical devices and processes. There are installations for the separation of all kinds of loose mixtures with the help of electrostatics. It has become widely used in the technological processes of printing, processing paper and films.

In the electrostatic field, painting, the application of abrasive particles, dry powders and even short fibers to all kinds of substrates. Electrostatic field and corona discharge are the main participants of the xerographic process for fast reproduction of texts and methods of contactless printing.

This is how industry enters into life and "electricity from friction", which was fond in the second half of the 18th century and which was little studied for the next 150 years. And in this rapidly growing practical application of electrostatics, the secret of heightened interest in electrostatic generators, necessary for the activation of all these important technological processes.

Generator on razor blade

The fact that static electricity for a long time did not find useful practical application, curiously affected the fate of electrostatic generators. While electromagnetic devices and devices quickly left the walls of laboratories and, gaining an "engine" look, were affirmed on telegraphs, factories and power stations, electrostatic devices vegetated on the shelves of educational offices.

Of course, we can not say that they did not improve at all: between the Leyden jar charged with a glass-wool frayed glass and the familiar electrophoric school machine, the distance is huge. But neither a glass rod and furs, nor an electro-electric machine can be found anywhere except for educational physical cabinets: sources of static electricity have been improved not for industrial devices for almost a century and a half but as demonstration devices.

So, in place of machines in which glass balls, cylinders and disks were electrified by friction against wool or leather cushions, so-called "induction" machines came. Their action was based on a phenomenon discovered by Volta and having nothing to do with the "induction" that made up the glory of Faraday.

Volta noticed that if, for example, a positive metal plate is close but not brought into contact with a metal disk isolated from the ground, negative charges will accumulate on its surface facing the charged plate. Charges are positive, seeking to move away from the same charges of the plate, will gather on the outside.

If this external side is grounded for a short time, the positive charges will go to the ground, and the disk, even removed from the field of the plate, will be charged negatively. Having discharged it to the Leyden jar, it is possible to repeat all this operation again and again, during which the charge initially imparted to the plate itself is not consumed, but continuously "induces" - induces - a charge in a metal disk.

The first car, in which all these operations were performed automatically, was built in 1831 by the Italian Belly. Then it was perfected by the German physicists Tepler and Goltz, and finally, in the 1870s, Wimshurst's induction electromotive machine appeared, which now adorns the school's physical cabinets.

The study of the electric discharge in rarefied gases gave the first impetus to the improvement of electrostatic generators, and it turned out to be the maximum that could be squeezed out of multi-disk machines, this 300 thousand in and 1.2 kW.

The study of the atomic nucleus, which required even greater stress, led to the appearance of new structures. In fact, the generators of Van de Graaf and Felici, created respectively in the 1930s and 1940s, did not fundamentally differ from each other. The heart of each of them was a huge hollow metal ball, reliably isolated from the ground, on the inner surface of which the electric charge was continuously supplied. Only Van de Graaf used the tape on two rotating pulleys to charge the charge, and Felici - a rapidly rotating plastic cylinder.

The first generator of Van de Graaf, with a tape electrified by induction, was launched in 1936 and at a power of 6 kW gave a voltage of 5 million c. Later, for the electrification of tapes and cylinders, corona discharge began to be used, and by the 1950s, scientists had two types of generators.

The generators of Van de Graaf gave high voltages - up to 10-15 million volts - at low currents up to 1000 cca, while the Felichi generators, on the other hand, produced comparatively high currents - up to 10,000 cu, at lower voltages, up to 1 million cu. But what an expensive price these characteristics get!

First, the size. A generator of several tens of kilowatts is a structure 5-10 m high. Secondly, a "crown" also appears near the spherical electrode on which the charge accumulates. To suppress it, you have to place the entire plant in a sealed steel casing, filled with gas under high pressure. Thus, the generators of Van de Graaf are filled with a mixture of nitrogen and carbon dioxide at a pressure of 30 atm, and the Felichi generators are hydrogenated at a pressure of 25 atm. Thirdly, relatively rapid wear of belts and cylinders leads to contamination of the internal cavities of the generator with dust

Not very significant, when it comes to unique generators for scientific research, these shortcomings become intolerant for industrial generators, which should work reliably for a long time. That is why in recent years, electrical engineers have been paying more and more attention to the development of cheaper, reliable, powerful and compact generators, primarily electrohydrodynamic generators.

Indeed, a moving charged tape or a rotating cylinder can be replaced by a flow of a charged dielectric liquid. Carrying a charge in its entire volume, such a flow should create a much larger current than a tape or a cylinder in which the charge is located only on the surface.

The main difficulty in creating an electrostatic generator working on a hydrocarbon-hexane stream was the communication of the electric charge of the liquid. Since the usual methods of electrification - corona discharge and radioactive radiation - caused an undesirable change in the properties of hexane, the researchers had to look for something better.

The prototype of the device, which served as the basis for further developments, was a pipe into which a cathode was inserted-a set of thin steel blades. To the edges of the blades with a small gap adjoined the grid anode, behind which the collector of the generator was located.

When the pump began to pump through the hexane tube, a high voltage was applied to the blades and to the grid, under which the electrons flowed from the blades into the thickness of the moving liquid and reported a charge counting in hundreds of microamps. Part of these charges immediately settled on a positively charged grid, and the rest flowed through the liquid through it to the collector, on which the accumulation of charge took place.

The main drawback of this scheme was the unproductive neutralization of "injected" in liquid charges on the grid. Therefore, in the following design, the blade points were located along the pipe walls along the liquid flow, and the mesh was rolled into a tube and placed along the pipe axis. Now the negative charges that drained from the blades did not have time to neutralize - the flow carried them to the collector faster than they could reach the grid.

When testing the first prototype model of an electrohydrodynamic generator created in England, a curious thing appeared. As the charge accumulates on the collector, its electric field increasingly resists the influx of new charges. Then comes the moment when the charges completely stop reaching the collector and begin to accumulate at the output from the injector. And since like charges repel each other, they begin to move to the walls of the pipe and eventually break it.

It is the danger of mechanical destruction that limited the maximum stress of the experimental model of 400 thousand people. But the next model, in the design of which the corresponding changes were made, allowed to create a maximum voltage of 2 million c. The pump of this electrohydrodynamic generator was driven by a motor of 10 liters. S, and the flow velocity in the working part was 5 m / sec.

According to experts, at the present time, sufficient experience has already been accumulated to begin the creation of the first industrial electrohydrodynamic generators.

Dusty Generator

In 1936, the famous Soviet historian of technology V. Danilevsky in the article "The history of technology as one of the factors of technical progress" persistently drew the attention of specialists to the amazing Armstrong steam-electric machine. "Now," he wrote, "it is necessary to study the principle of the Armstrong machine by Soviet electrical engineers in order to establish the possibility of a new design of the same principle ..."

According to the descriptions, it was possible to establish that this machine produced a maximum voltage of several hundred thousand volts and was undoubtedly the most powerful electric machine of its time. But the calculations also gave an answer to the question of why such a curious and effective idea was consigned to oblivion: efficiency. Machine was only 0.01%! The negligence of this figure has for a long time discouraged electrical engineers from engaging in steam-electric machines.

Only in the 1930s, apparently, under the impression of the successful work of electrostatic smoke cleaners, experts again returned to this idea. Indeed, an electrostatic cleaner is not difficult to convert into an electrostatic generator. To do this, you only need to turn it: with the help of an air jet, drive the dust particles, charged in the corona discharge, through the collector electrode. In the 1930s, French and Belgian electrical engineers built such a "dust-electric" generator. The war interrupted these works, and they resumed only 30 years later.

Scientists have drawn attention to the fact that in such installations charged solid particles are transported by gas. The changes that he undergoes can be described with the aid of long and well-studied thermodynamic cycles. So the idea of ​​an electro-gas dynamical generator was born for the direct conversion of thermal energy into electrical energy.

The main element of such a generator is a "turbine" - a channel in which the gas expands and performs work on the charged particles, causing them to overcome the resistance of the electric field and move to the electrode with a high potential. The same element to which electricity is supplied and, causing charged particles to accelerate, compresses the gas, becomes a "compressor". By assembling a turbine and a compressor with heaters and coolers, it is possible to obtain electrical similarities of widely known thermal engines - diesel engines, gas turbines, Stirling engines, Erickson, etc.

At the present time, there is not a single electro-gas dynamical generator working on any of these cycles. While there is working out only one element - the turbine. The existing prototypes of such turbines are still very imperfect. Their efficiency Does not exceed 15-20%, while the efficiency Modern steam and gas turbines reaches 90-95%. However, thermodynamics suggests ways to improve efficiency. Electro-gas-dynamic turbines: they, like steam and gas, must be made multistage.

True, at first glance, the comparison is not in favor of the novelty. Where the gas turbine is only 10-20 steps, an electric gas-dynamic one needs about 200! But it should be clearly understood how much these steps are simpler - in fact, each of them is not more than a section of the pipeline.

Preliminary studies have shown that a channel with a diameter of 50 mm with 200 turbine and 200 compressor stages develops a power of 50 kW. In order to get, for example, a capacity of 50 thousand kilowatts, you need to connect 1000 of these channels in parallel.

Great influence on efficiency. Gas-dynamic turbines and compressors are also provided by the speed of the dust-gas mixture, the size of the dust particles, the pressure, etc. If, as a result of taking all these measures into account, Such devices will be brought to 80-90%, then the total thermal efficiency Electro-gas-dynamical installation operating on the Erickson cycle, will be 46- 56%. That is comparable to efficiency. Modern power plants ...

On the diagram:

Electrostatic generator with rubber tape. A - electrostatic induction. A1 - scheme of the Van de Graaf generator: 1 - high voltage electrode; 2 - collector electrode; 3 - moving tape; 4 - insulator; 5 - charging system with corona discharge.

Electrostatic generator with dielectric fluid. B - scheme of communication of electric charge of liquid: 1 - blade - cathode; 2 - grid-anode; 3 - negatively charged ions; 4 - flow direction of the liquid. B1 - scheme of the liquid generator: 1 - injector; 2 - the collector's confuser; 3 - collector.

Electrostatic generator with dust and gas working body. B - Armstrong's steam-electric machine: 1 - steam flow; 2 - wooden cylinder with holes; 3 - collector; 4 - electrode. B1 - scheme of single-stage and multistage electro-gas dynamic generators: 1 - high voltage electrode; 2 - collector: 3 - conversion zone; 4 - injector with corona discharge; 5 - charge neutralizer; 6 - the fan.

Based on the materials of the journal Science and Technology