Problems: coulomb forces would be expected to attract a person into the "chamber" formed by the PP film, and the attractive force should increase linearly across distance. There should be no "wall" in the center, a discrete wall is repulsive, also nonlinear.
If for some reason a person was repelled from the center of the chamber rather than being attracted, there still should be no "wall," since the repulsion force should exist over a large distance; it should act like a deep pillow which exerts more and more force as one moves deeper into it. Large fuzzy fields, this is how magnets and iron behave, and this is how e-fields and conductive objects should also behave.
A thought: unspooling of film typically generates a much higher net charge on the long piece of film than on the small surface of the spool. However, since charge is created in pairs, and net charge is conserved, the imbalances of charge must be equal and opposite. The charge on the entire length of moving film must be equal in magnitude to the charge on the spool. Yet the charge on the film is very large and is continuously increasing. The limited surface-charge on the spool required that opposite charge is being lost through some unseen path.
Very probably the spool is spewing out enormous quantities of ionized air with polarity opposite that of the charge on the moving plastic film.
Charged air would be created by discharge in the cleft between film and spool as the film was peeled from the spool. I wonder if film was being peeled from the top of the spool, so that any ionized air created in the cleft would be launched into the "tent-chamber" region? (If it was peeled from the bottom of the spool, the charged air would end up outside the "tent.") Or, if a corona discharge arises in the cleft between film and spool, perhaps the UV and e-fields of this corona can ionize the air on both sides of the exiting plastic film, and spray the charged air everywhere.
So, if the charged "tent" of film is negative in the above situation, and if a large quantity of positively charged air is being generated by the spool, then perhaps the "invisible wall" is caused by a cloud of suspended air ions held in position by e-fields. Perhaps it's a pressure gradient created by ionized air trapped under the tent by electrostatic attraction. Yet again this effect would be expected to create a diffuse zone of increasing force, not a "wall", but an "invisible pillow." Added note: concrete floors behave as conductors (resistors) in this situation. Where megavolts at microamps are involved, the division between insulators and conductors is at 10^6/10^-6 = 1000 gigaohms. Concrete resistivity is in the realm of megohms, so it behaves like a grounded metal sheet.
However, a volume of charged air is somewhat analogous to iron filings near a magnet. If a solid sheet of iron filings is held in place by a magnet, then a literal "wall" is created, and this wall will resist penetration by nonferrous objects. If in the above manufacturing plant, a sheet of highly charged air is for some reason being held in place by the fields created by the charged film, then a transparent "wall" made of charged air would come into being. It might produce pressures on surfaces, and resist penetration by human bodies.
My question is this: if the entire situation could be turned on its side, so the "invisible wall" became an "invisible floor", could a person *stand* on it? Have we discovered the long-sought "Zero-G waterbed?" :) - B.B.
The_same_potato on December 19th, 2017 at 21:42 UTC »
(hug of death)
David Swenson of 3M Corporation describes an anomaly where workers encountered a strange "invisible wall" in the area under a fast-moving sheet of electrically charged polypropelene film in a factory. This "invisible wall" was strong enough to prevent humans from passing through. A person near this "wall" was unable to turn, and so had to walk backwards to retreat from it.
This occurred in late summer in South Carolina, August 1980, in extremely high humidity. Polypropelene (PP) film on 50K ft. rolls 20ft wide was being slit and transferred to multiple smaller spools. The film was taken off the main roll at high speed, flowed upwards 20ft to overhead rollers, passed horizontally 20ft and then downwards to the slitting device, where it was spooled onto shorter rolls. The whole operation formed a cubical shaped tent, with two walls and a ceiling approximately 20ft square. The spools ran at 1000ft/min, or about 10MPH. The PP film had been manufactured with dissimilar surface structure on opposing faces. Contact electrification can occur even in similar materials if the surface textures or micro-structures are significantly different. The generation of a large imbalance of electrical surface-charge during unspooling was therefore not unexpected, and is a common problem in this industry. "Static cling" in the megavolt range!
On entering the factory floor and far from the equipment, Mr. Swenson's 200KV/ft handheld electrometer was found to slam to full scale. When he attempted to walk through the corridor formed by the moving film, he was stopped about half way through by an "invisible wall." He could lean all his weight forward but was unable to pass. He observed a fly get pulled into the charged, moving plastic, and speculates that the e-fields might have been strong enough to suck in birds!
The production manager did not believe Mr. Swenson's report of the strange phenomena. When they both returned to the factory floor, they found that the "wall" was no longer there. But the production workers had noticed the effect as occurring early in the morning when humidity was lower, so they agreed to try again another day. The second attempt was successful, and early in the morning the field underneath the "tent" was strong enough to raise even the short, curly hair of the production manager. The "invisible wall" effect had returned. He commented that he "didn't know whether to fix it or sell tickets."
LINKS
ESD Journal Reddit TIL Another poly unwind setup (vid) Problems: coulomb forces would be expected to attract a person into the "chamber" formed by the PP film, and the attractive force should increase linearly across distance. There should be no "wall" in the center, a discrete wall is repulsive, also nonlinear. If for some reason a person was repelled from the center of the chamber rather than being attracted, there still should be no "wall," since the repulsion force should exist over a large distance; it should act like a deep pillow which exerts more and more force as one moves deeper into it. Large fuzzy fields, this is how magnets and iron behave, and this is how e-fields and conductive objects should also behave. A thought: unspooling of film typically generates a much higher net charge on the long piece of film than on the small surface of the spool. However, since charge is created in pairs, and net charge is conserved, the imbalances of charge must be equal and opposite. The charge on the entire length of moving film must be equal in magnitude to the charge on the spool. Yet the charge on the film is very large and is continuously increasing. The limited surface-charge on the spool required that opposite charge is being lost through some unseen path. Very probably the spool is spewing out enormous quantities of ionized air with polarity opposite that of the charge on the moving plastic film. Charged air would be created by discharge in the cleft between film and spool as the film was peeled from the spool. I wonder if film was being peeled from the top of the spool, so that any ionized air created in the cleft would be launched into the "tent-chamber" region? (If it was peeled from the bottom of the spool, the charged air would end up outside the "tent.") Or, if a corona discharge arises in the cleft between film and spool, perhaps the UV and e-fields of this corona can ionize the air on both sides of the exiting plastic film, and spray the charged air everywhere. So, if the charged "tent" of film is negative in the above situation, and if a large quantity of positively charged air is being generated by the spool, then perhaps the "invisible wall" is caused by a cloud of suspended air ions held in position by e-fields. Perhaps it's a pressure gradient created by ionized air trapped under the tent by electrostatic attraction. Yet again this effect would be expected to create a diffuse zone of increasing force, not a "wall", but an "invisible pillow." Added note: concrete floors behave as conductors (resistors) in this situation. Where megavolts at microamps are involved, the division between insulators and conductors is at 10^6/10^-6 = 1000 gigaohms. Concrete resistivity is in the realm of megohms, so it behaves like a grounded metal sheet. However, a volume of charged air is somewhat analogous to iron filings near a magnet. If a solid sheet of iron filings is held in place by a magnet, then a literal "wall" is created, and this wall will resist penetration by nonferrous objects. If in the above manufacturing plant, a sheet of highly charged air is for some reason being held in place by the fields created by the charged film, then a transparent "wall" made of charged air would come into being. It might produce pressures on surfaces, and resist penetration by human bodies. My question is this: if the entire situation could be turned on its side, so the "invisible wall" became an "invisible floor", could a person *stand* on it? Have we discovered the long-sought "Zero-G waterbed?" :) - B.B.dominusUmbrae on December 19th, 2017 at 19:51 UTC »
"He didn't know whether to fix it or sell tickets"
Thinking like a true entrepreneur
SFanatic on December 19th, 2017 at 19:43 UTC »
This would be a great Mythbusters episode.