ELECTROSTATICS NEWSLETTER
January/February, 2002 No.160
PRESIDENT’S MESSAGE
As you may recall, in the last Newsletter I posed the following question: “Is it possible to estimate the energy dissipated in a discharge that initiates from a charged insulator?” I received numerous responses, and as promised, I am including them as part of this issue’s President’s Message. Feel free to continue the thread of discussion with additional submissions. We will try to publish all responses.
Several of the responses applauded the idea of posing discussion questions on basic electrostatics in the Newsletter, so I’d like to present yet another perplexing question for contemplation. Throughout the 20th century, the discipline of electrostatics managed to push the frontiers of technology by embracing a wide range of size scales. The large industrial electrostatic precipitator, first pursued seriously around mid-century, involves size scales on the order of meters to tens of meters. Many manufacturing processes involve electrostatics on the meter to centimeter scale. At the other end of the size spectrum, the perfection of electrophotography as a method of document reproduction has required the application of electrostatic principles on a “tens-to-one micron” scale. Recent work in micro-electromechanical systems has demonstrated the importance of electrostatics on a micron to sub-micron scale, and the rapidly emerging science of nanotechnology is pushing the size scale to well below the sub-micron limit.
As part of the nanotechnology revolution, electrostatics has emerged anew as an important consideration in biology. Indeed, several papers at recent ESA conferences, including those by John Gagliardi, Heiko Jacobs, and others, have shown the important links between cellular biologists and “electrostaticians.” [The term “electrostatician” was first proposed by Juliusz Gajewski at the Electrostatics 2001 Conference in Zakopane, Poland]. My approach to electrostatics has always been through the eyes of a traditional electrical engineer, hence a recent Web search on biological electrostatics revealed a plethora of terms with which I am not intimate. For example, performing a Google search on the terms “electrostatics AND biology” yielded, among others, the following:
-Electrostatics and the “generalized Born” solvent model.
-Matching CHARMM's electrostatic approximations to environmental approximations.
-Electrostatic solvation.
-Electrostatic recognition in the interaction of cytochrome c and cytochrome b5.
-Actin filament electrostatic potential.
-Lennard-Jones potential operating over a span of single angstroms.
I am sure that some of you resonate with these terms, but they are probably foreign to those familiar with the lexicon of “traditional” electrostatics. They do point, however, to one fact: electrostatics works and is important at very small size scales. Electrostatics can be described mathematically by a general set of laws, namely the low-frequency simplification of Maxwell’s equations. These laws work well on large size scales, and also on small scales approaching those of cells and proteins. At some very small size scale, quantum mechanics must take over, and so the classical electrostatic laws that we apply routinely to problems in industry and in the lab should break down. So now, the question for this issue of the Newsletter:
“How small a size scale can we examine while still applying the classical laws of electrostatics?”
I think I know the answer, but I’d like to reserve comment until we hear from some of the physicists and biologists out there.
For the Friendly Society,
Mark N. Horenstein
ESA President
RESPONSES TO THE QUESTION POSED IN THE NOV/DEC 2001 NEWSLETTER
“Is it possible to estimate the energy dissipated in a discharge that initiates from a charged insulator?”
Disclaimer: All responses express the opinions of the writer and do not reflect any official position of the ESA.
· From John Chubb, JC Instruments, UK (jchubb@jci.co.uk)
Mark,
I have just received the Nov/Dec issue of the ESA Newsletter. I think it an excellent idea to generate a discussion on the question of assessing the energy released in discharges between an earthed projection and a charged insulator surface. A few preliminary thoughts and comments that occur to me:
1) Knowing the total electrostatic energy released may not be what is of actual interest. You may want to know whether the discharge will ignite flammable gases, damage semiconductor devices, or radiate upsetting electromagnetic energy.
2) I believe that discharges to charged insulators have a high-conductivity channel from the metal electrode that links to a gas discharge over the surface of the insulator. This surface discharge spreads out sort of radially, gathering charge until its propagation is curtailed by lack of driving electric field. The area is revealed by Lichtenberg figures.
3) Because much of the discharge will be close to what was the charged surface, the energy released here may well not be too effective in causing ignitions because of the cooling of the nascent flame kernel by proximity to the surface. (Hence my comment above that the energy released may not be equivalent to that of metal to metal sparks).
4) I think there will be influence to the character of the charged surface when its resistivity gets low enough that it can, a) affect the electric field at the edge of the propagating air discharge, and b) contribute additional charge/current into the discharge.
5) There is a question as to whether localized conductive/resistive threads spaced over a fabric surface can inhibit propagation of a surface discharge across a charged material and so usefully limit the energy and incendivity of the discharge. This of course is very relevant to flexible-intermediate bulk containers (FIBCs). [Ed Note: FBICs, used to transport bulk powders, granules, and pellets in industry, are a common source of electrostatic discharge.]
6) I have wondered whether some information might be gained on the energy dissipated by the sound or increase in local gas pressure generated by the heat liberated.
Best wishes, John Chubb
· From Graham Hearn, Wolfson Electrostatics, UK (glh@soton.ac.uk)
Dear Mark,
Many of your questions in ESA Newsletter No.159 can be answered by John Davidson and Adrian Bailey of Southampton University, UK who are working on exactly these problems. Please see Davison, Williams, Bailey & Hearn - J. Electrostatics, vol 51-52 (2001), 374-380.
The work of Gibson & Lloyd was good in its day but things have moved on. We now have the instrumentation to capture and analyze ESD waveform components and we now use special probes to accurately quantify other elements of the ESD apart from charge transfer.
A big question is, how do we determine if a material or situation is safe for use in sensitive flammable
atmospheres without performing several hundred gas ignition tests? The Crohmic Blue material developed by Linq in the US, for example, gives large ESD charge transfers but no ignitions. Charge transfer data alone is not sufficient in such analyses. Southampton University in the UK and PTB in Germany are looking at ESD evaluation techniques which do not involve ignition trials.
Best regards,
Graham Hearn
· From Jim Adams of Radio Frequency Systems, Corvallis, OR (Jim.Adams@rfsworld.com)
To understand the paradox of charged insulators, one needs to study the molecular matrix at the surface in comparison to material inside that surrounds it.Ê Insulators have a stable outer electron ring that prevents electrons from traveling, but the material at the surface has electron orbits that are exposed to the outer world, in a sense unbalancing the matrix and permitting free electrons to be attracted to the surface area. Charge can be built up and can travel over the surface, but not through, the insulating material, hence the situation is not really paradoxical at all. With other non-insulating materials [e.g. metals], electrons have more mobility within the structure, so any surface charge is bled off rapidly and equalized throughout the material. The charges on a insulator aren’t bonded tightly to the surface, and so are easily dislodged and moved across the surface area. The latter would not be current flow in the conventional sense.
Everything has some capacitance associated with it, but in the case of charged insulators, you would need to form two areas of charge separated by an insulator so thin that the electrostatic field can be expressed across it. It’s tempting to think of anything that charges up as having significant capacitance, but that's not always the case. Hopefully, this diatribe won't be too......shocking.
· From ESA Member Richard Hull (hullr@whitlock.com)
Hello, I have been a member of the ESA for a few years now and am pleased beyond your comprehension that you are forcing these tough theoretical issues. We know far less than we would like to think about electrostatics and charge in general. In a rush to electrodynamics in the mid 1800's, electrostatics was relegated to the parlor trick category it had occupied prior to that time.
Electrostatics had no real effective wheel work" up to the time of Maxwell. Coupled with poor insulation of the period, poor understanding of the underlying causes such as electrons, polarized materials at the electronic level, etc., and a general feeling that with electrodynamic and electromagnetic wheel work capable of real power, electrostatics was effectively dropped as a study.
It was only with the arrival of the modern hazards associated with protecting other processes and modern sensitive electronics that electrostatic studies really picked up. This often focused on "prevention of electrostatics" and rarely on use or application beyond simple pick and place or separation processes.
All material objects that have mass, even the best insulators, are electrical in structure and can supply electrons or support charge separation if some form of work is done to them or around them. All materials which are classically thought of as insulators and which have mass have a dielectric constant. Period! To me, charge application and retention need never have a conductor present, at least as we think of a conductor, i.e., metals.
Based on my experiments and thoughts, charge can be placed on and exist at any point in space where there is an interface between two differing dielectric constants. Mylar, a highly polar material, will hold a tremendous charge. Consider the following experiment: Make a capacitor of two 1-sq.-ft. metal plates and a 14” 14” 5 mil Mylar sheet. Charge the capacitor to 8000 volts. Very carefully separate the plates. Remove the plastic. Roll it up. Mail it to a friend in a cardboard tube. When she receives it, she can carefully insert it between two plates again, and POW! A large dangerous discharge is seen. While some of the charge energy is lost in separating the plates, a great deal remains associated with the highly polar dielectric. (I have done this experiment, by the way.)
Some might view this phenomenon as the charge residing inside the dielectric, and others as the potential energy stored at the air/Mylar interface. Regardless of which point of view one adopts, there is significant energy trapped in or about the Mylar dielectric. The metal plates used in the charging circuit have no [net] charge on them! Was the charge ever really there on the plates? Or, are the plates merely a means to an end of placing and removing the charge to a dielectric surface from a remote source of energy or to a load with a vengeance and in a manner amenable to seeing real work done in a conductive electrical circuit? Can metal really hold a charge, or is the charge extant at the metal dielectric interface which can itself evolve and transfer from interface to interface? Where does real charge reside?
I can tell you that you can, indeed, locally discharge the Mylar, leaving local areas of the material discharged and other areas not discharged or impaired at all. All of this is surely due to the highly polar nature of the material. Mylar is a superb insulator with a tremendous resistance to the flow of classic electrical current. You can place the charged sheet back on top of one of the square plates, take a 1"-diameter metal disk on a glass rod on the top of the sheet, bring a wire connected to the bottom plate into contact, and get a mini "SNAP"! You can "cookie cut" perfect circular areas of no charge into the sheet by following this procedure. Other dielectrics of a not-so-highly-polar-nature will behave differently.
Charge in insulators can move either through the body of the material or along the surface. This depends on the type of interface and the materials forming the differential dielectric junction. It appears that non-polar insulators have more of a surface flow of charge where the entire item is discharged from one spot, while polar insulators discharge through the bulk of the material locally leaving nearby charges buried and locked within the material itself. This is especially true if the polar material was charged through its body as in the two plate capacitor as opposed to surface charged by friction.
Good stuff, this questioning of first principles. Inquiring minds want to know.
And this commentary that I found in some stored e-mails, originally posted on electro-list in 1994:
· From Thomas B. Jones, U. of Rochester (jones@ece.rochester.edu)
Over the past few years, I have observed a steady increase in the number of electrostatic ignitions and nuisances seemingly caused by the substitution of plastic parts in components and subassemblies for manufactured items, from automotive batteries to prosaic stuff such as gaskets, couplings, funnels, etc. While many of these incidents are merely nuisances, others I have learned about have been downright scary. We all know that the literature on ESD hazards associated with polymers and plastics goes back many years and, at this point, much of the phenomenology is reasonably well-understood. Yet it should be of growing concern to us all that, because consumer items are now being manufactured with more and more plastic parts, an increasing fraction of these incidents is likely to occur in the business office or even in the home.
For our mutual benefit, I would like to recommend that we post information received on this subject, including brief summaries of published papers, reports, and maybe even anecdotal items (with proper attention to relevant details). Such postings might be extended to the mention of new products, such as conductive impregnants for extrudable plastics, conductive fiber filter bags, etc. (I know for a fact that I miss of lot of new product info.)
My first contribution to this exchange is the brief mention of a paper presented a year ago by Tolson & Tomlinson at the Lahnstein Electrostatics Conf. The authors described the explosion of a lead-acid battery caused by hydrogen gas, thought to have been ignited by a surface-tracking electrostatic discharge. The paper was well-delivered, well-received, and the subject of good discussion. The point I wish to make here is that a battery seems at first glance an unexpected site for an ESD ignition. There are undoubtedly other ESD surprises awaiting us, and we might have an interesting time identifying (and, for those of us employed in industry, dealing with them before the lawyers do). (Source: J. Electrostatics, vol.30, pp 149-157, 1993.)
· And so ends the first round of commentary on the subject of discharges from insulating surfaces.
On another note (“Just when you thought it was Safe to go Back into the Water,”) the discussion continues on whether the leaves of an electrometer repel each other or are attracted to remote, external charges. Consider the following contribution from ESA Member John Gagliardi:
· From John Gagliardi, Rutgers University (gagliard@camden.rutgers.edu)
In a recent issue (November/December 2001) of this newsletter, Mark Horenstein recounted the discussion that followed his talk on the leaf electrometer at the annual ESA Conference in June 2001. In my brief remarks at the end of the conference, I presented an argument that the electrostatic repulsion was strictly local, and we do not need remote negative charge (assuming positive charge on the leaves) to account for the electroscope's behavior. Since it happens that this particular case is an excellent example of the power of Gauss's law (GL) in such matters, I would like to review - and expand somewhat on - the presentation that I gave at the end of the conference.
You probably remember GL (also known as Maxwell's first equation in integral form) as a forbidding-looking surface integral of the flux of the electric field over a closed surface (Gaussian surface, GS) whose value is equal to the net charge enclosed by the GS divided by o. The GS is at one's disposal and is chosen to exploit the symmetry of the problem, so that there is usually no need to integrate at all, and the problem reduces to simple algebra. That is, the surface integral typically reduces to the unknown electric field magnitude E multiplied by some known area A, and GL becomes EA = q/eo.
Let's apply this equation to the problem at hand. In Fig.1, the two conducting plates are shown, and we assume that the spacing between the plates is small compared to the size of the plates, so that we can neglect fringing considerations. Gaussian surface 1 (GS1) is shown as a dashed line, and from symmetry considerations (since equal charges q are placed on the two plates), the electric field E is zero on plane PP midway between the plates. In addition, E is zero inside the conducting right-hand plate, and the flux of E through the other four sides of the rectangular box that we have chosen as the GS is zero, since E is parallel to each of these sides. Thus, we have EA = 0, and we conclude that there is no charge enclosed by the GS, so that the charge on the inner (leftmost) surface of the right conducting plate must be zero. A similar application of GL using GS2 leads us to conclude that the inside surface of the left hand plate is also uncharged. We conclude that all of the excess charge is on the outermost surfaces of the two plates. It is left to the reader to choose a GS and prove (by an entirely similar argument) that the electric field is zero everywhere between the plates.
Turning our attention to GS3, we have that 2EA = 2rA/eo, where r is the charge per-unit-area on either (outer) surface of the plates, so 2EA is the flux through the two outer ends of GS3, and the total flux through the other four sides of GS3 is equal to zero for the same reason as before: E is parallel to the other four surfaces. Therefore, the magnitude of the electric field outside the two plates is given by E = r/eo. The field points in the +x direction to the right of the plates, and in the -x direction to the left of the plates.
Thus, the original problem may be replaced by two sheets (in the mathematical sense, of zero thickness) of charge (Fig. 2), each with area charge density , separated by a distance that can be taken to be equal to the distance between the outer (charged) surfaces of the two plates (although this distance is actually not critical, because of the original assumption of a small separation between relatively large plates). This geometry reproduces exactly the zero-field condition everywhere between the outer charged surfaces of the two plates (including the zero-field region within the conducting plates), and the value r/eo elsewhere (outside the plates), as can easily be seen by applying GL to GS4 and using the superposition principle for electric fields. This will be left as an exercise for the reader. You will find that the field due to either sheet alone has magnitude r/2eo on both sides of that sheet. From superposition, the net field is zero between the sheets, and since there is no distance dependence for plate separations that are small compared to the size of the plates, the field magnitude is twice r/2eo "outside" the sheets. The magnitude of the force on either sheet will then be equal to the field due to the other sheet multiplied by the charge on the sheet in question, so F = qE = qr/2eo = q2/2eoA, where r = q/A, and q is the charge on either plate.
The important thing to notice about this derivation is that it rigorously demonstrates that there is no need to consider any charge external to the plates - all the charge needed by GL is on the plates (leaves). As I remarked at the ESA 2001 Conference, this situation bears some similarity to logical deductions in Einstein's theory of relativity (RT) in that once the veracity of the starting point is accepted (GL here, and experimental observations in RT), the conclusions follow from simple logic. As in RT, the logic here is not difficult to follow, but the results can be hard to believe. (The haunting possible neglect of external charge here; the conclusion in RT that the mass of objects moving relative to an observer increases as measured by the observer.)
The repulsive force between the surface charges on the farthest edges of the two plates can also be calculated if the plates are not replaced by two sheets of charge. Briefly, it can be shown that the electrostatic field at the precise location of the charge distribution (not immediately outside the conductor, where it is twice as much) at the surface of a charged conductor is given by r/2eo. Thus the force, F = qE = qr/2eo = q2/2eoA on each of the plates has the same value as that given above with the (simpler) two-sheet model.
Reference
1. See for example, Griffiths, D. J., Introduction to Electrodynamics, 3rd ed. (Prentice-Hall, Upper Saddle river, NJ, 1999), p. 102.
(Horenstein comments: Thank you, John, for succinctly stating in simple mathematical terms the concept that I was trying to convey with my physical models in the ESA2001 talk on the electrometer (electroscope.) I concur completely, but I’m sure others will have more to say on this continuing saga.)
ESA CONFERENCE 2002
Reminder:
The 30th Annual Conference of the Electrostatics Society of America will be held jointly with the Institute of Electrostatics Japan (IEJ) from June 26-28, 2002 on the campus of Northwestern University in Evanston, Illinois.
Deadlines for formal papers: Title to Program Chair by March 15; manuscript by April 1, 2002.
Deadlines for abstract-only talks: Title to Program Chair by April 1; prepared abstract by April 15, 2002.
Registration information will be forthcoming in the March/April Newsletter and will be available on the ESA Web site. For more information about the conference, visit www.electrostatics.org.
NEW MEMBER INVITATIONS TO ESA CONFERENCES
From ESA President Mark Horenstein:
Over the past few years, we’ve made it a point to identify individuals involved in research or development in electrostatics who have not been part of the traditional ESA membership. These individuals have received personal invitations from the ESA President to give papers at future ESA Conferences, and many have chosen to attend. If you know of someone who you think should receive one, please let me know and I will send out a formal invitation for this year’s conference. (mnh@bu.edu; 617-353-5437)
ESA CONFERENCE 2003
Update:
It’s official! The 2003 Annual conference will be held jointly with the Electrostatic Processes Committee of the IEEE Industry Applications Society. The details of this conference are in the process of being finalized, but it will be held in a late June time frame in the Little Rock, Arkansas area. To accommodate the expected larger number of papers, a four-day, rather than three-day, conference is planned, with publication opportunities available via a joint Proceedings. Watch for details in the near future.
ELECTROSTATICS IN THE NEWS
Sent in by ESA Member Stuart Hoenig
(Excerpted from THE WALL STREET JOURNAL - Monday, December 3, 2001)
Static Electricity Present in Anthrax Letters Made Spores Cling, May Have Saved Lives
By John J. Fialka and Gary Fields
“WASHINGTON - Investigators say whoever is behind the anthrax attacks may have missed a crucial deadly detail. They suspect the perpetrator failed to remove static electricity from the powder containing the deadly spores. According to scientists who have made anthrax for use in weapons in the U.S. and the former Soviet Union, the presence of an electrostatic charge may have saved American lives. While some of the charged particles can still become airborne -- where they are the most deadly -- much of the material tends to cling to surfaces. The sticking tendency may have made cross-contamination of mail more likely, according to one senior Federal Bureau of Investigation official involved in the investigation, because the spores would have been prone to attach themselves to envelopes and surfaces.
"Electrostatically charged materials are very hard to disseminate," explained Bill Patrick, a scientist who helped develop anthrax-loaded weapons for the U.S. in the 1950s and 1960s. While Mr. Patrick said he hasn't personally seen samples of anthrax, a scientist working on the investigation, he said, has described it to him ‘It's purified like our material and it has a small particle size, just as we did, but it has an electrostatic charge,’ he said. Some scientists cautioned that the electrostatic charge in the powder could have grown as it was handled. Richard Flagan, a professor of chemical engineering at the California Institute of Technology in Pasadena whose specialty is aerosols said that the mail-sorter machines could conceivably have transferred an electric charge by jostling the letters containing the powder.”