ELECTROSTATICS NEWSLETTER
November/December 2001 No.159
PRESIDENT’S MESSAGE
DEBATING THE FUNDAMENTALS
Greetings, and happy holidays to all. May the coming winter months find you healthy, happy, and free from static cling.
Those who were able to attend ESA 2001 at Michigan State University will likely recall the lively debate that followed my informal talk on the subject of the leaf electrometer (M. N. Horenstein and T. B. Jones, “The Foil-Leaf Electrometer Revisited: Can A MEMS Device Push?”, Proceedings of the Electrostatic Society of America Annual Conference, East Lansing MI: June, 2001.) This presentation was the end product of lengthy exchanges that I had with my good colleague, Prof. Tom Jones of the University of Rochester. Prior to the ESA meeting, several of our discussions appeared as articles in this Newsletter. On the key question of the talk, “Do the leaves attract or repel?”, I concluded, of course, that Tom was correct. One does not need external negative charges in order for the leaves of a positively-charged electrometer to separate. Rather, the deflection of the leaves can be explained solely on the basis of mutual repulsion of like charges. I am also aware that a sizable fraction of you, perhaps encouraged by John Gagliardi’s impromptu presentation on the last day of the meeting, are still convinced that the opposite is true: The deflection of the electrometer leaves is caused by mutual attraction between the positive charges on the leaves and their opposite, remotely located negative charges. I should tell you that I gave the same presentation last month at a brown bag seminar here at Boston University. The reactions of the audience were similarly split. Half of those present believed in my new-found unipolar view, while the other half insisted that my pre-Jones, bipolar picture had been correct. I’m now sure that I’m right. Are you? In any case, the conversations on all fronts have been great fun, and I look forward to more as the months roll on. They have exemplified the grand tradition of the Friendly Society.
Similarly friendly but “charged” discussions have taken place around the issues of whether one can define a voltage on the surface of an insulator, and whether the proper unit of surface resistivity is the ohm or the ohm-per-square. As with the electrometer discussion, the opposing sides of these debates are equally confident of their footings, and both sides are strident in their stances. I have concluded only one thing from these latter two discussions: We have about as much chance of reaching a consensus as we do agreeing on the merits of irradiated vegetables.
The purpose of this article is not to stir the pot of debate on these previous issues – there’s plenty of time to do that during the cookie breaks of future annual meetings. Rather, I’d like to initiate a new discussion on an equally thorny problem. We all know that the surface of an insulator can become electrostatically charged. If a grounded object (e.g., a finger) approaches such a charged surface, and if the electric field between the surface charge and the finger becomes high enough, breakdown will ensue. Some of the stored charge will flow from the insulator surface to the grounded object. As it does so, it will dissipate energy that will heat up the air around the discharge. This situation can become problematic if the insulator happens to be surrounded by flammable vapors or ignitable dust-air mixtures. The heat from the dissipated energy of the discharge can ignite the former and cause all sorts of nasty things. The issue of discharges from insulators is thus important from the standpoint of electrostatic safety, and it figures prominently into determining the ability of a particular system to produce troublesome ESD (electrostatic discharge) events.
Here’s a question that has eluded me for years: Is it possible to estimate the energy dissipated in a discharge that initiates from a charged insulator? Addressing this question stirs up some of our old friends from previous debates such as, “What, if any, is the capacitance of a charged insulating surface”, and “How can the charge on an insulator move if the surface cannot conduct?” In my quest for an answer to this question, I thought it would be interesting to solicit comments from the readership. Perhaps someone who is an expert on sparks knows the answer and will quickly be able to short circuit the debate. Yet others may have thought about the problem and have ideas to contribute. Others may have no idea at all, but just like to join in on ESA discussions. In any case, feel free to share your insights and responses with our Newsletter Editor Bill Smart, or send them to me via email. I’ll edit the comments and publish it in a subsequent Newsletter. For the purpose of these discussions, I’d like to use the term “insulator spark” to refer to a discharge that takes place from the surface of a charged insulator to a nearby grounded object. I realize that this nomenclature may not be representative of industry standards.
In addressing this problem, there are lots of issues to think about. One can clearly calculate the energy stored on a charged insulating surface from basic principals of physics. Only a physicist could get away with stating the problem in the following way: It takes 1 joule of energy to move 1 coulomb of charge over a distance of 1 meter against a field of 1 volt/meter. (Remind me to tell you the joke about the physicist and the spherical horse sometime.) Throw in some realistic scaling factors that involve large and small powers of ten, perform a few line and surface integrals, and voilà -- you’ve calculated the stored energy. The question remains: How much of this stored energy will be liberated when a spark occurs to a nearby grounded object?
A great deal of work has been done in trying to observe the nature of insulator sparks. Terms such as brush discharge and propagating brush discharge (sparks that occur from insulating surfaces with and without ground backing, respectively,) populate the literature. Numerous experimental studies have attempted to categorize the physical nature of insulator sparks under a variety of conditions. In the 1960’s, Gibson and Lloyd from the UK performed an oft-cited experimental study in which discharges from insulating surfaces were produced in the presence of vapors of known ignition energies. (N. Gibson and F.C. Lloyd, “Incendivity of Discharges
from Electrostatically Charged Plastics”, Brit. Appl. Phys, 16, 1965, p 1619.) The purpose of
these experiments was to try to quantify empirically the energy content of insulator sparks under
a variety of conditions and to compare them to capacitor sparks of know similar energy content and discharge current signatures. Their data have been cited many times and form a common thread through numerous discourses on the subject.
The experimental studies of Gibson, Lloyd, and others have provided us with valuable guidelines, but I believe it would be useful to be able to predict the energy content of insulator sparks from fundamental models. This ability would eliminate much of the guesswork that takes place in the areas of electrostatic safety, electrostatic discharge (ESD), and materials processing. Estimating the energy liberated by an insulator spark is no easy task, because nothing is clearly defined as is the case when a sparks occurs between two charged conductors.
Here are a few of the questions, some of them paradoxical, that I like to think about:
-When an insulator spark occurs, how much charge motion is there parallel versus perpendicular to the surface of the insulator? Some parallel charge motion must occur. Otherwise, the spark would involve only an infinitesimal amount of charge, and the discharge would dissipate zero energy.
-How much, by area, of the surrounding surface charge will be “dumped” into the discharge? Which charge regions will remain immobile on the insulator surface?
-Does charge relocate along the surface and fill in the spot where the discharge occurs, or does the spark leave a region of diminished charge?
-Is air breakdown the sole method of charge transport, or is surface of the material involved as well?
-Why does more energy seem to be liberated in a propagating brush discharge e.g., when an insulating sheet is backed by a ground plane, compared to an ordinary brush discharge? It might seem that with charge more tightly coupled to a ground backing than to an approaching grounded object, less charge would be free and available to participate in a discharge.
These are some of the questions I’ve come up with so far. Please do share your thoughts. Is this stuff old hat, and is your president naïve? Or, do these questions represent new ground? If you know the answers, feel free to share them with the readers. Send your contributions to the Editor or to me via e-mail. I’ll compile all responses and circulate them in the next Newsletter.
For the Friendly Society
Mark Horenstein
AIChE SAFETY CONFERENCE
The Safety and Health Division of the American Institute of Chemical Engineers will hold its next Loss Prevention Symposium March 10 through 13, 2002, at the Hilton Exhibition Center, New Orleans, LA. Of interest to ESA members: one topical session will address Static Electricity & Ignition Control Strategies. Also offered is a one-day short course, Understanding and Controlling Static Electricity, taught by Dr. Vahid Ebadat, Chilworth Technology, Inc. For conference and registration information, contact AIChE at 800-242-4363 or www.aiche.org/spring.
Bob Benedetti
JOURNAL OF ELECTROSTATICS
The publishing editor of Elsevier's Journal of Electrostatics has informed us of their wish to continue our arrangement whereby ESA members can purchase a personal Year 2002 subscription to the Journal at the drastically reduced member price of $99.
Orders for member subscriptions will be processed by the ESA, and Elsevier will mail your personal copy directly to the address that you specify. If you are interested, mail the required form, available at www.electrostatics.org/links/jestatlet.html, together with your payment of $99 made payable to the Electrostatics Society of America, to the ESA Secretary-Treasurer:
Tim Erin
723 Woodshire Way
Dayton, OH 45430 USA
LEFT OUT OF MEMS
You know those little things. The microelectromechanical systems that are all the rage. It isn’t a research area for me, and I’ve been feeling left out.
But now I have some help. A recent review article has appeared in Physics Today October 2001. I recommend it highly. There is mention of the very practical (sensing car accelerations for airbags) and it leads to the very abstract (that mysterious Casimir force that comes from the vacuum).
I have confessed my ignorance. But this article lifted me a tad. Can you use it? A key line says it all; “electrostatic activation is a standard technique for micromachines while it’s seldom used for macromachines”.
Glenn Schmieg
BENJAMIN FRANKLIN BOOK
H. W. Brands has written “The First American: The Life and Times of Benjamin Franklin”.
w In his lifetime he was the
best known American
w He began a model for the
American character. He
was practical, self-reliant,
unimpressed with wealth
and title, and had a sense
of humor.
w He was the first to have a
real sense of American
identity separate from
England.
Glenn Schmieg
FRANKLIN: FEMINIST OR WOMANIZER ?
There is some evidence to support both claims. Of course, the words are not mutually exclusive. In today’s U.S. Senate some of the most ardent advocates of women’s rights have been philanderers.
By eighteenth-century standards, Franklin was a feminist before the word was known. At age seventeen in “The Silence Dogood Letters,” Franklin shocked Puritan Boston with the notion that women might be able to manage shops, preach sermons, and read law better than men.
Still, the sexist comments expressed by his fictional character Poor Richard were in the chauvinist spirit of the times. One thing is certain: Franklin respected the intelligence of women when few in his day did. He took seriously women’s opinions when others only indulged them.
Taken from The Wit & Wisdom of Benjamin Franklin