ELECTROSTATICS NEWSLETTER

November/December 2000 No.153

PRESIDENT’S MESSAGE

For the last several years, I've worked in the field of micro-electromechanical systems, or "MEMS". Many of these tiny silicon micro-devices rely on electrostatic forces to initiate mechanical motion. I entered this field after having been asked to join a Boston University MEMS team of mechanical, manufacturing, and systems engineers as the "electrostatics expert." The experience of working with this multidisciplinary team has confirmed one of my long-held suspicions: Issues from one person's field are often left as "exercises to the reader" by individuals from an another field. For example, as a die-hard electrical engineer I sometimes find myself modeling a 32-part mechanical structure as a simple mass and restoring spring. Conversely, my colleagues usually model a complex electrode structure as two "parallel capacitor plates." Similarly, I've often been asked by coworkers to "just build some circuit" that can produce 1 kV at 10 mA from a 9-V battery.

One common misconception that I hear often from mechanical engineers is the notion that applying "two" voltages of "opposite" polarity to two conductors will cause them to attract one another, while applying "two" voltages of the same polarity will cause the conductors to repel. This faulty conclusion, of course, results from watching too many science museum demonstrations involving charged, insulated ping-pong balls. As our electrostatics fundamentals tell us, when two conductors are held at different fixed potentials (i.e., separated by a single voltage difference,) the force between them will vary as the square of the applied voltage. The force will be unrelated to the polarity either conductor relative to ground and will always be attractive. I've spent many hours trying to convince my non-EE colleagues (and even one EE who is a systems type) that it's impossible to get two conductors held at fixed potentials to repel. The counter argument that I often hear cites the example of the traditional foil-leaf electrometer, in which two pieces of conducting foil are suspended from a conducting rod inside a

sealed jar. Applying voltage or charge to the rod causes the two foils, which are electrically connected, to repel. Hopefully you saw this demonstration in some science class or museum exhibit. If you think

about the problem, you quickly realize that, although the two foils do indeed separate when charge or voltage is applied to the rod, the force cannot be repulsive. The two foils are at the same potential, hence the electric field between them must be zero, because an electric field can only extend from a region of higher potential to a region of lower potential. The foils separate due to the attractive forces exerted between the charged foils and the outside world -- charges of opposite polarity induced on either the surface of the glass or on other conductors outside of the jar. (We usually think of glass as being an insulator, but glass actually has a small surface conductivity due to adsorbed moisture.)

These debates on the topic of electrostatic repulsion of conductors have been stimulating, and they have reminded me of a fascinating dialog what went on among the aging alumni of the MIT EPSEL lab (all

former students of Profs. James Melcher and Gerald Wilson.) The topic concerned two parallel metal plates held at different potentials and immersed in a dielectric liquid. As is well known, applying a voltage between the plates will cause the liquid to rise between them. If a hole were punched in one plate above the liquid level, could the system be used as a pump? (The final conclusion, still not accepted by all who participated, was that the structure will not act as a pump. Perhaps we can publish the friendly but heated e-mail exchanges at a future date.)

These two discussion threads are reminiscent of the countless poignant discussions that have taken place during the breaks and mealtimes of our annual ESA meetings. Many of us find these informal exchanges to be just as valuable learning experiences as the formal talks and papers. In the spirit of these exchanges, I'd like to start (or restart) a tradition of including "food for thought" questions in the bi-monthly ESA Newsletter. If you are inclined to offer input, send it to me at mnh@bu.edu and I'll start a discussion thread. We can publish some of the more interesting discussions in subsequent issues of the Newsletter. Similarly, if you have a food-for-thought question that you’d like to pose, send it along as well.

Discussion Question of the Month:

As noted above, two conductors held at different potentials will always experience an attractive force. When one of the conductors is held at constant charge rather than constant potential, a repulsive force can be obtained. Under what conditions, if any, can two conductors under constrained voltages be made to repel one another?

One possibility has been suggested: Ground conductor #1 through a resistor of value R. If conductor #1 has capacitance C relative to conductor #2, then its charging time constant will be RC. If a voltage

having combined ac and dc components is applied to conductor #2, the dc voltage component will induce a fixed charge on conductor #1, causing an attractive force between the conductors. If the frequency of the ac voltage is much higher than 1/RC, the ac voltage will not induce any additional charge on conductor #1, hence the charge on conductor #1 will remain at the value induced by the dc voltage. Will the force component associated with the ac voltage be alternately attractive and repulsive,

or will it be only attractive?

By the way, the entries from last month's President's Message attributable to James Clerk Maxwell: A Treatise on Electricity and Magnetism, 1873 were numbers (3) and (6).

For the Friendly Society,

Mark Horenstein

ESA President

ELECTRIC FENCES AND GRASSHOPPERS

The Associated Press recently reported on a forest fire in Washington State which was blamed on a grasshopper landing on an electric fence. Let’s think about this in two ways. First, is it electrostatics (in the sense most of us use)? And, second, is this story really credible?

An electric fence puts out brief pulses of high voltage. There is certainly a motion of charge, and a small current exists. But everything important relates to charges and the electric fence. Surely magnetic fields are unimportant in the operation of electric fences. So -- I would answer, yes -- this is an example of electrostatics. Compare it in your mind with electrostatic spray painting or flocking or xerography.

And second is it credible? A grasshopper could alight on a fence wire and feel nothing. But it could span the distance between wire and a close weed, to be electrocuted. Could that start a fire? Do any of our readers use electric fences? Have you ever seen anything ignited? Does anyone have accurate information on the energies involved?

And a related postscript. Do copy machines occasionally ignite dust or a sheet of paper?

Glenn Schmieg

RECHARGEABLE BATTERIES

In the November issue of Popular Science, there is an interesting article about a potential breakthrough in rechargeable batteries. It reminded me of a presentation I made at our ESA conference back in June '81, in which I presented my thoughts on what I called "Electrostatics, the Subtle Force". One of the items I speculated on was the remote possibility of using capacitors to store energy for driving our cars. I suggested that a tankful of gasoline (enough to drive a car about 300 miles) stored 2.1 x 10⁹ Joules of energy. Since gasoline engines are only about 25% as efficient as electric motors, we could drive the same 300 miles on 5.2 x l0⁸ joules of stored electrical energy. I recall computing the size of a capacitor capable of storing that much energy, and I showed an overhead sketch of a 1 ton car carrying 16 tons of capacitors on its roof. . . . Hardly a practical solution to give us clean, efficient automobiles. That led me to think about the plain old electrochemical battery as an extremely compact capacitor, in which my 3 mil mylar film capacitors are replaced by positive and negative charges separated by molecular distances, instead. The one advantage of conventional capacitors, however, is that they can be charged, discharged, and recharged quickly and indefinitely. Rechargeable batteries require up to 8 hours or more for recharging, and can be recycled only several hundred times, at most.

The breakthrough the Popular Science article describes is a 12 volt "proton polymer battery" that is being developed by Japan's NEC Corporation. In this battery, the current is carried by protons and electrons, instead of complex molecules of conventional rechargeable batteries, substantially increasing its speed of charging and discharging, and causing less degradation of the electrodes than the electrochemical reactions of conventional batteries. Reportedly, it can be recharged in about 5 minutes and is projected to last for tens of thousands of cycles. And it is capable of delivering much more power in a shorter period of time. NEC is showing its battery to other manufacturers, but no date nor any certainty of production was announced.

This would not only make battery driven automobiles more attractive, but its ability to deliver power in short pulses would also make it applicable to "Bluetooth" short range radio technology, wirelessly linking of notebook PC's, mobile phones, and even wirelessly "patchcording" our VCR's, TV's, and CD players and recorders together.

If this proves viable, future batteries may be more like electrostatic capacitors than we had imagined.

Bob Gundlach

STATIC ELECTRIC FIELDS AND QUANTUM GRAVITY

To couple these terms together seems like quite a stretch. And, like most of our members, my interests

certainly lie on the side of electrostatics. But connections in research over many decades offer surprises. Let’s follow a trail from A. D. Moore up to the most modern abstract research in string theory.

In the 1930’s and beyond, A. D. was working with industrial people who needed solutions to complicated electric field problems. These problems had complex boundaries, not the simple ”plane and sphere” of textbooks. He worked hard on a mathematical method called curvilinear squares and an analog method called fluid mapping. Both of these have disappeared with the coming of fast numerical iterations of Laplace’s equation on a computer. But don’t forget fluid mapping yet.

Spreading water between two parallel sheets of glass allows flow patterns, and if the sheets are close together, say one millimeter separation, these patterns satisfy Laplace’s equation. When A. D. first told me about this, and showed me some wonderful flow patterns in his laboratory, he said there was some early work by H. Hele-Shaw.

Now just lately, Hele-Shaw flow has been connected with a set of abstract integrals. And these in turn connected with another set of equations in quantum gravity!

It sounds bizarre, but the next progress made in electrostatics could affect progress in quantum gravity (or vice versa). Now that’s something to think about.

Glenn Schmieg

THANKS TO PROFESSOR KAZUTOSHI ASANO

The following letter was sent to Professor Asano by Mark Horenstein on September 25, 2000.

Dear “Kazie”

On behalf of the ESA I want to thank you for the wonderful hospitality extended to us by the membership of IEJ at our recent Joint Symposium in Kyoto. The technical sessions were well organized, the food was great, and, as always, we enjoyed renewing friendships with our colleagues and acquaintances. Although attendance from ESA this year was smaller than we had expected, those who were fortunate to attend found the conference stimulating and very enjoyable. We all felt that the meeting was a great success.

I hope that we can continue the tradition of hosting a joint conference every other year. We would like to invite the IEJ to the USA in 2002 in a city still to be determined. As soon as we have made plans for a specific site that is convenient for travel from Japan, I will let you know. May the long relationship between ESA and IEJ continue.

Sincerely yours,

Mark Horenstein

ESA WEB SITE

Apologies for the long delay in updating the ESA Web site at www.electrostatics.org. The work schedule of the Webmaster has been daunting for the past two months, leaving little time for webmastering. For those of you who have submitted requests for links or e-mail address additions, rest assured that they should be added to the Web site by the time you receive this newsletter in the mail.

Did you know that you now can apply for ESA membership over the Web? In the future, we also hope to arrange for a method to pay for ESA conferences and other items via credit card over the Web using a third party carrier. Stay tuned for more details.

WELCOME NEW ESA MEMBERS

Kevin Haynes Ries Van Twisk

Anthony Dalla Villa James R. Reppermund

Gabriel Laryea

JOURNAL OF ELECTROSTATICS

Are you having trouble with your subscription to the Journal of Electrostatics paid for via the ESA? Several members have contacted me with problems over unreceived issues despite the cashing of their

checks. These difficulties can be attributed to start up transients. Rest assured that payments for any unreceived issues will carry over to next year. No one's subscription fees will disappear into the ether.

If you are having difficulty with your subscription (or if your issues are regularly appearing,) we'd like to know that. Send an e-mail to mnh@bu.edu.

ESA 2001 ANNUAL CONFERENCE

Plans continue for our 2001 Annual Conference at Michigan State University from June 27-30. Many thanks to Marty Kashef of Delsys Pharmaceutical Corp. for volunteering to be Local Host Chair. Volunteers are still needed for various odd jobs, including on-site registration, tour organizing, and outreach mailings. Contact ESA President Mark Horenstein (mnh@bu.edu; 617-353-5437) if you'd like to volunteer. Remember, the ESA has no paid employees (that's why are dues remain so unreasonably low) and runs solely by the efforts of its members.

WOULD YOU CONSIDER HAWAII?

In thinking about sites for future Electrostatics Society of America/Institute of Electrostatics Japan joint meetings, would you consider going to a meeting in Hawaii? Some discussion occurred at the recent

IEJ/ESA meeting in Kyoto about choosing a compromise site as a way of increasing overall attendance from both sides. Would you go to Hawaii if our regular meeting were held there? Would you go if a separate meeting, held in addition to our usual June meeting, were to be held? Please share your thoughts with any of the ESA officers or Executive Council members.