ELECTROSTATICS
NEWSLETTER
May/June 2001 No.156
PRESIDENT’S MESSAGE
Greetings to all.
We have several new members this month, many having signed up via our
on-line membership application form. A list of new members may be found
elsewhere in this newsletter. I look
forward to greeting all members, old and new, at our upcoming conference,
ESA2001, to be held on June 27-30 at Michigan State University. Note that you can now register ahead of time
on line (and pay by credit card, if you wish) right from the ESA Web site. See www.electrostatics.org
for details.
One of the hallmarks of ESA conferences is the open
and non-confrontational atmosphere in which individuals can bring up technical
issues for discussion. This collegial
atmosphere has earned us our reputation as "The Friendly Society." In
that spirit, I must admit that I made an error in my discussion of the gold
leaf electrometer in my Jan/Feb President's Message. In that article, I made the statement that the leaves of the
electrometer, which lie at the same potential, cannot repel one another and
must instead be attracted to opposite polarity charges lying outside the
instrument. After numerous e-mail discussions with my colleague, Dr. Thomas
Jones of the University of Rochester, one of which we published in the last
Newsletter, I have newly reached the conclusion that the equipotential leaves
actually do repel each other. For a
well designed instrument, this repulsion will be independent of the location
any opposite-polarity charges external to the device. For the curious, I will present the details in more depth at the
upcoming ESA meeting in June. But for now, I trust that the spirit of the
Friendly Society will prevail and that you'll forgive me for my previous
erroneous statement!
Sometimes,
friendly discourse in the ESA tradition happens outside the forum of the annual
conference, often by way of group e-mail exchanges. One such exchange happened recently on the subject of measuring
surface charge on insulators. In keeping with my theme in recent President's
Messages of highlighting core issues in electrostatics, I thought I'd share key
elements of the discussion with you this month. At the request of some of the
participants, I've left out identities and have paraphrased the exchange in my
own words, but I think I've captured the essence of the discussion. Feel free
to add your "two cents" by sending in letters to the editor or by
commenting on the topic at the upcoming annual conference.
Summary of E-Mail Discussion on Measuring Surface
Charge
From Alpha:
To all:
I have three questions on the subject of
capacitance:
1. Can one
define the capacitance of an insulator?
2. Can a
charged insulator be characterized by a quantity expressed in volts?
3. Can one
measure the voltage of an insulator using a non-contacting voltmeter or a
hand-held
field
meter?
I accept the
notion that the classic description of a capacitor requires that it be made up
of two conductors. However, I do find this definition very limiting in the
field of electrostatics. If anyone has a reference or can help me with some
mathematical expression for the equivalent capacitance of an insulator, I would
appreciate it. I've checked my old text books and everything about capacitors refers
to conductors.
From Beta:
Dear Alpha,
If you point a hand-held field meter at a charged
insulator, one can obtain a wide range of readings simply by changing the
relative positions of the insulator,
meter, and ground. Hence, it would seem
that such measurements would not be very useful. The problem is exacerbated because most hand-held fieldmeters are
calibrated in "volts."
Indeed, in this general context, a voltage reading has little meaning
unless the location of the meter is carefully chosen. Conversely, if the meter is correctly positioned, then the
voltage reading obtained from the meter can be used to calculate the field
between the meter and insulator, and, in turn, the surface charge on the
insulator and the change in voltage when the insulator is moved to another
location. Knowledge of the surface charge is important for determining the
likelihood of brush or other types of discharges.
The better meters are calibrated for a fixed
distance d between a conducting test electrode at fixed potential Vo and the
meter, so that the approximate electric field E measured by the instrument is
known. (The exact value of electric field usually will be affected by fringing
effects and won't be given by just V/d.)
Suppose that a process engineer wants to obtain an
order-of-magnitude estimate of the surface charge on a web that spans a region
with no nearby grounded surfaces. [Ed
note: Here "web" refers to a sheet of plastic, film, textile, etc,
rather than an Internet site.] If the
fieldmeter is held at the manufacturer's specified distance d from the web
surface, the meter reading can be translated into an (approximate) electric
field reading E. This value then can be
used with Gauss's law to calculate the approximate charge per unit area on the
web.
A second situation exists when the web rests against
a grounded metal surface such as a plate, roller, or drum. In these situations, the upper, charged web
surface becomes one electrode of an
equivalent capacitor, and the grounded drum or plate
functions as the second electrode. When the meter is brought near the web, a
second equivalent capacitor is formed between the web surface and the meter.
Because the web surface is common to both conductors, and because the drum or
plate and the fieldmeter are both at ground, these two capacitors appear in
parallel in the equation Q = VC.
For thin webs, the meter-to-web capacitance is only
a small fraction of web-to-grounded surface capacitance, hence the additional
capacitance introduced by the meter can usually be ignored. For thicker webs, the capacitance of the
meter can become important and must be taken into account. In either case, the fieldmeter can be used
to determine the charge on the web and is therefore a very useful instrument.
From Gamma:
Dear Beta,
Thanks for your good explanation of the difference
between an electric field measurement and a surface potential measurement. I find that some confusion arises when
people who are not well versed in electrostatics use fieldmeters that are
calibrated in "volts." They
quote measurements in volts and easily slip into thinking they are measuring a
potential, rather than remembering that the measurement is calibrated for a
fixed distance and that they are actually measuring an electric field, or
voltage per unit length.
In a similar vein to the recent ESA Newsletter
President's Message discussing ohms vs. ohms/square for surface resistivity,
[See NL- 155 March/April 2001], I think it would be preferable if people quoted
fieldmeter readings in volts per unit length.
From Beta:
Dear Gamma:
I think the practice of marketing hand held
electrostatic fieldmeters with units displayed in volts came about when some
company's marketing department realized they would have trouble selling hand
held fieldmeters to people who do not work in electrostatics. The concepts of electric field and
surface-charge density mean little to people who aren't trained in
electrostatics. These folks do, however,
know a lot about volts. They have 12 of
them under the hoods of their cars, 3 of them inside a common flashlight, and
120 of them in the electrical outlets of their homes, and therefore are willing
to buy a device that measures "volts." But they are likely to be puzzled by a device that measures
electric fields.
From Delta:
To Alpha:
Many thanks for copies of your correspondence. Of course, I guess I will have a problem
with the well-known concept of "capacitance loading" if dielectrics
cannot have a potential! If I approach
a charged, insulating surface with a fieldmeter (or voltage probe,) and if I
get the same reading that I get from a metal surface held at a fixed potential
(and can check for uniformity of response over the area of the insulator as
well,) then surely will I not have an equivalent
potential?
How does the meter know, for example, if the surface is a charged
dielectric or an energized conductor?
Many of the problems from "'static" arise
not from the charge on surfaces per se but from the influence such charge has
on things nearby via the electric fields that the charge creates. These effects may include induction charging
of items, electrostatic sparks, or the attraction of dust, dirt and thin
films. The effects are the same as if
the dielectric surface had a potential - so why the distinction?
If one tries to emphasize that potential varies with
circumstance (such as proximity of nearby grounded surfaces,) then that is
good. But my feeling is that a
dielectric surface does most surely have a potential that one can define and
measure. I will be interested to hear
your further feelings.
From Epsilon:
To all who have contributed to (or merely read
patiently) the on-going discussion of electrostatic measurements, etc., kudos
all around! Discussions like this one
put us on the same wavelength, same page, same
whatever. Nice going!
From the President:
And there you have it! The measuring of charge on
insulators using electrostatic instruments has long been a much misunderstood
and controversial subject. Feel free to add your own comments or insights.
For the Friendly Society,
Mark N. Horenstein
ESA President
WELCOME TO NEW AND RETURNING ESA MEMBERS
Daniel Chadwick
Riccardo Cocciolone (consultant)
Frank Czechowski
Buckminster
Desmoulin/Caley (consultant)
Bruce H. Easom (LSR
Technologies)
Maximuk Evghenii (Inst.
of Appl. Phys)
Bruce R. Forsyth (3M
Center)
Darin K. Fowers (Thiokol
Propulsion)
Daniel Gagnon (Electro
Static Tech.)
Mel Gannon (consultant)
Chris Gnehm (MycoFerm)
Rebecca D. Harman (S.C.
Johnson & Son)
Kevin Haynes
Bert Hickman (Stoneridge Eng. )
Ollie Lachance (Corning
Life Science)
Larry Levit (Ion)
Ricardo Lopez (Flextronics)
Andreas Marquard (IMVM)
Muhammad Mohsin (academia)
David L. Myers (Kimberly-Clark)
Robert Ogburn
Hirohiko Sakai
John S. Salmento (F.L.Smidth
Inc. )
Annette Schiel (student)
Charlie Sloan (consultant)
S.Thangavelu (Chennai Petroleum)
Ries van Twisk
Anthony Dalla Villa
These individuals all signed up for membership on
the ESA Web site at www.electrostatics.org.
COMMENTS ON A. D. MOORE
I recently met a fellow named Bruce McCubbrey who,
in commenting on my association with the ESA, revealed that he had been an
undergraduate at the University of Michigan at Ann Arbor under A. D.
Moore. His description of A.D. included
the following comments: “A bear”, “He gave the hardest exams I ever had”, and
“He was the greatest professor I ever had.”
Mr. McCubbrey also noted that A.D. managed to work some electrostatics
into every course he taught, regardless of the subject.
Mark
Horenstein
PAY THE ESA BY CREDIT CARD
The ESA is now able to accept secure on-line credit
card payments for conference fees, membership dues, and personal subscriptions
to the Journal of Electrostatics. This
feature is now available as a registration option for the ESA2001
Conference. See www.electrostatics.org
for details. Please note that non-US
users of PayPol must register under International Accounts.
Sorry, we are not yet able to accept credit cards by
phone, mail, or fax at this time.
Mark
Horenstein