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Re: PHYS-L: Magnitude of feet-scuffing voltage?

On Sat, 23 Jan 1999, Karl Trappe wrote:

I think the static voltage misconception is discussed in Aaron's book on
teaching introductory physics. I recall that the problem is reversed, ie,
that students don't think a "battery" or "power supply" will supply the
charge (voltage) of static machines, like Van de Graaff's.

I agree, since both misconceptions are based upon a belief that "static
electricity" is electricity which is static: where "unmovingness" is the
main important characteristic of the charges. How might students arrive
at such an unnatural perspective? Obviously it is taught to them by the
science textbooks which they encounter early in their careers (K-6,
somewhat less in gr. 6-12.)

If static electricity is electricity which is static, then obviously a
high-voltage power supply has little to do with static electricity, since
it supplies electric current. And obviously the contact-electrification
which arises in the everyday environment has nothing to do with the
voltage of the "current electricity" devices such as batteries and
generators.

I like to say it this way:

STATIC ELECTRICITY IS NOT ELECTRICITY WHICH IS STATIC AND UNMOVING.
INSTEAD, THE WORD "STATIC ELECTRICITY" MEANS "ELECTROSTATICS", AND IT
ALSO TRANSLATES AS "HIGH VOLTAGE PHENOMENA." IF WE WANT TO DISCUSS
SURFACE CHARGE, WE SHOULD SAY "SURFACE CHARGE" RATHER THAN "STATIC
ELECTRICITY."

My own feelings on this topic are strong, because in 3rd grade, when the
class was following the textbook and rubbing balloons upon arms to cause
arm-hair to rise, I asked the teacher for the name of the "stuff" that
exists in the space between the balloon and my arm. I don't remember her
words, but I still remember the emotions: being dismissed and belittled in
front of my friends. (She didn't know the answer, and so desparaged both
the question and the questioner.)

Heh. For those who don't know what drives my physics interest, a major
component is entirely Freudian: a desire for vengence regarding childhood
emotional damage caused by science teachers! I certainly must thank
those teachers though. Seriously. Without them, I might be a very
normal, boring person with no strong opinions, science interests, or
interesting "scars."

a large :)


The four stages of the misconception-overturn process:

1. You're WRONG!!!!!

2. Well, you're TECHNICALLY correct, but that has no implications.

3. You're entirely right, the implications are staggering, and it is
now clear that I've been totally blind for most of my career!

4. What are you talking about? It never happened! I never had any
misconceptions to begin with.



Bill's alternate version:

4. The people who taught me the errors? I'll get them for that! :)

(Which is rapidly followed by the realization that my teachers are no
better than I, since they are victims of a continuing infection, and
therefor the solution is to wipe out the disease, not to blame its other
victims for being in an infectious state.)

Regarding the humidity leaking charge explanation: What is the accepted
mechanism? I know my electrostatics demos don't work when the
humidity is high, so I am guilty of saying that the charge leaks
off....Karl

I'm impressed. I find that most people opt for dishonesty in order to
save face, and will silently change their language, but they will never
tell anyone that their earlier assertions were wrong. This converts my
attacks upon educator-misconceptions into a vast uphill battle, since I
don't usually attract others to join the crusade. If human beings
constantly courted embarassment in order to avoid dishonesty, then the
situation would be far different. Unfortunately, we humans usually court
dishonesty in order to avoid embarassment.

The charge-imbalance DOES leak off, but it goes along surfaces. When
humidity is high, the conductivity of the surfaces of objects becomes far
larger than when the humidity is low. High humidity converts everything
into "metal." (This can be cured by warming the surface with a hot air
blast from a hair dryer. If the cause of discharge was the conductivity
of the air, then eliminating adsorbed water from the surfaces would do
little good.)

Here's a rule of thumb for conductivity. If an object has a capacitance
of 100pf, and the time constant of its discharge curve to ground is 1
second, then the general ohms-range of series resistance between the
object and the earth is T/C, or 10^10 ohms. "Conductive" is ten billion
ohms! If I recall correctly, capacitance increases faster than surface
leakage as object sizes grow, so smaller objects would discharge faster.
I think. Or do all sizes of objects act the same, leakage-wise?

In order for airborne water droplets to render the air conductive, they
would each need to carry an imbalance of charge and so provide mobile
charge carriers. In other words, the air would be full of ions, and
therefor become somewhat conductive just like any ion-containing fluid.
Water CAN make things conductive, but it does so by providing a conductive
electrolyte, not by simply providing pure water.

My current misconceptions list ("present," not "current!"), at
http://www.amasci.com/emotor/stmiscon.html contains:

- 'Static Electricity' is electricity which is static?
- Friction causes 'static electricity?'
- 'Static electricity' is a buildup of electrons?
- Neutral objects have no charge?
- 'Electricity' is a form of energy?
- 'Charging' a capacitor fills it with charge?
- 'Static' is a rare event: sparks and dryer-cling?
- Ben Franklin's kite was struck by lightning?

Still need to add:
- 'Static Electricity' has nothing to do with voltage.
- Humid air is a conductor.


PS, here's an interesting electrostatics tidbit.

Part of the 1997 Nobel prize in chemistry went to Boyer and Walker for
illuminating the nature of ATP-Synthase, the central component of
biological energy systems. They discovered that ATP-Synthase is a
nano-robotic assembly line which is driven by a tiny rotating motor. An
ELECTROSTATIC motor. The motor sits embedded in a cell membrane, and is
driven by proton concentration difference across the membrane. It
discharges a proton current, violating the popular concept that
"electricity" is made of electrons. It has a circular "work table" where
it takes in three ADP molecules in sequence, processes them one at a time
to combine them with phosphate for a sort of "compressed spring" energy
storage, then releases each one as it rotates, with a "discharge current"
of four protons per ATP created. The motor turns at around 100HZ. In our
bodies, the total operation of these motors processes approximately one
human body mass of ATP/ADP per day, and far more during continuous
exertion. Like most electric motors, the system can be reversed and
operated as a generator: it takes in ATP, pops off the phosphate, and uses
the released energy mechanical energy to drive the electrostatic motor
backwards as a charge pump.

Now isn't that the most intensely cool idea you've ever encountered before
in your entire life? Living organisms are like a substance made from
electric motors! Don't come around telling ME that electrostatics is a
fairly useless backwater of science, important mostly for laser printers
and for explaining why, inside the clothes dryer, the sweaters grab up all
the socks! :)

http://db.bric.postech.ac.kr/review/biochem_biophys/98/1_12.html

http://www.lbl.gov/~hwang/animation/index.html

((((((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
Seattle, WA 206-781-3320 freenrg-L taoshum-L vortex-L webhead-L