Re: Voltaic Pile of Confusion

[William Beaty <billb@ESKIMO.COM>]

On Sat, 29 Jul 2000, Tom McCarthy wrote:

> I have been reading an account of Volta's reasoning for creating a pile and
> I don't think it is quite right (this is not Volta's reasoning but the
> author's interpretation of it).  If one cell genterates a given current,

Most probably this is the "current electricity" misconception.  Textbooks
in the early grades inform us that batteries produce an "energy" named
"current electricity" which is made out of electrons.  This is wrong on
many counts.  Among other things, it causes people to believe that
batteries generate constant current (and that batteries supply
a "stuff" called "current.")  That author might believe that individual
cells produce current rather than voltage.

> then several cells connected in series would produce a larger current under
> the same conditions. I would think this should read voltage, rather than
> current. The current would remain fixed but it would be more energetic.
> What do y'all think?

Right.  If that author doesn't have physics training, then he/she might
not understand voltage.  Or, if the material is aimed at K-12, the author
might be intentionally avoiding the word "voltage."

> I have another similar question. What is the best way to view how the
> electrons all achieve a specific voltage in an arrangement where x batteries
> are hooked up in series.  In other words, how is an electron produce in one
> battery affected by the presence of another battery?

Individual electrons are not altered by the battery, any more than
individual water molecules are altered by going through a water pump. It
is the RELATIONSHIP between electrons (and between the positive nuclei of
metal atoms) which is changed by a battery.  By packing extra electrons
into a metal wire, the voltage of that wire is changed. If a metal is
given more electrons than protons, then that metal will have a negative
voltage with respect to ground.

You have to think in terms of static electricity.  One cell wil
electrostatically l charge its positive terminal to a potential of ~1v
with respect to its negative terminal.  If you connect another cell to its
positive terminal, that entire second cell will be charged to a potential
of ~1v... but then the positive terminal of the second cell will be
charged to a potential of ~2v with respect to the first negative terminal.
Stack a bunch of cells, and the ends of the stack will develop large
static charges and high potential.  Voltage is an electrostatic
phenomenon.

  ZAMBONI PILE
  http://www.amasci.com/emotor/duluc.html

Many authors point out that voltage resembles water pressure.  This
analogy is not perfect, but it does give us a handle on understanding
batteries.

Suppose we have a constant-pressure water pump which turns itself off
whenever the outlet pressure-difference attains 1 ATM.  Fill that pump
with water, connect a short hose between the pump's inlet and outlet, then
turn the pump on.  It runs, and continuously pumps water in a circle.  Now
clamp the hose shut so the flow stops.  The pump will run until the until
the pressure-difference between the plugged hoses is 1 ATM.  If you loosen
the clamp a bit so water can leak through, then the pump will run just
enough to keep the pressure at 1 ATM (so that the pump just exactly
supplies the water needed by the leak.)

OK, now WHAT HAPPENS IF WE HOOK TWO OF THESE PUMPS IN SERIES?  Hook them
in a circle, and clamp the hose shut.  Each pump will run until the
pressure-difference between its inlet and outlet is 1 ATM.  But this means
that 2 ATM is applied across the clamped part of the hose.  If the first
pump's inlet is at 0 ATM, then its outlet will be pressurised to 1 ATM,
which feeds into the second pump's inlet.  The second pump's inlet will be
1 ATM higher, or 2 ATM with respect to the FIRST inlet.

By stacking upt these constant pressure water pumps (and then plugging the
hose), we can stack up the pressure difference.

Pressure is a HYDROSTATIC phenomenon, and to understand voltage, you have
to realise that batteries generate "static electricity."  This fact is
missing from most K-12 textbooks.  Here's an excellent paper about the way
electrostatics applies to simple circuits:


  http://cil.andrew.cmu.edu/rchabay_extra/circuit.pdf



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