In true A.C. electrons flow both ways in the circuit. In household
electricity the incoming black wire is"hot" & electrons flow from the
power co. to my "load".  The white wire from the power co. is
connected to ground & no electrons flow from the power co. to my
house.  (Forget the fact that there are two black wires from the
source of power & forget the safety ground wire - they are not
relevant) If my circuit is a light bulb connected to the black & white
wires what is the source or origin of the electrons flowing up the
white wire?  If the source is the earth, why do they flow?  (What
creates the difference in potential? )  Or, is the answer to all this
that I really have fluctuating DC and notAC?

Dear alanj-ga,

The trick to this one is that, as you say, potential differences drive
the flow of electrons, but absolute potential has little meaning:

"Note that the potential energy and hence also the electrical
potential is only defined up to an additive constant: one may
arbitrary choose one position where the potential energy and the
electrical potential is zero."
  - Wikipedia article on Electrical Potential
    http://www.wikipedia.org/wiki/Electrical_potential

Where household wiring and the electrical power grid are concerned, we
arbitrarily declare "ground" as our zero point, and measure potential
from that point.

The neutral (white) wire is - as you say - grounded at your house:

"One of the other wires will be white (or black with white or yellow
stripes, or sometimes simply black).  It is the neutral wire. It is
connected to the "centre tap" (CEC; "center tap" in the NEC ;-) of the
distribution transformer supplying the power. It is connected to the
grounding conductor in only one place (often inside the panel)."
  - "What does an electrical service look like?"
    Electrical Wiring FAQ
    http://www.faqs.org/faqs/electrical-wiring/part1/section-15.html

Now, when AC is generated, the potential difference is induced in the
generator coil between the live (black) wire and the neutral wire. (In
actuality, there are usually three lives and a single common neutral -
three-phase power - but for simplicity, I'll only deal with one. I've
included a link below to an article on three-phase power, if you're
interested in the full details.)

This potential difference is induced by the rotation of the generator
coil within a magnetic field. An excellent simulation of this process
can be seen at:

http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/generatorengl.htm
(Java applet)

As you can see from this, as the wires of the coil change direction
relative to the magnetic field, the induced potential difference also
changes in direction. Since one side of the coil - neutral - is tied
to ground, as we have said, which is defined as zero, the potential of
the other (live) side alternates between positive and negative;
alternating current.

And having finished the background, I can now get to the meat of your
question. The source of the electrons flowing up the white wire is the
earth; and they flow because of a potential difference induced by the
generator. It's just that in that half of the cycle, the black wire
has a potential lowered from that of the earth, thus causing electrons
to flow towards it.

Additional Links:

Wikipedia article on Three Phase Power
http://www.wikipedia.org/wiki/Three_phase

If this answer isn't quite what you're looking for, please feel free
to request a clarification.

Hope this helps,

cerebrate-ga

Search Strategy:

alternating current electron flow -
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potential difference -
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electrical generator -
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One thing that might surprise you is that the drift velocity of
electrons in your wires is very very slow.  If the circuit you
described is made up of a 120 W light bulb and copper wires 1.8 mm in
diameter, the current in the wires is 1 amp and the associated drift
velocity is 2.9 * 10^-5 m/s, or 11 cm/hour.  If you think of electrons
shifting back and forth at 60 Hz, the distance they move is
approximately a ten-thousandth of a millimeter--less than one
hundredth the width of a human hair. However, in reality, the
electrons don't stay in an orderly pattern and shift back and
forth--they zip around in random thermal motions. The drift velocity
calculations are easy to do once you know that copper has about one
conduction electron per atom and about 8.5 * 10^28 atoms/m^3.

In any case, I thought this fact might put your question about where
the electrons come from in a new light.


Also want to point out re:

"It's just that in that half of the cycle, the black wire
has a potential lowered from that of the earth, thus causing electrons
to flow towards it."

electrons have a negative charge, and so would flow away from a
lowered potential, not towards it.