# basic ac theory question

Discussion in 'Electronic Basics' started by Jerry, Oct 8, 2006.

1. ### JerryGuest

If electron flow with alternating current keeps changing direction, then it seems that current doesn't move along a path from one point to another, but instead moves back and forth. Yet AC wiring books describe current as moving from the hot/black side, through the load, and returning via the neutral/white side.

How does AC move along a path when the direction of current keeps changing?
Thanks for any insights,
Jerry

3. ### John O'FlahertyGuest

The current moves to and fro, and does its work. A light bulb, for
example, gets just as hot when the current is going right-to-left as
when it goes the other way. No net movement of electrons is necessary.
Then why call the wires different names? To keep track of which wires
have voltage with respect to each other. At any instant in time,
whether they are plus or minus with respect to the other kind of wire,
two hot wires will have no voltage between them, and the same is true
of two neutral wires. Also, the neutral is connected to ground
somewhere, and its important for safety reasons to know which wire has
little or no voltage with respect to ground.

4. ### JerryGuest

Simplification makes sense but doesn't answer the question. Does AC move
along a path or just "back and forth"?

5. ### SkepticGuest

Think of sound. The air molecules move back and forth but do not travel from the source to the listener. Yet we still think of the sound traveling from one point to another. Don't confuse the wave with the medium. Waves move though the medium doesn't.

If electron flow with alternating current keeps changing direction, then it seems that current doesn't move along a path from one point to another, but instead moves back and forth. Yet AC wiring books describe current as moving from the hot/black side, through the load, and returning via the neutral/white side.

How does AC move along a path when the direction of current keeps changing?
Thanks for any insights,
Jerry

6. ### John PopelishGuest

That they do.
Current isn't the movement of any electron, but the net
movement of charge through the conductor, at any point.
While an electron is going one way (and moving a tiny
fraction of an inch) the current it is part of is moving in
that direction, from the power generator to your house.
when that electron turns around, the current anywhere along
that path from generator to house goes the other way. This
is because the distance from the generator to your house is
a small part of a wavelength at that low frequency.

When you raise the frequency to the signals arriving through
the cable television system, this assumption of uniformity
direction all along the path, is no longer valid.
Then they are mistaken.

The distinction between hot and neutral is that the
potential (voltage, not current) on the neutral wire with
respect to Earth is low (they are connected together at the
power panel and at the transformer on the pole). The
voltage between the hot conductor and Earth is much higher.
So connecting a load between hot and ground is quite
similar to connecting it between hot and neutral. If you
are standing on a dirt in your bare feet and you touch the
hot wire, you will understand why they chose this word to
describe that side of the circuit.

8. ### Daniel MandicGuest

It shakes, with a small head on that direction, where all the flow
begun.

Best Regards,

Daniel Mandic

9. ### Alan BGuest

then it seems that current doesn't move along a path from one point to
another, but instead moves back and forth. Yet AC wiring books describe
current as moving from the hot/black side, through the load, and returning
via the neutral/white side.
First of all, you mistake electron flow for charge flow. Current is not
electron flow, rather it is charge flow. One amp is defined as one Coulomb
of charge per second passing a given point.

simplistic description of the connections being made in a circuit. The
supply side and the return side are necessary labels, helpful when building
or maintaining circuits.
If there is no path, charge cannot move at all. So if charge is moving,
there must be a path. Think of a million hikers on a single-file trail.
The path is there, and the hikers are there, but they cannot decide which
way to go, so they sway back and forth. The ends of the trail show the
movement of the individual hikers within the trail; the energy of their
movement goes back and forth, and is noticeable.

10. ### Tom BiasiGuest

.. Think of a million hikers on a single-file trail.
Sounds like a lot of fun, do they sing Koom-ba-ya?

11. ### WayneGuest

I have always understood charge as the flow of electrons if it is a
metal conductor. If it is not a metal conductor then charge is not
necessarily electrons.

12. ### PuckdropperGuest

I'm sorry, did you say something?

Puckdropper

13. ### jasenGuest

a similar question is

how do wheels travel when they just go round in circles?

the answer to your question could be that it travels like a wave with an
extremely long wavelength, but that could confuse more than it enlightens.

Bye.
Jasen

14. ### Alan BGuest

Charge is the energy produced when electrons move. A potential is
introduced at either end of the circuit path, which imparts energy to the
electrons in the conductors, causing them to move, which in turn creates a
net flow of charge. Charge has no mass and moves fast, close to c.
Electrons have mass, and move rather slow when excited. I wonder what
fundamental element moves in a non-metal conductor? Neutrons?

OK, best to leave the non-instructive, derisive satire to Phil Allison.
Er, except he's too dull for satire. Ah well, sorry. Charge does not flow
easily in non-metals (insulators) because elementally, they have few free
electrons, which means it is difficult to jostle them loose with potential
energy. Thus, you can apply voltage to a ceramic, or glass, and its
electrons will not budge, hence no charge will be produced.

Now, if you're talking about hole flow in a semiconductor, that's a whole
new bucket of worms; are you sure you want to go there?

15. ### Alan BGuest

Heh. Explaining things that cannot be directly observed is a dicey
enterprise!

16. ### FinGuest

yes, you're sorry.

17. ### Tom BiasiGuest

I know, I was just having some fun. It really wasn't a bad example;
pretty good actually.

Tom

18. ### Michael A. TerrellGuest

Those wiring books are describing the physical path of a circuit in a
around. They don't need to know if it is AC or DC, worry about phase or
anything else. They just need to know how to hook up and outlet or a
light and switch. Some electricians don't know any more than that, yet
they can do good work that passes inspections.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

19. ### Stanislaw FlattoGuest

They don't tell you the whole story, (I don't know who wrote this!). AC
as my teacher told me is a sinusoidal effect consisting of "positive"
(whatever it means) and "negative" half-waves. So what is "hot" and
"neutral" changes every half-cycle. Consider a steam engine piston,
moving forth AND back and yet doing "work".
See above.
HTH

Stanislaw

20. ### jasenGuest

energy is not produced when electrons move
charge is a a number of electrons (or equivalent)
Nope. The potential exerts force on the electrons.
electrons have mass... potential has no mass, and propogates (like a wave) at
speeds upto c.
electrons (in most cases) or ions (in ionic fluids) or holes
(in p-type semiconductors), neutrons may move but they carry no charge.
potential energy doesn't jostle, (but heat does), potential actually exerts
no force,

however a difference in potential that creats the force proportional
to the difference in potential and inversely proportional to the distance.
huh?

Bye.
Jasen