Re: Trying to understand AC by analogy to water waves.
- From: Andy Resnick <andy.resnick@xxxxxxxxxxx>
- Date: Mon, 18 Dec 2006 09:25:12 -0500
Colin wrote:
When first learning about electric circuits, we've all been encouraged
to think about water flowing through pipes. This combined with the
understanding that it's the energy that's used up in a circuit, and not
charge or electrons, helps a lot to put a picture with the math used in
circuits.
But I don't remember the analogy being carried over in any way to AC.
In DC, using the water-pipe analogy, I imagine electrons are actually
flowing through the wires, doing work as they lose potential energy on
the way to ground, like water from a water fall being channeled through
turbines.
AC electronics can be thought of as the very low frequency domain of optics. So instead of thinking about water waves, think about light.
<snip>
1) AC travels through wires like a water wave through a cannal. In this
analogy zero potential is like "sea level" or the level of the water
surface when there's no waves. Then the "below zero" potential is just
part of what's needed to make a wave. Just as in ripples of water
where the crests are possible because there's a trough just before it,
the positive potential in the wire is possible because there's a
negative potential preceeding it.
I think what you mean here is simply the amplitude of the wave. And there is a phase to keep track of as well- think of three-phase 240 VAC lines.
2) Whereas DC is something like a waterfall, in AC, the energy is
transmited as a wave without flow of electrons, until the waves sort of
"break on the shore" and "spill over" into puddles or streams to lakes,
or whatever.
Again, it's more like transmission and reflection. "Impedance matching" is the correct term.
<snip>
Some questions:
I. Physically, how is AC created? What is the device called and in
simplified terms, how does it work? Can you stretch the mechanical wave
analogy to describe what is happening in the creation of AC?
Oscillating charges. Move an electron sinusoidally, and create a sinusoidal electric field (and a sinusoidal magnetic field).
II. What are the electrons in the wire actually doing as the "potential
wave" propagates through the wire.
They move in response to the field, however one defines "move" given a suitable definition for an electron. The resistance to movement is called the "polarizibility" of the media, and one has dielectrics with no free electon flow, and conductors, where electrons will flow, and from this one may construct all kinds of things- anti-reflection coatings (impedance matching connectors), single-mode fiber (single-mode waveguides), etc.
III. How does an AC/DC converter work (again in simplified terms). Can
you convert a mechanical wave to a flow of gas or liquid? How would you
create a wave-free current of water starting from water waves? How
would you create a quiet wind starting from sound waves? How good are
AC/DC converters anyway, do they really completely remove the AC wave?
I had to look up some diagrams, but it appears to simply rectify the signal- there's a diode element that has differing resistances for forwards and backwards current. Don't know much about the specifics.
IV. I have a lot of questions about how AC is actually used.
Yes, you do...
a.Do most electronic devices have AC/DC converters in them? Devices
that come with power adapters that transform the voltage also seem to
convert to DC power too, but do most other electric devices without an
external adapter simply contain an internal power adapter that converts
AC to DC?
b. Can some electric devices or appliances, like light bulbs or
refridgerators, work directly on AC current?
c. Does the light bulb or other AC device, in using the energy
available in the AC wave actually reduce it to a DC current just before
going to ground anyway even without an explicit AC/DC converter
included in the circuit.
d. Do lights actually flicker along with the AC at 60hz? I don't
notice anything like that from regular light bulbs, but florescent
lights sometimes seem like they flicker very rapidly. But, don't
florescent lights have power adapters included that convert to DC?
For most devices (i.e. non-diode-containing devices), AC works fine. Incandescent light bulbs work just fine- they are not much more than a resistor. I don't think they flicker due to the rates of heat flow.
Fluorescent lights have a giant transformer (called a ballast) that is needed to generate the large voltage required to create the arc. Flicker is not caused by AC voltage- it means the ballast is not generating the high voltage, or the bulb itself is faulty.
V. Since the AC potential is just as often below ground as above, is
the average actually zero?
Yes, but the power is proportional to the *square* of the field, and is not zero.
Doesn't seem like it since connecting an AC
wire directly to ground produces a huge current. I would think AC
moving from power plant through wires to ground to be analogous to a
pond connected by a cannal to an enormous lake both at sea level where
neither have significant natural waves.
<snip>
Again, think of AC more in terms of optics than water.
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
.
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