Re: Trying to understand AC by analogy to water waves.
- From: "Sorcerer" <Headmaster@xxxxxxxxxxxxxxxxxx>
- Date: Mon, 18 Dec 2006 01:33:09 GMT
"Colin" <colinkeenan@xxxxxxxxx> wrote in message news:1166396110.534585.66160@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
| 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.
|
| At first, I was thinking, why would there be any flow of electrons at
| all if the potential is just as often below ground level (below zero)
| as above ground level.
|
| But then, I thought more about how the potential is often described as
| a sign wave or squared off sign wave. So then, is it fair to say:
|
| 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.
|
| 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.
|
| 3) The form of the wave is actually more like a sound wave than a water
| wave. Or, what is the form of the wave in terms of electron movement?
| Side question: can you ever generate a wind starting from sound waves?
| Maybe at low frequencies? In other words, is there some sort of
| one-way barrier possible that would let the crests of the sound waves
| through, but not let them go back, so there would be an overall flow of
| air?
Yes. Use a clack valve.
http://www.gasballooning.net/images/ValveDiagram.jpg
This used a clack valve.
http://www.historyplace.com/worldwar2/ww2-pix/v1.jpg
http://www.srl.org/shows/austin/show/v1-5.JPEG
http://en.wikipedia.org/wiki/Image:V1FlyingBomb.JPG
| Some questions:
|
| I. Physically, how is AC created?
By a moving magnet going back and forth or rotating. Rotation
is a good way of moving the wire past the magnet or the magnet
past the wire over and over again.
This one is three-phase, you'll learn about that later.
http://www.ewh.ieee.org/soc/es/Nov1997/09/B_WOU1.JPG
http://www.ferret.com.au/articles/images/4b/24002f4b.jpg
For now, just realize that you get AC from any two of the copper
slip rings.
For DC, you need a commutator:
http://www.k9ape.com/new/CommutatorA00228A.jpg
| What is the device called and in
| simplified terms, how does it work?
It is called a generator, it works by moving a wire past
a magnet or a magnet past as wire.
| Can you stretch the mechanical wave
| analogy to describe what is happening in the creation of AC?
Make the water flow through the pipes both ways.
|
| II. What are the electrons in the wire actually doing as the "potential
| wave" propagates through the wire.
Changing direction.
http://www.androcles01.pwp.blueyonder.co.uk/AC/AC.htm
|
| III. How does an AC/DC converter work (again in simplified terms).
With a rectifier, often called a diode.
It's like a valve that only lets water go one way.
This is fluid going both ways in the pipes:
http://www.viss.lv/dati/tehnikasnoma/jcb_160.jpg
If you want a DC/AC converter you need an oscillator.
|Can
| you convert a mechanical wave to a flow of gas or liquid?
Yes.
http://www.viss.lv/dati/tehnikasnoma/jcb_160.jpg
It is a slow wave, but still a wave. The driver can wave to you
with his bucket as you can wave with your hand.
| How would you
| create a wave-free current of water starting from water waves?
http://www.riversevernbore.co.uk/
This is the OPPOSITE way to the flow of the river.
How
| would you create a quiet wind starting from sound waves?
Clack valve. Dunno about quiet, though. A fan does a better job,
but this is not exactly quiet:
http://ttjstk.diy.myrice.com/f14/Tomcat-11.jpg.jpg
You can see AC superimposed on the DC thrust.
How good are
| AC/DC converters anyway, do they really completely remove the AC wave?
For the best water analogy a capacitor is used, that's like a water tank, you fill
the tank with drips and take out a steady stream.
http://en.wikipedia.org/wiki/Image:Gelmersee04.jpg
In this case the drips are rainfall.
http://130.58.92.218/webstuff/Phys81/Phy81Session1_2/Bridge.jpg
AC in, DC out.
Unfortunately it wasn't known when the diode was first drawn
that the electrons flow the opposite way to the arrows, but that's
convention.
| IV. I have a lot of questions about how AC is actually used.
| a.Do most electronic devices have AC/DC converters in them?
Yes. Your computer is the first to look at.
| 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?
Yes.
| b. Can some electric devices or appliances, like light bulbs or
| refridgerators, work directly on AC current?
Most do, the domestic supply is AC.
| 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.
No. the filament in the bulb works with the current alternating direction.
| d. Do lights actually flicker along with the AC at 60hz?
Yes, but the filament in an incandescent bulb is so hot it doesn't
have time to cool beforwe being heated again so the effect is minimal.
A fluorescent tube has a noticable flicker.
| 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?
No, they have a choke to limit the current, but it's still AC.
|
| V. Since the AC potential is just as often below ground as above, is
| the average actually zero?
Yes. Don't rely on averages. The average dog seldom bites, but one
bite is all you don't want.
| Doesn't seem like it since connecting an AC
| wire directly to ground produces a huge current.
Yep, forwards and back.
| 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. The enormous lake is ground
| and the pond supplies water for the power plant. If there's no barrier
| in the cannal, waves generated in the pond shouldn't produce a current
| in the cannal, just waves propagating through the cannal to the lake.
You are adding ripples to a river.
With AC, the water goes up to the clouds, falls back to the ocean,
that's two ways, up in air, down in air.
The sun evaporates the water, the water falls as rain, the rain fills the
lake from streams, the lake makes a steady river, the river goes
out to the ocean. That's one way.
| If there's a barrier, large waves in the cannal could throw water over
| the barrier producing a current in the cannal. Why would there then be
| a huge flow of electric current when connecting the power plant to
| ground when there's no barrier and the net voltage is zero with an AC
| wave flowing through the wire?
Don't use averages. Your average distance from your bedroom
is zero every day until you are buried or move house, but each day
you leave it, if only to go to the bathroom.
.
- References:
- Prev by Date: Re: Andros' Rephoton (2)
- Next by Date: Re: Which way do you turn the twistie?
- Previous by thread: Re: Trying to understand AC by analogy to water waves.
- Next by thread: Re: Trying to understand AC by analogy to water waves.
- Index(es):
Relevant Pages
|
