basic capacitor question?

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
new guy
Posts: 126
Joined: Thu Oct 30, 2003 1:01 am
Location: philadelphia
Contact:

basic capacitor question?

Post by new guy » Fri Aug 25, 2006 5:10 pm

People say "DC" electricity is blocked and "AC" electricity passes through a capacitor. BUT since the plates inside a CAP. are seperated by insulators how can electrons pushed by "AC" voltage travel from one plate to the next (in one side and out the other) Are they actually "PASSING THROUGH" or does it just look like that? What is actually happening with a capacitor in an "AC" circuit??

User avatar
Externet
Posts: 1869
Joined: Fri Feb 01, 2002 1:01 am
Location: Mideast USA
Contact:

Post by Externet » Fri Aug 25, 2006 5:41 pm

Hi.
Insert an elastic diaphragm in a water pipe.
If you force water into the pipe, the diaphagm will deform a given amount, acording to the water pressure (voltage).
Some water will come out at the other end (current), due to the deformation of the diapragm even when there is NO flow path trough it.
That is for direct current DC, "charging" or deforming the capacitor dielectric.

If you repeatedly push and pull water alternately in an out with a piston, the diaphragm will deform forth and back alternatively, water will MOVE inside the pipe, but still no flow path across the diaphragm.
That is the alternating current AC.

Miguel :grin:

dyarker
Posts: 1753
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

Post by dyarker » Fri Aug 25, 2006 7:57 pm

Good one! I've never heard the water analogy to electricity extended to AC and capacitors before. "diaphagm", perfect. It would have taken me 3 times as many words to explain it.
Dale Y

User avatar
Externet
Posts: 1869
Joined: Fri Feb 01, 2002 1:01 am
Location: Mideast USA
Contact:

Post by Externet » Fri Aug 25, 2006 8:44 pm

Thanks. A rare lucid moment... I wish someone had explained me things this way when I was a kid... took me too many years of experience to put it in simple words.

And many, many other clear analogies I wish could be created for oscure concepts misleading the first learning stages.

Miguel

dyarker
Posts: 1753
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

Post by dyarker » Fri Aug 25, 2006 9:58 pm

Just thought of an extension to the analogy.

The stiffness of the diaphagm is kinda' like the reactance/impedance of a capacitor. A stiffer diaphagm is like smaller capacitor. Energy is lost flexing the diaphagm.

I think it ends there though. Can't cover phase shift because water is not compressable.

Do you have one for inductors? Every once in a while I work on web page of really basic electronics. If I ever get it done, I will be able to answer questions like newguy's with a link instead of typing the same thing over and over again.

C U L,
Dale Y

User avatar
philba
Posts: 2050
Joined: Tue Nov 30, 2004 1:01 am
Location: Seattle
Contact:

Post by philba » Fri Aug 25, 2006 10:03 pm

and the analogy works with parallel caps but not series.

User avatar
Chris Smith
Posts: 4325
Joined: Tue Dec 04, 2001 1:01 am
Location: Bieber Ca.

Post by Chris Smith » Fri Aug 25, 2006 10:19 pm

The water analogy is the best, bar none.

But your leaving out the fact that there is water on the other side of that diaphragm, [exactly like the electron exists on the other side of the cap] so that as the diaphragm moves, that energy is translated to the water on the other side through the diaphram causing motion, force, movement, and energy transfer.

Because it is "Back and Forth" energy and not actually traveling down a line type of DC force [its AC], so goes the water on the opposite side, push then pull, and its all a small distance or reference of energy type of movement.

All of this action is energy transferred, and yet the water on one side of the diaphragm never comes in contact with the water on the other side of that diaphragm.

But a force is exerted, it moves, and it is transferred and used.

dyarker
Posts: 1753
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

Post by dyarker » Fri Aug 25, 2006 10:22 pm

If it works for parallel, it has to work for serial.

The lower pressure after the first diaphagm pushes the second less, and the pressure is lower still after the second diaphagm; and so on ...

Cheers,
Dale Y

User avatar
Externet
Posts: 1869
Joined: Fri Feb 01, 2002 1:01 am
Location: Mideast USA
Contact:

Post by Externet » Fri Aug 25, 2006 11:05 pm

Inductor... hmmm... I would say inserting in a water pipe; a propeller, spinner, fan, turbine, helix, or something like that behaving with momentum, somewhat hard to start and to stop.

The wider the blades or more massive the helix, the higher the inductance.

A resistor, obviously inserting a wire mesh of varied amount of obstruction for minimal to great resistance to flow.

A diode... a one way valve, spring loaded closed that needs a pressure of 0.2 for a Ge or 0.7 for Si to overcome the spring.

Now someone should come up with a transformer.... Two turbines, linked by a shaft inserted in separate pipes ?

A large diameter wire; a large diameter pipe.

A battery.... a plain water pump

An alternator.... a reciprocating piston pump

Amperimeter... a water flow rate meter

Voltmeter.... a pressure gauge

Ohmeter... a tiny pump with a flow rate meter?

Miguel

cato
Posts: 366
Joined: Sun Sep 07, 2003 1:01 am
Contact:

Post by cato » Sat Aug 26, 2006 4:40 pm

Inductor Water Analogy:

replace a section of the pipe with a soft rubber tube-->increase pressure the tube expands so flow out the far end doesn't increase right away-->decrease the pressure the tube collapses so the flow out the other end doesn't drop off right away...


this is also known as the weeb waaa analogy, for those who remember spanky and the gang.

User avatar
Chris Smith
Posts: 4325
Joined: Tue Dec 04, 2001 1:01 am
Location: Bieber Ca.

Post by Chris Smith » Sat Aug 26, 2006 7:51 pm

The inductor is the Flywheel.

#1 It resists any instant change
#2 It maintains any flow, resisting #1
#3 It is mass like a pipe FULL of water
#4 It is slow to speed up and slow,... to slow down
#5 It has the ability to store kinetic energy because of its mass.

It is mass and momentum.

cato
Posts: 366
Joined: Sun Sep 07, 2003 1:01 am
Contact:

Post by cato » Sun Aug 27, 2006 4:47 am

How does a fly wheel maintain flow? What flow?

What does mass have to do with an inductor?

What does kenetic energy have to do with an inductor?

User avatar
MrAl
Posts: 3862
Joined: Fri Jan 11, 2002 1:01 am
Location: NewJersey
Contact:

Post by MrAl » Sun Aug 27, 2006 5:47 pm

Hi there,

An inductor looks the same as a mass mathematically if you use
the "voltage:force" analogy, where a voltage in the electrical system
equals the force in the mechanical system. You can use either
rotational (flywheel) or the translational (a simple mass) systems.
Translational systems are a little more simpler to work with.

I like to use the "force:current" analogy myself, which makes the
capacitor look like a mass and the inductor look like a spring
in the translational system.
When a force is applied to a mass (current applied to cap) the
mass doesnt move (voltage across cap doesnt change). After a
little time passes the mass moves (voltage changes) and begins to
pick up speed (voltage increases).
So we can see the current looks like a force and the voltage level
looks like distance. The three elements, cap, inductor, resistor,
look like mass, spring, and damper respectively.
If you know how to transform mass into capacitance and spring
constants into inductance you can simulate your entire mechanical
system using an electrical circuit analysis program. You can also
transform the results of the electrical system back into the mech
system so you will know how it behaves with a given input.

A quick look at the inductance and capacitance with resistance:
Say you have a heavy rod welded to a spring welded to another rod
and the two rods rub on something. You apply a force to the first
rod and it doesnt move because of it's mass, but then it starts to move
and it compresses the spring (because the second
rod doesnt move yet). Next the second rod starts to move and then
the force is stopped, which means the first rod eventually stops,
and the mass of the second rod keeps it moving a little and the
energy in the spring keeps it moving too.
The system oscillates a little and damps out to rest eventually, depending
on the 'resistance' (the amount of rubbing action) in the system.
This would be equivalent to two caps, two resistors, and an inductor
in between.
LEDs vs Bulbs, LEDs are winning.

Will
Posts: 310
Joined: Tue Feb 11, 2003 1:01 am
Location: Katy Texas
Contact:

Inductance/Capacitance

Post by Will » Mon Aug 28, 2006 2:42 am

I think a fairly good mechanical analogy is a centrifugal compressor going into surge. If you have a long pipe on ther discharge and the flow gets throttled to the point it is close to 'the hump' on the head/flow characteristic then the compressor will probanbly go into surge. For a period the gas in the pipe keeps moving (Due to its kinetic energy - W = v^2/2g) until, at one end of the pipe, it is compressed to a pressure higher than that of the compressor discharge, when it reaches some peak pressure it stops - the compressed gas is then somewhat like a charged capacitor (Energy stored = PV similar to QV = Cv^2/2 ?) and it then discharges by forcing the gas back through the compressor. It will continue this reciprocating motion, with the compressor trying to jump off its foundation, until something is done to stop it. I have been stood alongside (For about a half a second until I got out of there) a 3,500 hp that went into surge - I would'nt want to do it too often
BB

User avatar
Chris Smith
Posts: 4325
Joined: Tue Dec 04, 2001 1:01 am
Location: Bieber Ca.

Post by Chris Smith » Mon Aug 28, 2006 7:03 am

How does a fly wheel maintain flow? What flow?

What does mass have to do with an inductor?

What does kenetic energy have to do with an inductor?


Cato,

In mechanical engineering, all of the above is true.

A fly wheel is flow with out stopping because of its mass, its weight, its kinetic energy.

You cant stop the flywheel once it is accelerated, hence its purpose.

To smooth out the jerks and bumps and maintain a even flow.

Same as any motor.

The flow is the mass of the electrons flowing and ensuring they will continue with out a hitch.

A inductor aids in the energy increase in any tank circuit, because it is a flywheel of electrons.

A tank circuit shows more voltage out than voltage in,... for the reasons of physics.

Post Reply

Who is online

Users browsing this forum: No registered users and 27 guests