Capacitors, how do they work, or why?

[Tyso_Bl]

Okay then, this has been bugging me for a while, and working on the assumption that there are no stupid questions only answers here goes...

I know what a capacitor is, what it does, how to make one, how to calculate its properties, how to use it and so on.

Now if I take a couple of metal plates, separate them wire em up I get capacitance, move em further apart I get less capacitance, stick a ceramic tile in between I get more capacitance.

Take all the air out between the plate there is still capacitance, if the capacitance is a function of what is between the plates as well as its thickness, why does a vacuum capacitor have capacitance? If the energy stored in a capacitor was a only a matter of how many electrons were pumped from one plate to another why would the distance between them matter?

Am I seeing trees and no forest?

T

[Herald1360]

Hmmmph! In 40 odd years as a practising electronics engineer I've never even thought about how one works. I "know" that the relative permittivity of nothing (aka free space) is 1 and that any other insulator is >1.

Any physicists about?

[Skywave]

When it comes to a "How does a capacitor manage to do what it does?" type of question, I think in terms of energy: nearly all of physics is concerned with energy, one way or another.

I see things this way: you'll notice my classical approach.

The two plates accumulate -ve and +ve charges: unlike charges mutually attract. The greater the area of the plates, the greater the charge; the closer the plates are to each other, the greater the charge. Why should those facts be true? It is because greater plates enable more charge per plate to be accumulated than smaller ones; plates close together 'pull harder' on each other than those spaced further apart. (Think of magnets.) The 'pull' between a pair of plates is a manifestation of stored energy - potential energy in this case - energy that is capable of 'doing work': energy that is transformed when those two plates are connected to a resistance, whereby a current flows and that energy (now 'kinetic energy') gets converted into heat. So a dielectric is not essential, but if one is present, then energy can be stored in that dielectric. This increases the amount of energy that can be stored in a capacitor (whose plates have been determined) but that does not mean that 'no dielectric' = 'no stored charge' for reasons stated above. Different dielectrics have different 'abilities' of just how much charge they can store per unit volume.

Well, that may not be a perfect answer, but, by-and-large, it makes sense to me! (Based on my level of education in Physics to GCE A-level in 1968. Was that really such a long time ago? "Time flies like an arrow; fruit flies like a banana" ).

Al.

[joebog1]

HHMMM OK I will ask a question!

Please tell me the frequency of a permanent magnet!

While you are about it, and I believe they are the same frequency,
what frequency is gravity ?

Joe

[paulsherwin]

There is a good Wikipedia article on the subject: https://en.wikipedia.org/wiki/Capacitance

[Argus25]

Originally Posted by Tyso_Bl

Take all the air out between the plate there is still capacitance, if the capacitance is a function of what is between the plates as well as its thickness, why does a vacuum capacitor have capacitance?
T

The basic answer to this question is that a vacuum or empty space in not empty. It just looks like it. It is an electromagnetic and gravitational medium and electromagnetic waves and gravity waves propagate through it. This property is permittivity for the electric field and permeability for the magnetic field. In fact with just these two constants the speed of light is easily calculated as:

1/Sqrt(permeability x permittivity).

(but gravity is more mysterious)

In a sense physical devices like capacitors or inductors concentrate these properties into a smaller zone of space by locally increasing the permeability and permittivity with "magnetic materials" and "dielectric materials".

If you could alter the properties of space, say by lowering these permeability and permittivity constants, light passing through could speed up, but as far as anyone knows, they are a constant throughout the universe and therefore the speed of light is too. When these values are increased of course electromagnetic waves slow down, like they do in coaxial cables.

The impedance of space for example is about 377 ohms, which has implications in the design of antennas to transmit and receive. One notion is, if there are any Aliens out there, they will have been forced to design their antennas to look just like ours because they are stuck with the same space as us. So if you see a flying saucer with a small parabolic dish on it or a baby Yagi antenna, don't be surprised.

[Radio Wrangler]

The 'impedance of free space' can be deceptive. It is a ratio. To make sense, you need to be able to visualise it.

A propagating electric field wave will induce a magnetic field wave to accompany it. also a propagating magnetic field wave will induce an electric field wave to accompany it.

So whatever started out as a wave of purely one type will mature into a pair. They will stabilise at levels where the transfer of energy from one to the other balances the transfer from the other to the one.

With the permittivity and permeability constants of our universe, this equilibrium has an electric wave of 377 volts-per-metre balancing a magnetic wave of 1 ampere-per-metre.

If we treat this as a simple ratio; 377/1 then the units are (volts/metre)/(amperes/metre)
The 'per metre' bits cancel leaving volts/amps and we know another name for that, Ohms.

So it is usually said that the field ratio of a mature electromagnetic wave pair is 377 Ohms, and it's spoken of as the impedance of free space. Students have forever been wondering where to stick the avo prods to measure this.

By the way, if we multiply the two field strengths together we get (volts/metre)*(amperers/metre) which simplifies to watts/metre-squared thich is the power density of the electromagnetic wave. If you make that a vector multiplication, then it properly accounts for the transverse, at right angles to each other direction of the electric and magnetic components, and that power density points in the direction of propagation.

As Argus says, empty space isn't empty. It has properties. We really don't have a handle on how these propertied do what the do, though we have a lot of knowledge of what they do. The current fashion in physics seems to be to invent and assign 'carrier particles' to every known property and then to try to find evidence to support them. We can't see them directly, just their effects and then try to deduce what the causatory thing might be.

If Physicists start trying to apply their science to psychology, within a femtosecond they will have invented carrier particles for smugness, smarm and fear.

Astronomers have done experiments trying to find if the speed of light and other 'constants' are any different at great distances from us and it all looks uniform. Any differences would have quite huge consequences. There are changes in areas of strong gravitation, but normal service is resumed as you go away from them in any direction, and in those areas the changes fit with the calculated distortion of time and space themselves.

Again, as Argus said, space aliens come to have a bit of fun making tabloid headlines will have antennae designed around the same constants we use. Otherwise they'd find they were subjected to our constants and that could have nasty consequences in terms of other bits of their flying saucer falling apart or exploding unstably. A colleague had a book collecting tabloid headlines. "Alien *** slime killed my begonias" has to take the prize... but is it any weirder than some of the latest physics?

How does a capacitor work? We have ideas, but we're guessing in the dark. We may someday get a breakthrough, but we may never really know.

David

[joebog1]

SSSOOOO !!! at the end of the day, haven’t we just USED the phenomena to suit ourselves, WITHOUT really understanding "space and time" ?.
Paul posted a very interesting "semi thesis" from the wikipedia, WITHOUT any real proof!
After all, ANY theory is believed true UNTIL its proved wrong. Take Einsteins first theory of relativity for instance.

My dad ( a master of math and science AND electricity) always said that we use it, but don’t know what we are using.
Go one further and we have Tesla, AC doesn’t work he was told, THEN charged one gold crown to the professors at the polytechnic to hear him make them look fools, after all they told him poly phase was a fantasy!! This is history.
I have seen some math, and some very good suggestions, BUT
WHAT is the frequency of gravity? OR permanent magnets?

with respect to those who are far better mathematicians than I
Joe

[cmjones01]

Oddly, I accidentally discovered a way of observing the impedance of free space a few weeks ago while playing with my time domain reflectometer. It generates a pulse into any transmission line you connect to it, and allows you to observe any reflections returning with femtosecond precision. A perfect short circuit returns an exact mirror of the orignal pulse, and an open-circuit returns the pulse in the same polarity as the original. By comparing the original and the reflection, the time domain reflectometer gives a direct readout of the impedance at any point on the transmission line.

Just out of curiosity, I connected a pair of wires to it and experimented with twisting them together and pulling them apart to see how the impedance varied. If I pulled the two wires apart into a 'V' shape, so they were diverging out into free space, the readout showed 377 ohms. I leave the analysis of why this should be so to others...

OK, so it's not an Avo, but it's still a measuring instrument!

Chris

[Radio Wrangler]

All antennae exhibit impedance transformation, so it comes down to needing an analysis which shows that your wires, Chris, are 1:1 ratio and broadband.

Flared things tend to exhibit impedance transformation and broadened bandwidth.

377 ohm open wire feeder? Easy! Coax would be a bit extreme. We could sell 377 Ohm feeder to the audio crowd as being 'in-tune with the universe'.

David

[dseymo1]

When asking questions such as this, it's often useful to consider what sort of answer we're hoping for. As has been very ably illustrated above, ultimately, we can only say 'It exploits the nature of the Universe'.
It's usually more helpful to ask what underlying mechanisms are shared with better-understood phenomena, as Argus 25 has done. From this, we can sometimes deduce further links, and synthesise hypotheses about other observations.
It's a case of 'Big fleas have little fleas...", and, to mix a metaphor, turtles all the way down!
Excuse my ramblings...just a thought!

[Radio Wrangler]

We can't give a definitive answer on what matter is.
We don't know what electricity is.

We can't define art for that matter or prove the existance or otherwise of any particular deity.

"We demand rigidly defined areas of doubt and uncertainty!"

Nope, those are rather wooly, too. Even the amalgamated union of sages, luminaries and other professional thinking persons were off the ball.

Pratchett and Adams.... yeah!


David

[JacKam_]

Unfortunately I am not a good English speaker, anyway let's start :

Physics is like observing the Universe, experimenting and describing the results in a language called mathematics.

The Universe likes equilibrium.

What do we know ? :
Electrons like to move in metals.
Electrons do not like electrons crowd, but they like to be together in even numbers 2,4,8 etc..

INTRO :
For some reason (research quantum physics if you like) the electrons feel that there is more space for them somewhere else, but sometimes they cannot travel to that spot easily, as their energy is too low. So they create something called electric field to attract free or more abundant spots and/or to tell another electrons about the opportunity to do equilibrium.

The worst medium for the electric field to propagate is vacuum (so far I guess).


DIELECTRICS ? :
Let's imagine a crowd of nice boys and girls staying on two banks of a river. Then put 40m wide magnifying lenses vertically in a middle of the river. What would happen ?

Answer is : More boys and girls will gather on both banks, even if they do not like their friends on the same bank.
Some of them will decide to swim across, that will make capacitor resistance < infinity.

That's how dielectric works, creating larger capacitance C.


DISTANCE :
The wider is the river , the less of them will be on the banks, as they do not see clearly others on the other side.


Q=C*U :
Guess you can explain that in your next post !


Does it help ?

Thanks,

Jacek

[G8HQP Dave]

Originally Posted by joebog1

After all, ANY theory is believed true UNTIL its proved wrong.

No, most theories are believed to be wrong from the outset. That is because most theories are wrong - especially the theories you see on the web. Other theories are believed to be true because evidence supports them. Sometimes they are later proved wrong; more commonly they are extended. For example, Newton's classical mechanics was not proved wrong by Einstein's special relativity - it was extended. Special relativity was not proved wrong by general relativity - it was extended.

As has been said, a capacitor can be considered to be a device for storing electrical potential energy. Similarly, an inductor stores magnetic kinetic energy. Put the two together and you have a resonator. It turns out that a resonator is almost any arrangement of stuff which allows for the relatively lossless interchange of potential and kinetic energy. The maths of a weight bouncing on a string is near enough the same as an LC tuned circuit.

[Al (astral highway)]

Interestingly, and perhaps counter-intuitively, the dielectric constant for a
vacuum is 1.0000 and for air, 1.0006

So there is an immeasurably small distinction between the capacities of vaccuum and air-dielectric capacitors with the same areas of plate that are holding the charge and same distance between plates.

Other materials have dielectric constants that are 2 to ten times higher than a vacuum.

[JacKam_]

I like that "No" Dave.

I didn't mention any theory. Zen is Zen.

[Herald1360]

Originally Posted by joebog1

HHMMM OK I will ask a question!

Please tell me the frequency of a permanent magnet!

While you are about it, and I believe they are the same frequency,
what frequency is gravity ?

Hmmm. What about 42MHz/Tesla? Ah, that's just NMR frequency for hydrogen in a magnetic field.....

[Neutrino]

Originally Posted by astral highway

Interestingly, and perhaps counter-intuitively, the dielectric constant for a
vacuum is 1.0000 and for air, 1.0006

So there is an immeasurably small distinction between the capacities of vaccuum and air-dielectric capacitors with the same areas of plate that are holding the charge and same distance between plates.

Not quite immeasurably small. If a wireless set was tuned to a short wave station and then put in a vacuum the drop in capacitance (assuming an air spaced tuning capacitor) could be enough to tune to a different station, although this would be a moot (or mute?) point because the loudspeaker would be inaudible in a vacuum.

I wonder if this effect was observed on airborne radio transmitters and receivers.

[ITAM805]

Does current flow through a capacitor - ask Dave

[G8HQP Dave]

Originally Posted by joebog1

WHAT is the frequency of gravity? OR permanent magnets?

Gravity can have any frequency, but we experience mostly DC (from the Earth) with a little 11.57uHz (from the Sun) and 0.41uHz (from the Moon).

Permanent magnets have a frequency of 0Hz.