Capacitors, how do they work, or why?

It must do, current in is the same as current out, it has only two wires and no other place to go. This neglects stray capacitance which will be in the order of 10^-12 of the capacitor proper i.e. ****** all.

Permanent magnets have a frequency of 0Hz.

Unless you wiggle them.

[jamesinnewcastl]

Hi

Admittedly I haven't read all the replies but the thread wobbled a bit!. I have the following thoughts for what they are worth...

Capcitance is the ability to create charge on conductors that are in the same universe as the electric field between them causes the electrons to gather on one side and I guess a deficit appears on the other (because that's where the electrons were). The electric field is caused by a potential difference.

In the act of 'charging' the electrons rushing to occupy the conductor constitute a current - but nothing jumps between the conductors. The act of leaving the other conductor is also a current. Thus the current passing 'through' a capacitor is actually electrons shuffling back and forth on their own side of the infinite resistance between the conductors.

If something is put between the conductors and more electrons gather on the conductor it must be because that something is increasing the electric field strength - probably by the act of its own electron structure lining up. Thus different materials have different internal make-ups and so one is better than another. Consider why a 'dielectric' is called 'di-electric' ie having a +ve on one side and a -ve on the other and so matching up with the charge on the plates.

No reason why the dielectric necessarily needs to make a positive contribution - perhaps some materials will weaken the field strength?

The capacitance is also controlled by how the electric field can be strengthened or weakend - so make the conductors into plates and bring them closer together and whoa!!!! Nothing special about a vacuum in any case.

Is there any mystery at all? Well apart from how the heck an electric field works!


James

[Argus25]

While we are on the topic of capacitors, I have found it interesting the notion that capacitors can act as substitutes for batteries. The idea came about after the invention of very high Farad value "super capacitors"

But here is an interesting quirk:

If a capacitor is charged via a constant voltage source or power supply V (which always has some internal resistance) the final energy stored in the capacitor is VQ/2 where V is the voltage and Q is the charge. Since Q=CV this is the same as CV^2/2.

However, the catch is that the work done by the charging power supply is VQ as the charge Q has moved across the supply potential V.

This means that half the total energy taken from the supply, in the charging process, is wasted as heat generated in the series resistance in the charging circuit and only half of the energy stored in the capacitor.

This doesn't happen charging a battery where the energy is stored by an electro-chemical process because the battery has a relatively uniform terminal voltage throughout the charging process, unlike the capacitor where its terminal voltage is directly proportional to its charge. In other words a battery can absorb charge without its terminal voltage elevating significantly, a capacitor cannot do this.

One way to help the situation of this massive energy loss charging a capacitor is to do it with a supply that tracks the capacitor's terminal voltage. In essence it needs to be a current source, not a voltage source. But this is all food for thought for those who think capacitors can replace batteries. Also of course, as a capacitor discharges, its terminal voltage is much less stable than a battery.

[emeritus]

I seem to remember that there was an extensive correspondence and a couple of articles in the pages of "Wireless World" in the late 1970's about "displacement current" and its relevance to charging capacitors, but I don't have the back numbers.

I remember my school maths teacher observing that, when people come across something difficult, they sometimes give it a name, and then people think they know what it is. That was in connection with irrational numbers such as Pi, but the principle seems to hold true in other fields. I agree with other posters about the point that many things are really only mathematical models that work and do not necessarily represent what is actually happening.

I remember when my son was young, he asked how radio waves travel through space, and I told him that I didn't think anyone actually knew.

When you go back to basics, Maxwell devised his famous equations that allow you to determine mathematically what happens when you energise a conductor of given length and orientation with an alternating electric current, and to calculate what voltage will be induced in another conductor of a given orientation and length at a given distance from it. This was the sort of thing we had to do when I was a student, and although I have long forgotten how to do it, I understand that Maxwell's equations do work.

But what is electricity? One way of looking at the equations is that a current flowing in a wire, generates an electric field around the wire. Another way of looking at the equations is that an electric field can be launched in the space surrounding a wire and will cause an electric current to flow in the wire. There is an obscure type of transmission line (the "Surface Wave Transmission Line") invented by a Mr Goubau ( I think) that is described in the 4th edition of the "STC Reference Data for Radio Engineers" where a single dielectric-coated conductor does act as a sort of inside-out waveguide that makes use of the guiding property to support a non-radiating wave. The dielectric layer is essential for the wave to be non-radiating.

My understanding is that Maxwell's equations rely on the presence of a medium to support wave motion, and that it how the concept of the "Aether" came about, a mysterious fluid that pervades everywhere, including vacuum, and allows electro-magnetic radiation to traverse empty space devoid of matter. Physicists have devised experiments which proved to their satisfaction that the Aether does not exist, but I have not seen any explanation as to why Maxwell's equations, which rely on the existence of an all-pervading medium to support wave motion, work. Likewise, what is light? The wave theory and the corpuscular theory are chalk and cheese, but some phenomena are explained by one, and other phenomena by the other. You can use the appropriate mathematical model to give accurate results for the situation you are interested in, but as to what light actually is, is anther matter.

[G4_Pete]

https://www.youtube.com/watch?v=gPXv063O5B8

This clip demonstrates an interesting phenomenon of capacitance with some insight into observable characteristics.

As stated we are often just putting maths round observable phenomenon Q=CV for example dictated how the microphones I used to make for Bat ultrasound worked. But I really did not care any more than wiggling the capacitance by impingement of a sound pressure wave wiggled the voltage providing that Q was held constant. Held constant by high impedance charge.

One thing I find fascinating is watching my van der graph generator in action charge going from the inside of the dome to the outside and streaming away from any sharp edges.


Pete

[Radio Wrangler]

Originally Posted by astral highway

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

Those are RELATIVE dielectric constants.... relative to that of a vacuum. So 1.000000000000.... is no surprise for a vacuum relative to a vacuum!

David

[JacKam_]

After an exam of quantum mechanics my collegue came out of a professor office and said , "Got A*" !
I asked: "How did you do that ?, how did you learned ?" And he responded something like "I read about Buddha, there is no knowledge, but experience".

At this time he was 2 dan Taekwondo.

[G4_Pete]

Reading my last post I may have been a bit flippant but if you read the complete life achievements of Paul Dirac and have performed the standard university physics experiments on electron charge mass ratio, Einstein’s photo electric effect and light partial/wave duality as I have done it is difficult to hold lasting credence to quantum theories in the knowledge that they get overrun by new theories. However taking this in the context of the famous historical quote “standing on the shoulders of Giants” insight is gained step by step albeit that occasionally some are looking in the wrong direction.

So with regards to capacitors and charge, maths formulas describe the phenomena but as to the ultimate quantum mechanics explanation perhaps the answer is indeed 42.

Pete

[MichaelR]

Thinking more about the concept of energy is it not true that energy cannot be destroyed or for that matter created it just "transforms" from one type to another ........ what is energy

[julie_m]

Originally Posted by joebog1

WHAT is the frequency of gravity? OR permanent magnets

It's the same as the equivalent number of ounces in a metre ....

[G8HQP Dave]

Originally Posted by emeritus

I remember my school maths teacher observing that, when people come across something difficult, they sometimes give it a name, and then people think they know what it is. That was in connection with irrational numbers such as Pi, but the principle seems to hold true in other fields.

We do know what Pi is: the ratio between diameter and circumference of a circle, or other ratios coming from circles or spheres. We have to call it Pi because we cannot write down its value as a rational number, not because we don't know what it is. We can write down its value to as many decimal places as you want, but we don't have enough paper to write down the infinite number of digits we would need for completion.

One way of looking at the equations is that a current flowing in a wire, generates an electric field around the wire.

Magnetic field. The electric field comes from the charge/voltage on the wire, not the current.

My understanding is that Maxwell's equations rely on the presence of a medium to support wave motion

No. For quite a while Maxwell thought this and used this idea to help him develop his equations, but it turns out that he was wrong. There is no medium, and the equations do not demand that there should be a medium.

Originally Posted by MichaelR

what is energy

Energy is the potential for doing work e.g. moving stuff around.

[MichaelR]

Ok the potential for doing work..... moving stuff around ...what is energy?

[Oldcodger]

Originally Posted by 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?

Likewise for time and interest in the inner workings. I remember some mumbo jumbo at college on charge transfer etc, but to me the basics suffice- it should block DC and pass ( to a certain extent, depending on value) AC. It can AC couple stages or decouple resistors.
To me - it's the same as the argument on RC time constant circuits. Are we on the Engineering or Scientific side of the fence.
e.g For this, an Engineer will regard the TC as R*C, whereas a Scientist will use a complicated formula to calculate this to many decimal points.

[julie_m]

Well, if you want to get really abstract and philosophical about things, you could point out that we don't really know for sure that the universe is made out of particles. We have conceived a model of a universe made out of particles that should behave in a certain way, and that happens to be very close to what we observe in reality; but that isn't to say that there isn't another plausible model of the universe, totally incompatible with any of the ideas we currently use, that would equally explain it. We just haven't found either it, or a good reason why this model should be the only one that works. (How can x ** 2 be equal to 25, if x is not equal to 5? It was only by asking questions at the edges that we found out about something as trivial as negative numbers .....)

By convention, we go along with the scientific consensus; because having one explanation that Just Works nearly all the time as opposed to a whole bunch of fragmentated theories that only apply to certain domains of the universe is like being able to play one song perfectly from start to finish as opposed to being able to play several different but disconnected intros, riffs, solo parts and codas.

[G8HQP Dave]

Originally Posted by Oldcodger

To me - it's the same as the argument on RC time constant circuits. Are we on the Engineering or Scientific side of the fence.
e.g For this, an Engineer will regard the TC as R*C, whereas a Scientist will use a complicated formula to calculate this to many decimal points.

As someone who has worn both hats I ought to be confused, but both the scientist in me and the engineer in me agree that RC is the time constant. Where the two might differ is in deciding exactly what is R and what is C in any given situation. The scientist might need more precision so may need to be more fussy.

'Energy' is one of those things which a physicist develops a somewhat intuitive understanding of. To say what it 'is' means explaining it in terms of more fundamental concepts, but energy is a fundamental concept so is what it is. Physics is full of fundamental conserved quantities, but the general public has not heard of most of the others so does not feel the need to ask what they 'are'.

[julie_m]

A mathematician will calculate everything down to the last decimal place.

An engineer will calculate to one more decimal place than their least precise measuring instrument. (So will a physicist, but they will also quantify the tolerance.)

As for energy, it's like chips around a poker table: freely exchanged between the players, but the total amount never changes during the whole game unless you introduce or remove players.

[G0HZU_JMR]

but to me the basics suffice

Yes, I think the most valuable knowledge comes from experience of working with the various types of capacitor in the real world. eg ceramic, tantalum, electrolytic etc etc. For example, gaining experience of what can go wrong with each type if not used appropriately.

I think the (RF) properties of real world inductors are much more interesting than real world capacitors but maybe that's for another thread

[Tyso_Bl]

This is more complicated than I thought, for the time being can we stick to things related directly to capacitors and forget about gravity etc.

From what I read it appears that "charge" is stored in the dielectric, not the plates.

When I mentioned vacuum, what I meant was the absence of air or other elements or compounds.

How is the dielectric affected by the pumping round of electrons? If a vacuum, what properties does a vacuum share with gases or solids that give rise to capacitance? Are we to take it that a vacuum or empty space isn't really empty? (I do recall reading about quantum foam at the edge of black holes giving rise to Hawking radiation), would quantum foam have something to do with being a dielectric?

Thinking about the connection between electric and magnetic fields, if they are static and not changing in intensity do they exist independently of each other?

What about "edge effects" in the calculation of capacitance, why are they usually ignored, and is there any connection with how capacitance arises?

I'm almost begining to think I shouldn't have asked, too late now though!

T

[Radio Wrangler]

Actually, the charges ARE stored on the plates of the capacitor.

David