Does current flow through a capacitor?

[IanBland]

Persons of a delicate disposition may feel free to replace references to "condom" with "child's balloon".

As to electrons, the question of what they do is dependent on the problem that, when not observed, they do not exist; ergo what is actually going on inside a capacitor depends on whether you've got the back off the unit and are looking at it, or not.

[Tyso_Bl]

Try keeping in mind that current can only flow into or out of a capacitor, if you do that quickly enough, say by connecting it in series with a load to an ac supply, it can look like a current flows through it.

If you can find a Fourth edition of Scroggies Foundations of Wireless, it to my mind has some of the best explanations of the hows and whys of the things we concern ourselves with here.

[G8HQP Dave]

Electrons still exist when not observed - conservation of charge (and other quantum numbers) requires this. It's just that we don't know exactly where they are and what they are doing.

It can be dangerous to base understanding on textbooks aimed at complete beginners or aimed at people who need only a working knowledge of a subject. In some cases the teacher has oversimplified to aid initial understanding; in other cases the teacher himself learnt what he knows from such a book so he doesn't really understand either.

[IanBland]

Not according to the Copenhagen Interpretation they don't. Bear in mind that you can only check with charge or another quantum number has been conserved by observing the system.

Plus, it was a joke.

[ms660]

Originally Posted by G8HQP Dave

Electrons still exist when not observed - conservation of charge (and other quantum numbers) requires this. It's just that we don't know exactly where they are and what they are doing.

It can be dangerous to base understanding on textbooks aimed at complete beginners or aimed at people who need only a working knowledge of a subject. In some cases the teacher has oversimplified to aid initial understanding; in other cases the teacher himself learnt what he knows from such a book so he doesn't really understand either.

That last sentence sounds familiar.

It's the three folks principle.

Those that know.

Those that don't know.

Those that don't know that they don't know.

I've been all three at various points in my life.

Lawrence.

[dseymo1]

Well we're into proper philosophy now.
Can anything not directly observable be said to 'really' exist?
I thought that the reason for quarks' having frivolous-sounding names was as a reminder that they don't 'really' exist. Electrons don't sound so silly (or perhaps we're too used to them), but are they any different?
I don't know!

[IanBland]

I will just repeat that it was an attempt at humour, hence "the back off the unit and looking at it".

But also that according to the Copenhagen Interpretation, you cannot talk of the existence of entities between observations. There is only the wave function.

[Tyso_Bl]

Originally Posted by IanBland

But also that according to the Copenhagen Interpretation, you cannot talk of the existence of entities between observations. There is only the wave function.

Oh not that again, can someone please remind me what counts as an observation?

Can one electron observe another? What exactly do we mean by observe? Does the wave function uncollapse when an observation ceases? Why not if not, isn't time symmetrical?

[turretslug]

Putting aside the semantic implications of "through", a real-world consequence is that the capacitor foil sees "ripple current", which can cause actual heating.

[Tyso_Bl]

Originally Posted by turretslug

Putting aside the semantic implications of "through", a real-world consequence is that the capacitor foil sees "ripple current", which can cause actual heating.

What about losses in the dielectric? Wouldn't that be more significant, the foils are likley to have a much larger effective cross section than the connecting leads.

[kalee20]

Actually, current can flow through a capacitor.

A capacitor consists of a couple of plates with an insulating dielectric between. If you push an electron onto one plate, there is a small but nevertheless non-zero probability that it will appear on the other plate, according to quantum theory. That is how tunnel diodes work.

Luckily, this probability is very small indeed, else our capacitors wouldn't be much good.

[PJL]

I don't think that an understanding of quantum physics is required to understand capacitors. A vacuum capacitor still has plates in which the charge is held. The vacuum is simply the insulator. Displacement current also still applies as electrons are driven away from the +ve potential to create a charge on the plate. A vacuum also has permittivity.

[Radio Wrangler]

Physics is always changing. The idea of a physical Aether as a carrier medium for electromagnetic influences has gone, but the new physics is trending towards having a carrier particle for everything. I quite enjoy joking that there is a particle/antiparticle pair, one of which carries smugness and the other, embarrassment. Sort of like little Robert Robinsons and Frank Spencers.

Anyway, though you might need quantum physics to almost understand capacitors. (almost- because I think no-one ever really understands anything. There is always some new theory waiting in the wings, ready to pounce), you certainly don't need it to use them

If you think of what's going on inside one, then doubts creep in and it gets contentious. But if you think of the outside of them, then they certainly seem to carry current.

David

[G8UWM-MildMartin]

Thanks everyone for such a marvellous philosophical discussion.
I must admit that I'd always been happy with the explanation in post 42, with the additional gross simplifications of it acting as a differentiator if in series or an integrator if in parallel in a circuit, then learning about the (more interesting to me) practical differences between the different dielectrics used over the ages.
Martin.

[julie_m]

Originally Posted by Radio Wrangler

If you think of what's going on inside a capacitor, then doubts creep in and it gets contentious. But if you think of the outside of them, then they certainly seem to carry current.

And I think that's the best answer so far.

At the microscopic level, there are all these tiny charged particles whizzing about doing crazy stuff. And there are too many of them; plus you've got Heisenberg's uncertainty principle meaning you can only determine where they are most likely to be, not exactly where they are.

At the macroscopic level, where the weirdy quantum effects tend to cancel one another out, there are discernible rules governing the behaviour of a capacitor. We have mathematics we can apply which describes an apparently counter-intuitive phenomenon -- current flowing through an insulator -- but it delivers the right answer nonetheless.

And to an applied physicist, the most important thing is for the numbers to add up as expected.

[mole42uk]

Originally Posted by ajs_derby

And to an applied physicist, the most important thing is for the numbers to add up as expected.

I thought that was accountants?

[dseymo1]

To bring down the tone further, it's rather like ropelights and such: does the illumination travel from one end of the string to the other, or are the lamps simply flashing in sequence? Is there any difference? Does it matter? ;-)
Or consider a radio system: Announcer - microphone - /'stuff'/ - loudspeaker - ear
Is what the listener hears the original sound? Even if he were to visit the studio, little if any of the announcer's breath would reach his ear, so what's the difference?
Likewise: AC source - cap plate -dielectric - plate - sink
'Same current' or not?
The models can mean whatever you want them to (or at least whatever the consensus).

[julie_m]

Originally Posted by mole42uk

I thought that was accountants?

No, accountants can subtly redefine "expected" to match their figures. Coins don't play by quite the same rules as fundamental particles, and can undergo a sort of quantum tunnelling into the accountant's pocket ...

[kalee20]

Really, to answer the OP's question, you have to agree what you mean by 'current' and 'through'.

If you push a few electrons into one lead of a capacitor in a circuit, you'll get a few come out of the other. Same as a piece of wire, or a resistor.

But whereas for a piece of wire, if you keep doing this in the same direction, eventually you'll see the first electrons emerge from the second lead, in a capacitor you won't - you'll break down the dielectric first. In most circuits the effort needed to shove more and more electrons in the same lead becomes too great (the capacitor has charged to the circuit's voltage). You can reverse the flow of electrons, and it's conceivable that the same electrons you pushed in the first lead will come out of the first lead.

So although a capacitor can be considered to pass a current, a given electron will never pass right through (except for quantum tunnelling through the dielectric as per my earlier post, which I guess is outside the scope of the question anyway).

[ms660]

What I mean by current and flow (as per the original question) is current that flows along a conductor as that's how I've always imagined to be.

As for through, I mean through in the sense of water going through a pipe one end to the other in an unbroken flow being egged on by a force ie: gravity.

Lawrence.