Does current flow through a capacitor?

[Dave Moll]

If by "current flow" one means flow of electrons, then for them to flow through a capacitor, they would need to flow onto one plate, then through the dielectric and on from the other plate. Barring leakage, surely they don't. The flow onto one plate, building up a charge, then discharge back the way they came when the current reverses.

[julie_m]

Then of course, in the case of an alternating current, we have to address the subtle difference between the flow of actual charged particles (it's the same ones that keep coming and going back again) and the consequent transfer of energy (in a simple circuit consisting of an alternator, capacitor and resistor, there is clearly a source and a load).

Energy can be transferred from one plate of a capacitor to the other, via the electric field, without any actual particles crossing the dielectric. And the actual underlying maths works exactly as though a current were flowing through an insulator.

I suppose we gloss over the difference for the sake of convenience, and talk of the equivalent current that, according to mathematics, seems to be flowing through the capacitor as though it were a current actually flowing through the capacitor.

[dseymo1]

Originally Posted by Dave Moll

If by "current flow" one means flow of electrons...

That's just the point I was trying to make earlier: it depends on what you mean by 'current' and 'flow'. In answering that, you also answer the question.

[mhennessy]

https://www.youtube.com/watch?v=ppWBwZS4e7A

[Tyso_Bl]

Q: Does current flow through a capacitor?

A:Yes*


*No

[Oldcodger]

Originally Posted by Station X

When explaining to a beginner how a circuit works I say that:-

1. AC flows through a capacitor.

2. DC is blocked by a capacitor causing a charge to build up on its plates until they are discharged.

Any more complicated explanation can wait until later.

I'd suggest that this is sufficient for most practical purposes, and unless someone is going to a high level of education in electronics, it's sufficient. I'd possibly go a bit further for the newcomer to electronics and say that for most practical purposes ( realising that impedance = AC resistance IS PROPORTIONAL TO FREQUENCY) , a capacitor is short to AC and O/C to DC.

[ms660]

Originally Posted by mhennessy

https://www.youtube.com/watch?v=ppWBwZS4e7A

Ah...Dave's rave...That's the video that prompted me to post the question.

Lawrence.

[Tim]

Yes and no.

[mhennessy]

Originally Posted by pwdrive

Ah...Dave's rave...That's the video that prompted me to post the question.

In which case, could you explain specifically what the sticking point is?

[G8HQP Dave]

It all hinges on whether you regard displacement current to be current. Displacement current has all the external appearance of a current (e.g. it generates a magnetic field just like an electronic current). Note I said "electronic current" because the charge carriers don't have to be electrons. You could have protons (or positrons) going the other way and it would still be an electric current, but not an electronic current. So if we accept that an electric current does not have to involve electrons we can ask what we would call something which has all the external properties of an electric current but does not appear to us to involve an identifiable charge carrier. If this is a current then yes, current flows through a capacitor.

If we insist that this is not a current, then the answer is no. But then we have created a new problem for ourselves, as we would have difficulty in applying Kirchoff's current and voltage laws to AC circuits. How can the currents sum to zero at a circuit node if any leg includes a capacitor, which we have just decided will not let any current through it? How can the voltages around a circuit loop sum to zero, if the loop includes a capacitor - as a component which will not allow current through it must be an open circuit, so we don't have a loop after all?

This is one of those problems which is not a problem for people who know no physics, as they don't see the problem. It is not a problem for people who know sufficient physics, as they know the solution to the problem.

Going back to a point someone made earlier, science does not merely construct models which happen to correctly predict the outcome of experiments (although I admit that even some scientists believe that this is all they are doing). The models must in some sense correspond to reality. Hence there really are particles which we call electrons.

[ms660]

Originally Posted by mhennessy

In which case, could you explain specifically what the sticking point is?

The..."*"

Although I like Dave's blogs I was unable to arrive at a conclusion as to what I was asking myself, hence posting the question on the forum.

For what it's worth, in the search for an answer, one bloke on another forum somewhere asked the same question, a reply came "Can a man pass through a wall"

Lawrence

[Diabolical Artificer]

It depends....Your question is a bit like " What's the answer to life, the universe and everything" Douglas Adams was a wise man.

I was going to post the link to the EEVblog. When I watched it I was no wiser from a physics point of view. However, it's easy to over think some aspects of electronics. I find it it better to stick to what a component does in a practical application. I have trouble getting my head everyday electronics without worrying about quantum physics - it's beyond me anyway.

A good example is current flow. Does current flow from negative to positive or the opposite? It's academic if your just fixing the odd radio or amplifier.

Sorry didn't want to belittle your question Lawrence, your heading into deep waters and a braver man than I.

Andy.

[ms660]

No worries Andy, those that know me know I have a thick skin both metaphorically and physically (at least on my hands anyways)

I have read various answers to the question here and there, the bit that sticks in my mind is that the dielectric is an insulator under normal conditions, that means in my mind current can't flow through it in the normal way that I understand and what I was taught.

I agree with what some other members have said about explaining capacitors to say a newcomer if they asked, eg: a capacitor blocks a steady DC and passes AC, that's how I would explain it, on the other hand that's not the answer to the question I posted, maybe I should accept "42" and leave it at that

Lawrence.

[mhennessy]

Originally Posted by pwdrive

...the bit that sticks in my mind is that the dielectric is an insulator under normal conditions, that means in my mind current can't flow through it in the normal way that I understand and what I was taught.

But air is an insulator under normal conditions. So how do you feel about valves?

[ms660]

Not very good with air in them

I sort of get the thermionic emission stuff.

For the moment I'll stick with what's in the navymars link I posted.

It's one of those questions I guess, interesting though.

Lawrence.

[richrussell]

If you only consider dc, then the answer is that no, conventional current does not pass through a capacitor. However once you start to consider ac, capacitors, resistors, inductors and even lengths of wire behave differently to that as the frequency increases.

It can be quite a jump once you thought you were quite good at understanding voltage, current and resistance at say A-level, but then once you get to looking at the maths once an alternating current is present at say degree level, you suddenly realise that everything you thought you knew was a massive simplification.

But with some of the nice analogies around (I like the condom in the pipe one), you can get your head around it without being bogged down with the complex numbers and calculus.

[mhennessy]

It's no more weird a concept than current flow caused by thermionic emission. Yet in general people seem much happier with the latter...

As I said earlier, could you be more specific about the exact thing that is causing you a problem? Please don't take this the wrong way, but your responses so far gave been vague. For example, it's not clear if you are happy or unhappy with the concept of displacement current? Maybe the confusion arrives before you get that far - I honestly can't tell.

As it's lunchtime, I've just taken the time to re-watch Dave's video. I think it's a good "hand-waving" explanation that is pretty good for its intended aim and the intended time-frame, and it arrives at a very pragmatic conclusion. Perhaps give us the timecode of the bits you're not happy with and we can take it from there?

[G8HQP Dave]

Originally Posted by pwdrive

I have read various answers to the question here and there, the bit that sticks in my mind is that the dielectric is an insulator under normal conditions, that means in my mind current can't flow through it in the normal way that I understand and what I was taught.

Insulators contain electrons. These electrons can wiggle from side to side, but not move away significantly from where they are (except when the insulator breaks down). Electron wiggling is all that is needed for an AC current, as even in a conductor that is all the electrons do. So no mystery about current flowing through a capacitor with a dielectric.

The real mystery (which Maxwell himself was puzzled about) is what is it that wiggles in a vacuum? What about a vacuum capacitor? Then you have to have displacement current, although that is a misleading name as there is nothing to displace. The name arose when people thought there was something to displace: the aether. Now we know there isn't anything, but it still behaves as though it is.

[mole42uk]

Originally Posted by Radio Wrangler

.....electrons are artificially-created models designed to fit observed phenomena...

[ms660]

Mark, no worries, I'm not taking anything the wrong way, as a bloke that fell down a ladder and then blacked out after igniting a fused dynamite charge and survived the episode I would say I'm fairly resilient to date, anyways less of that and more on the capacitor and current:

Since posting the original post I've been doing the usual Googling, some book texts and training texts I found state that current does not flow through a capacitor eg: in one end, through the dielectric and out the other, to that end I can understand (within my limits) the displacement stuff and the current shuffling from one capacitor plate to the other via the circuit/source or whatever we want to call it, the same texts go on to explain the reasoning behind the voltage/current phase shift of 90 degrees (perfect capacitor) so having read all that it has now become clearer to me at least why there is a phase shift between voltage and current, I've known since about 1964 that there was a phase shift of 90 degrees and that has stuck with me ever since, both when I was in the trade and since I retired from it but I never actually questioned why and how that phase shift arises.

I guess the key word in the original question is "through"

Lawrence.