EEVblog #486 - Does Current Flow Through A Capacitor?

[EEVblog]

I just want Dave's busted Ampere analogue meter shirt! That is far-riggin cool!

It's not my design, but my mate Rogers. He took the actual photo in an old power station.
If there is sufficient interest I could ask if he wants to run a Teespring crowd funded campaign?

[wbeaty]

Here's a way to measure displacement current.   Use a really thick dielectric.  But make it narrow.

Get yourself a rod of PZT ferroelectric ceramic insulator.  That's the material used in ceramic capacitors. 10cm long by 1cm wide would be good.   Plate the ends with metal, and hook up some wires.  You've built a capacitor.

Now connect this capacitor to a high-volt signal generator, and send a few KHz at a few mA through the device.

Next, connect a clamp-on ammeter probe around the rod.  Take a measurement.  You'll see a few mA.

Finally, slide the clamp-on probe all around the circuit.  The milliamps will be the same in the wire, in the insulating rod, and in the metal plating.   Well duh, current is constant in all parts of any basic circuit loop.  That includes the dielectric of capacitors.

Some may point out that the insulating rod is full of mobile electrons, and these electrons are wiggling.  Thus, a current exists.   Sure, but look at the original concept:  DOES CURRENT FLOW THROUGH CAPACITORS?   Is this rod thingy not a capacitor?  Of course it is.   Is there zero current?  Nope.  So, yes, current flows through capacitors.  And the rod device shows that we can even use apparent insulators as a kind of AC conductor, see: http://amasci.com/elect/mcoils.html.

OK, what about VACUUM-DIELECTRIC CAPACITORS?      (I bet there are some people who actually believe that currents do flow in ceramic capacitors and electrolytics, but not in vacuum capacitors.)

[c4757p]

I bet there are some people who actually believe

Doesn't matter what comes next, you'll always win that bet.

[wbeaty]

Doesn't matter what comes next, you'll always win that bet.

Heh.  The real question is, are they students, or do they see themselves as students?   If not, then very probably their ignorance is willfully maintained, and they'll never learn anything new.   We should all try to be like Einstein and RP Feynman, both of whom saw themselves as students, and tended to look down on "experts."


Also...



Does any water flow through the tank above?  Trick question!  Yes, there is a current, because whenever we force any water into one pipe, an exactly equal volume will come out of the second pipe.   Also, NO, no water flows through, because part of the current is made out of flexing rubber!

And that gives a clue to this whole mess.  It easily becomes a pedantic fight over mis-heard terminology.   Do charges flow through capacitors?   Not necessarily (but see the PZT rod above.)   Yet that's a very different question than this one:  is there a current in the space between the capacitor plates?  Yes, there is.  The current in the dielectric is always the same as the current in the capacitor leads.  A clamp-on ammeter could even detect it.  But the amperes in the dielectric need not be made out of flowing charged particles.

Pedantic Nitpicking is required!  "The ill and unfit choice of words wonderfully obstructs the understanding." - Francis Bacon

"Many errors, of a truth, consist merely in the application of the wrong names of things." - Spinoza

"Lest you think that I am quibbling over minor points of language, I note that in my experience many of the misconceptions people harbor have their origins in imprecise language... Precise language is needed in science, not to please pedants but to avoid absorbing nonsense that will take years, if ever, to purge from our minds."- Dr. Craig F. Bohren

[John Coloccia]

I wonder why everyone keeps treating "displacement current" as a real current?  It was misnamed by Maxwell because Maxwell was completely wrong about it's source.  It refers to nothing but a changing electric field giving rise to a magnetic field.  He imagined it a wiggling around of the aether, I suppose.

But let's talk about current flowing through the dielectric.  What's the resistance of the dielectric?  What happens to the all the Watts you should be dissipating?  Now even more laws fall apart, including Ohm's law.

And yes, water flows in the tank.  You can sit there and count the number of water molecules that scoot by a given point.  If you try to count the number of charges that scoot by a given point in the dielectric of a capacitor, hopefully you'll count 0 (and you'll actual count the leakage current).  So the tank analogy is a good one for the basic concept, but it breaks down when you look too closely.

Anyhow, I don't want to ruffle any feathers.  It think this is a fun discussion.

[IanB]

How big should it is?

In order to derail the thread even further, let's take a minor detour into the subject of language.

The correct phrasing above is, "How big should it be?"

Given the conditional nature of the question we should use the subjunctive mood (il congiuntivo in Italian, as I just learned).

In English, the present subjunctive of the verb "be" is also "be". As famously expressed in the song, Que Sera Sera:

When I was just a little girl
I asked my mother, what will I be
Will I be pretty, will I be rich?
...

However, you will find, as a student of English, that the subjunctive tense is an endangered species. English is being dumbed down, and "proper English" is heard less and less in England these days.

References:

[1] http://italian.about.com/od/verbs/a/italian-verbs-present-subjunctive-tense.htm
[2] http://en.wikipedia.org/wiki/Subjunctive_mood

[c4757p]

And yes, water flows in the tank.  You can sit there and count the number of water molecules that scoot by a given point.  If you try to count the number of charges that scoot by a given point in the dielectric of a capacitor, hopefully you'll count 0 (and you'll actual count the leakage current).  So the tank analogy is a good one for the basic concept, but it breaks down when you look too closely.

The rubber plate in the tank is the dielectric, and if you count water molecules scooting through the rubber... that ain't rubber.

I wonder why everyone keeps treating "displacement current" as a real current?  It was misnamed by Maxwell because Maxwell was completely wrong about it's source.  It refers to nothing but a changing electric field giving rise to a magnetic field.  He imagined it a wiggling around of the aether, I suppose.

1) Newton was wrong about many things as well but we still use his equations and terms when they are sufficiently good approximations. I don't need relativity to figure out how far something will go if I throw it.
2) That's exactly what we're saying displacement current is. We've established already that we are drawing a distinction between current and electric current, with the latter one type of the former. To paraphrase the bastard on YouTube who started all of this, you're saying I don't have fruit in my basket because you don't consider my tomatoes to be sufficiently fruity to be called fruit, when for the purposes of basket-measuring convenience, I've defined them as such.

[duskglow]

I've been following this topic for a while and staying out of it, because I don't have a whole lot to add mathematically.  It's been very educational to me, though, in multiple ways.

I've learned that there is a such thing as displacement current.  I learned how capacitors work in a better way than I did previously.  I even learned how inductors work in a better way than I did previously.

And I learned that in every discipline, there's always *someone* who insists on splitting hairs in such a way that newbies will get hopelessly confused if they listen to that person.

Here's the deal, from my perspective.  If you want to take electric current and displacement current, add them together, call it current, and suddenly all sorts of equations work and things light up and buzz and doesn't release the magic smoke, then trying to split hairs about it from an electronics perspective really isn't helpful.  It may be, in some obscure and very deep field of study, technically correct, but it's utterly useless in the context of getting things to beep, buzz, and release the magic smoke.

I know as a Linux guy, there are a few "traps for young players" too.  For example, if someone were to ask me what "tar" does and I were to go into a huge dissertation about tape drives and streaming data and all that...  yes, that's what tar originally did (tape archive) but all a hobbyist really wants to know is, how to I turn this .tar file into a directory I can do stuff with?  "tar xvf" and move on.  Read the man page if you care.  But there is absolutely nothing essential about the history of the tape drive and tape archiving, etc., and all it does is confuse the issue. 

And there is absolutely nothing essential in this case about knowing where the individual electrons are going.  The important thing is "how do I make the circuit with this capacitor in it do what I want?"  And the answer is...  you think of electrons as flowing through it.

Please, for the sake of beginners and newbies and people like me who have been at it for a while and are only just now learning some of the deeper stuff - don't confuse the issue.  It doesn't help.

*gets off my soapbox* 

[IanB]

I think this thread has exhausted the subject of electricity, and definitely should be moving on to the subject of language:

[c4757p]

I know as a Linux guy, there are a few "traps for young players" too.  For example, if someone were to ask me what "tar" does and I were to go into a huge dissertation about tape drives and streaming data and all that...  yes, that's what tar originally did (tape archive) but all a hobbyist really wants to know is, how to I turn this .tar file into a directory I can do stuff with?  "tar xvf" and move on.  Read the man page if you care.  But there is absolutely nothing essential about the history of the tape drive and tape archiving, etc., and all it does is confuse the issue. 

FWIW, that wouldn't answer the question anyway, tar doesn't really do tapes. (OK, it has a few tiny little "helper" functions for them, like tape-swapping, but most of it is medium-independent, taking advantage of the Unix device file concept.) The correct answer is "concatenates files, retaining metadata, and separates these concatenated streams back into individual files".

And using that word - "concatenate" - explains why you can easily append files to an existing archive, but removing a file or extracting one from in the middle of a thousand others takes ages.

Also - screw tar. What a vile pain in the ass. I only use it because it's the only format that I always remember (without a Google search) supports all the Unix file attributes.

You know, this forum would stay on topic about 75% more if I left.

[duskglow]

 

[IanB]

What's Unix? 

[c4757p]

Wait, did I brainfart?

What's Unix? 

Castrated men.

[duskglow]

Wait, did I brainfart?

No, just took the one paragraph I wrote that could have been left out without changing the meaning at all, and gave a really great example of what I was asking people not to do wrt the displacement current argument.    

[c4757p]

I suppose so. My point was that it wasn't really a close analogy - going into a discussion about tape backup when describing tar is more analogous to going into a discussion about pre-electron theories of electricity. It's even more irrelevant than arguing about current types, because the distinction between currents is the underlying behavior, but the niche applications of tar are more historical facts.

And I thought my description involving concatenation was pretty short and not confusing.

But you're probably right.

gave a really great example of what I was asking people not to do

I'm spectacularly good at not doing what people ask me to do. That's why I derail so many threads. I'm not sure if it's belligerence, stupidity, forgetfulness, or a mixture of both.

[ejeffrey]

And there is absolutely nothing essential in this case about knowing where the individual electrons are going.  The important thing is "how do I make the circuit with this capacitor in it do what I want?"  And the answer is...  you think of electrons as flowing through it.

Yes.  My point is that 'current flows through the capacitor' is not just right at a practical convenience, but also at a fundamental level.  The people who try to split hairs are not just unhelpful, but also fundamentally wrong.

[duskglow]

I agree it wasn't the best analogy.  I just pulled something out of my rear in an attempt to tie it to something from *my* field of expertise.  I wasn't trying to do an exact one for one.  I was just trying to make the point that this discussion, while I'm sure very interesting and esoteric and I'd love to learn more about it, just isn't very helpful when it comes to the topic at hand, which is making things beep and buzz.

Sorry if I came off a bit harsh there, I have a killer headache.

[duskglow]

Yes.  My point is that 'current flows through the capacitor' is not just right at a practical convenience, but also at a fundamental level.  The people who try to split hairs are not just unhelpful, but also fundamentally wrong.

But even if they were right, that wouldn't matter, I guess is my point; for the practical purpose of getting things to beep and buzz, it's not helping.

[c4757p]

Sorry if I came off a bit harsh there, I have a killer headache.

Ah, it's fine. I think I came off a bit harsh too - I wasn't feeling eloquent enough to make it sound polite, and I wasn't feeling intelligent enough to just decide not to say it 

The more of my posts you see, the more you'll start to realize that I generally fail to understand that people can't hear the tone of voice I hear in my head... 

[duskglow]

Ah, it's fine. I think I came off a bit harsh too - I wasn't feeling eloquent enough to make it sound polite, and I wasn't feeling intelligent enough to just decide not to say it 

And I just took another ibuprofen, so my crankiness should ebb at some point.

I probably shouldn't have spoken up, I'm not adding a whole lot.  Things like this kind of rub me wrong, though.  I get confused enough.

[c4757p]

You're doing better than I would. When I have a massive headache, the only things I understand are "aspirin", "air conditioning", "shut the hell up" and "piss off".

[IanB]

And there is absolutely nothing essential in this case about knowing where the individual electrons are going.  The important thing is "how do I make the circuit with this capacitor in it do what I want?"  And the answer is...  you think of electrons as flowing through it.

No, no, no, no. You think of current flowing through it. If you think of electrons when doing basic circuit design, then you have sprung a leak in your abstractions and your analysis has failed. Electrons do not exist in basic electronic circuit theory.

[duskglow]

I was going to reply, but then I realized I'd just be continuing the argument, so I used the wrong term.   It didn't take away from the point I was trying to make, though.

[IanB]

I was going to reply, but then I realized I'd just be continuing the argument, so I used the wrong term.   It didn't take away from the point I was trying to make, though.

I agree with the point you were making, but the whole existence of the word electron in the dictionary does major harm to the subject of circuit analysis. If you are not studying physics, electrons do not exist.

[croberts]

It's time to bring Larry Fine of The Three Stooges into this.

Teacher to Larry Fine:
If I give you a dollar and your father gives you a dollar how many dollars will you have?

Larry Fine To Teacher:
One dollar.

Teacher to Larry Fine:
You don't know your arithmetic.

Larry Fine to Teacher:
You don't know my father.