AC flowing through a cap. What actually happens?

[bryan1]

well guys I was going to stay out of this thread but eh I just can't resist.



Back when I was playing around with F&P motors as wind generators my mate was putting CAPS in parallel with the 3 phase output. I went and put them in series and found when the CAPS weren't connected I was getting around 5 amps output. I connected the caps in series and got 9 amps output. So on my own testing YES current DOES flow thru CAPS.........................

 

[Ratchit]

MrAl,

'...the same current
reaches the surfaces of both capacitors, lets put it that way.'

I think it would have been clearer if he said "the same charge imbalance occurs on all capacitors in a series string".

Mr RB,

Plate charge is irrelevant, as is anything that happens inside an electronic component.

False. What happens inside determines the kind of component it is, and how the component presents itself to the outside.

Current is clearly demonstrated from the OUTSIDE of the component, whenever the component has 2 legs and 1 amp is going in legA and 1 amp comes out legB.

False assumption. The current only appears to pass through the capacitor. As Mr Al and I have said, if the charge passes through the cap, then it does not accumulate or deplete. If it does not accumulate/deplete, then there is no capacitor characteristic. You cannot have it both ways. You seem to have difficulty comprehending that.

I thank you and the professor for proving my case.

The prof disproved your assumption. He specifically said that current does not pass through a capacitor. The electromagnetic fields pass through, but not current. You are confusing a changing charge imbalance with constant charge flow.

Ratch

 

[Ratchit]

bryan1,

Back when I was playing around with F&P motors as wind generators my mate was putting CAPS in parallel with the 3 phase output. I went and put them in series and found when the CAPS weren't connected I was getting around 5 amps output. I connected the caps in series and got 9 amps output. So on my own testing YES current DOES flow thru CAPS.........................

I don't think any of us know from your description what you did, and how you come to your conclusion. But why don't you tell us how current can pass through a capacitor without the capacitor not appearing electrically like a plain resistor. I believe technicians throw capacitors away when they start to emulate a resistor, don't they?

Ratch

 

[ljcox]

MrAl,

You did a good job of explaining why current must accumulate/deplete on the cap plates instead of going through the capacitor. That is the same argument I have been making. Who said that charge does go through the dielectric? If you re-read my posts you will find that I never said that.

ljcox,



Your explanation borders on mysticism. The Theory of Everything (TOE) is not going to rescue you from your belief that current may/is something other than a flow of charge. This is not a problem for theoretical physics. It is well known what happens when a capacitor is energized. The term "displacement current" may be a misnomer, but it is surely a representation of the changing electromagnetic fields that are in play when the energy storage level of a capacitor changes. As I replied to you in post #164 of this thread, "Until the different branches of physics are all unified, a theory of everything is just a musing of the mind. In any case, it will not change what the definition of current (charge flow per time) is. How do you know this? Are you psychic? If another quantity has to be defined, then that quantity will have a new name."

Ratch

I think we have to, as Mr Al said, agree to disagree on this issue. Time will tell.

 

[ljcox]

Hello again,

Well thank you Rachit, but wait till you hear this one, which im also sure you will agree with. This tops them all and offers no wiggle room for Brownout this time.

This is a direct quote, quoted from a lecture by Professor Viken, "Capacitors and Circuits Theory".
We enter the lecture while he talks about capacitors (two of them) in series...

Now keep in mind that these are not my words, these are the ACTUAL words of Professor Viken:
Notice how he goes through a LOT of trouble to avoid the use of the word "through" when talking about the current in the capacitor circuit.

(Note also there are no typos in this next passage, and so he does actually repeat himself a couple times)

"Two capacitors, connected together end to end in such a way that the same current,
they have the same current that, ah, goes (pauses) I shouldn't say that the same
current goes through them, because the current doesnt really go through a capacitor,
but i should say that the same current goes through, ah (pauses), the same current
reaches the surfaces of both capacitors, lets put it that way."

Sorry Brownout but i think you will have a very hard time wiggling around this one

What is there to wriggle out of?

The good professor obviously is more familiar with the subject than I am (I studied physics in 1969) but he does not know what the "Theory of everything will show.

 

[Ratchit]

ljcox,

Your explanation borders on mysticism. The Theory of Everything (TOE) is not going to rescue you from your belief that current may/is something other than a flow of charge. This is not a problem for theoretical physics. It is well known what happens when a capacitor is energized. The term "displacement current" may be a misnomer, but it is surely a representation of the changing electromagnetic fields that are in play when the energy storage level of a capacitor changes. As I replied to you in post #164 of this thread, "Until the different branches of physics are all unified, a theory of everything is just a musing of the mind. In any case, it will not change what the definition of current (charge flow per time) is. How do you know this? Are you psychic? If another quantity has to be defined, then that quantity will have a new name."

How do I know this? I do know how current is defined now in this age. If you propose that current will have a different definition in the future, then it is up to you, the proposer, to show why the definition will change. And the evidence should be something other than a vague ability of a proposed theory to describe an already well explained phenomena. Energizing a capacitor is not rocket science, and there is no mystery about what is happening that needs an esoteric theory to explain.

Ratch

 

[BrownOut]

Actually, the prof concluded his lecture by stating that displacement current flows through the capacitor, which validates Mr. RB's assertions. This is not unexpected, because Kirchoff requires it. And the fact that this discovery led to further discoveries of E-M waves is importand, significant and validates the concept of current flow. When did a pretend mathematical artifice led to important discoveries? That would not make any sense at all. Also, we've shown the definition of current, which includes the dispalcement term. No logical reason for anyone to continue to insist that we show something that's already been shown.

 

[killivolt]

What I don't get is the run on sentence that seems to be a dichotomy. I changed the words to see where it would go. Now I want to change the words from, "through to Moves" and connect the last 2 statements that seem to be non-cohesive.

As before, I replaced "Current with Flow" and "through to Moves"

I see 2 separate regions generating a sympathetic region (in between) co-equal in nature, when he states it.

"Two capacitors, connected together end to end in such a way that the same flow,
they have the same flow that, ah, goes (pauses) I shouldn't say that the same
flow goes through them, because the flow doesnt really go through a capacitor,
but i should say that the same flow moves, ah (pauses), the same (flow, moves)
reaches the surfaces of both capacitors, lets put it that way."

When you connect the 2 parts of this from, " but I shouldn't say" it terminally connects the 2 statements together. So, for my own benefit at least for me to see a verbal depiction this, I see the 2 regions.

How, is better explained with the 2 points or plates being conditioned to translate the energy from "state to state" or " Location to Location ". If there is a photon release, it may generate a thermal attribute.

At first thought you want to see "flow, move" with respect to water. However, if I were only dealing with water which we are not, when we did mention photon release I thought of this.

Van Hove singularity - Wikipedia, the free encyclopedia

These discussions may inspire someone here on the forum into nano research or maybe even development with Carbon Nano tubes.

Measurement of the quantum capacitance of interacting electrons in carbon nanotubes : Abstract : Nature Physics

I'm really enjoying this hope I'm still on topic and no one will put to big of a smack down on me.

I just really like physics. Even if it's only 2 inches away from my finger tips.

Capacitive Reactant

Edit: Just for fun. From wiki Speed of Light.

https://upload.wikimedia.org/wikipedia/commons/thumb/2/27/Frontgroupphase.gif/220px-Frontgroupphase.gif

The blue dot moves at the speed of the ripples, the phase velocity; the green dot moves with the speed of the envelope, the group velocity; and the red dot moves with the speed of the foremost part of the pulse, the front velocity.

 

[ljcox]

ljcox,



How do I know this? I do know how current is defined now in this age. If you propose that current will have a different definition in the future, then it is up to you, the proposer, to show why the definition will change. And the evidence should be something other than a vague ability of a proposed theory to describe an already well explained phenomena. Energizing a capacitor is not rocket science, and there is no mystery about what is happening that needs an esoteric theory to explain.

Ratch

Ratch,
I have never said that it WILL change, all I suggested is that it MAY change.

My point was that the TOE should resolve the paradox between conduction current & displacement current.

I'm not psychic either, I don't know what the TOE will reveal.

And I don't think any one else does either at this point in time.

 

[ljcox]

well guys I was going to stay out of this thread but eh I just can't resist.



Back when I was playing around with F&P motors as wind generators my mate was putting CAPS in parallel with the 3 phase output. I went and put them in series and found when the CAPS weren't connected I was getting around 5 amps output. I connected the caps in series and got 9 amps output. So on my own testing YES current DOES flow thru CAPS.........................

Bryan,
I think we would all agree that it is acceptable to say that current flows through a capacitor when describing the operation of electric circuits.

Where we differ is on the physics of what is happening inside the capacitor.

On the one hand, some argue that the modification of Ampere's Law by Maxwell is a mathematical artifice to resolve an inconsistency in the maths. Therefore, displacement current is not "real" since there is no flow of charge carriers in the dielectric.

On the other hand, Brownout, myself & others have argued that Displacement Current has the dimensions of current & is numerically equal to the conduction current going into/out of the capacitor. Therefore it is a current. We question why a current can only be defined as a flow of charge carriers.

My original point was that the "Theory of everything" will resolve the issue.

 

[MrAl]

Hi again,

killivolt:
You accidentally changed one of the words of the professors quote, from "shouldn't" to "should". That changes the meaning of the whole quote

Mr RB:
The basic underlying concepts to AC current are similar to DC current, which are both the flow of charge, That kind of subatomic action uses only 1 out of 29 electrons in the atom. On the other hand, Mica for example, may have as many as 40 electrons displaced on average in the different atoms of the substance. Thus one action uses a small fraction of the electrons in the atom, while the other uses a large fraction of the electrons in the atoms. Thats the difference between conduction and displacement.
So that's about as clear as you can get. The two actions are on a sub atomic scale different, and that's why it is called something else.

What if i said that AC current is a "Horse of a different feather". What do you think i would mean by that?
Well, if you didnt know (im sure you know already though) i could hand you the following theory:
"Birds of a feather flock together".
Now im sure you have heard this one before too, so you know they are not really talking about birds are they.
But you could infer from that that the reason it makes sense in the first place is because birds that have the same color feathers (or common to their species) will know each other by color. Thus you could infer that "Horse of a different feather" really means "Horse of a different color".
Now the whole point of that statement is to say that something must be different in some situation. But how did we know this? We didnt say anything about the curernt yet, and it certainly isnt a horse or a bird, and a horse does not have feathers (that i know of he he).
So how would anyone know what i meant to say there?
They would infer first that birds flock together when they are have the right color feathers, second that a horse doesnt have feathers, and from that since the birds have colored feathers if horses did have feathers they would be colored too, that i must have meant instead of 'feather', the word 'color'.
So we've managed to use a theory that had nothing to do with AC current, and even said that horses had feathers (an obvious play on phrases), but just being told an assuming for a brief period of time that horses do have feathers we were able to figure out that if they did we would know they were colored and thus the horse would have color too.
So figuring out what the writer meant by "Horse of a different feather" was possible through the use of a theory that linked feathers and color. But does the horse really have feathers? Certainly not, i hope.

But anyway, let me ask you a question now. What if someone asks you this:
"Mr RB, does current flow through a capacitor?", and you reply, "Yes",
and then they ask you this:
"Mr RB, what is displacement current?".
Are you going to tell them that displacement current is the same thing as conduction current?
They then ask:
"Mr RB, then why do they call it displacement current?".
What do you tell them?


ljcox:
Well, the professor came right out and stated that current does not flow through a capacitor, and although he was tempted to say that it did, he always corrected himself. Why would he have to correct himself if it was ok to say that?

Brownout:
See reply for ljcox above. Also you need to watch his other lecture too.

 

[ljcox]

ljcox:
Well, the professor came right out and stated that current does not flow through a capacitor, and although he was tempted to say that it did, he always corrected himself. Why would he have to correct himself if it was ok to say that?

Brownout:
See reply for ljcox above. Also you need to watch his other lecture too.

As I have said before, we don't know what the Theory of everything will reveal.

So it is pointless quoting prof X & prof Y because they don't know either.

 

[The Electrician]

ljcox:
Well, the professor came right out and stated that current does not flow through a capacitor, and although he was tempted to say that it did, he always corrected himself. Why would he have to correct himself if it was ok to say that?

Because he imprecisely said "...current does not flow through a capacitor...", when what he really meant was "...conduction current does not flow through a capacitor..."

Most of the dispute in this thread seems to me to derive from lack of proper, precise, terminology.

The word "current" is often being used without qualification. If the disputants would always say "conduction current" or "displacement current", as appropriate, and not just "current", much of the disagreement could be avoided.

In this article:
Displacement current - Wikipedia, the free encyclopedia

the first paragraph under the heading "Current in Capacitors", contains the statement "Although current is flowing through the capacitor, no actual charge is transported through the vacuum between its plates." The next two sentences say "Nonetheless, a magnetic field exists between the plates as though a current were present[/COLOR] there as well. The explanation is that a displacement current ID flows in the vacuum,"

These three sentences exhibit the problem with imprecise terminology that I'm talking about.

The first use of the word "current" is without qualification and it is asserted to flow even though no charge is transported; later we are told that what was meant was "displacement current".

The next use of the word "current" is in the phrase "as though a current were present". In this case the author apparently means "conduction current", a current consisting of charge transport. If he meant the same thing as he meant by "current" in the first sentence, he wouldn't have had to say "as though a current were present". He should have said "as though a conduction current were present"

A theory of everything is not necessary to settle the issue of whether "...a current can only be defined as a flow of charge carriers". It's not a matter of theory; it's a matter of semantics.

Maxwell's equations give us the answer to any (non-relativistic) question about how electricity behaves.

For the issue at hand, the equation:
[latex]\oint H \cdot dl = I + \frac{\partial \Phi_D}{\partial t}[/latex]

tells everything we could want to know. This equation provides a definition of "current" as the integral of H dot dl. In other words, we integrate the magnetic field intensity around a closed curve and that will be the value of the "current" passing through a surface bounded by the curve. The equation tells us that there may be two physical processes contributing to the magnetic field--a transport of charge carriers or an electric field which is changing in time. We could choose to use the unqualified word "current" to represent the integral of H dot dl. We could use qualified words to represent the two component parts of the "current", such as "conduction current" and "displacement current".

So, if we want to know the current passing through a region of space bounded by a closed curve, for example the circumference of a round wire, or a curve surrounding the space between the plates of a vacuum capacitor, we need only provide a means of measuring that integral. The tongs of a clamp-on ammeter do just that. If we could have a really small clamp-on ammeter that could enclose just the space between the plates of a vacuum capacitor, it would measure the (generalized) "current" (more precisely the "displacement current") passing "through" the capacitor.

It's a mistake to assume that in a conductor the [latex]\frac{\partial \Phi_D}{\partial t}[/latex] component doesn't exist. Except in a superconductor there is still a "displacement current" component; it may be very small, but it is there. In resistive materials the "displacement component" may be substantial; in a near vacuum it will be dominant and in a perfect vacuum it is all there is.

Maxwell chose to call the [latex]\frac{\partial \Phi_D}{\partial t}[/latex] component current "displacement current" because he believed in the "aether" and a "sea of vortices" interpretation of the "electric fluid". It was an unhappy choice for us. Perhaps less confusion would result if it were called something like "virtual current".

A similar situation exists with the choice of the name "imaginary" for one of the components of a complex number; the name causes consternation.

This "displacement current" phenomenon shouldn't be as puzzling as it apparently is. It is an experimental fact that with a time-varying electric field is associated a magnetic field, and with a time-varying magnetic field is associated an electric field.

The latter phenomenon should seem just as mysterious, but it's greater familiarity in everyday experience has robbed it of most of its mystery.

What I'm referring to is this: imagine a two foot long piece of 10 gauge magnet wire. This wire has a resistance of about 1 milliohm per foot. Further imagine that you connected the probes of your voltmeter across the ends of the piece of wire and measured 1 volt AC @ 60 Hz. You would probably say that there must therefore be a current of 500 amps flowing in the wire; how else could there be a 1 volt drop across a big copper wire like that?

[latex]\oint E \cdot dl =- \frac{\partial \Phi_B}{\partial t}[/latex]

The right hand term of this equation should perhaps be called the "displacement voltage".

A closed curve surrounding a region of space wherein there is a changing magnetic field will have an electric field along it which will integrate to a non-zero value. That non-zero electric field will be able to produce separated charge in a conductor.

What I'm describing is a situation where the 10 gauge wire is wrapped a few times around a magnetic core, a transformer core where the primary is energized by a 60 Hz line voltage. In this situation the wire can have a 1 volt difference between the ends of the wire even though there is no current in the wire. Why don't we think that is mysterious? The ends of the wire are points between which there is a 1 volt difference, and the resistance between those two points is .002 ohms. Why isn't there a 500 amp current between the ends of the wire? Isn't that mysterious?

 

[ljcox]

The electrician,
Thanks for your post, it is very intersting.

Did you see my post #86? It started this part of the discussion. Here is the relevant part.

I presume that terms & concepts such as current, current flow, charge and discharge, etc. were coined in the 19th century by people such as Gauss, Faraday, Maxwell, etc. These terms & concepts have been accepted by thousands of us since.

I daresay that they knew of the shortcomings of these terms and concepts.

I suspect that when the physicists derive the "theory of everything", some of these concepts & terms will need to be revised.

For example, if string theory proves to be correct, then current may be shown to be a flow of strings.

I say this because the concept of current flow being the flow of charge carriers does not cover the concept of displacement current.

I believe this concept is derived from Maxwell's equations which, as I understand it, show that there is a displacement current through the dielectric of a capacitor even though there is no flow of charge carriers inside the dielectric: note that the dielectric could be a vacuum.

This raises the question in my mind - why do we need a conductor to carry current?

Obviously the charge carriers in the conductor play a role of some kind.

I'll leave the answers to these questions to minds that are greater than mine.

You wrote "A theory of everything is not necessary to settle the issue of whether "...a current can only be defined as a flow of charge carriers". It's not a matter of theory; it's a matter of semantics".
I don't agree that it is semantics.

Nor do see why a "...a current can only be defined as a flow of charge carriers".

My point is that the Theory of everything MAY show that the flow of charge carriers is a consequence of something else that we don't surrently know about and it should hopefully resolve the paradox between conduction & displacement currents.

 

[Ratchit]

The electrician,

The ends of the wire are points between which there is a 1 volt difference, and the resistance between those two points is .002 ohms. Why isn't there a 500 amp current between the ends of the wire? Isn't that mysterious?

No, the inductive reactance is the primary inhibitor of current in this case.

I think you are agreeing with what I said all along. In a vacuum capacitor, no conduction current is present. But changing electromagnetic fields are present in the free space between the plates. These changing fields are physical, but can be calculated and interpreted mathematically as a virtual current. Would you agree with that?

Ratch

 

[The Electrician]

You wrote "A theory of everything is not necessary to settle the issue of whether "...a current can only be defined as a flow of charge carriers". It's not a matter of theory; it's a matter of semantics".
I don't agree that it is semantics.

It's a matter of semantics because that's what "defining" is. Defining is a matter of giving a name to objects or concepts.

It would be a different matter if we were attempting to "explain" or offer a "theory" of current flow.

Nor do see why a "...a current can only be defined as a flow of charge carriers".

Are you taking what I said:

'A theory of everything is not necessary to settle the issue of whether "...a current can only be defined as a flow of charge carriers."'

to mean that I believe that "...a current can only be defined as a flow of charge carriers."? That's not what I said. I made no assertion one way or the other. I only said that the matter doesn't need a TOE to settle it.

My point is that the Theory of everything MAY show that the flow of charge carriers is a consequence of something else that we don't surrently know about and it should hopefully resolve the paradox between conduction & displacement currents.

There is no paradox between conduction & displacement currents. The fact that both produce a magnetic field unifies them. If the unqualified word "current" is used to describe the result of integrating around a contour:

[latex]\oint H \cdot dl[/latex]

then we have taken account of both contributors to "current".

If one chooses to define "current" as only the flow of charge carriers, then this semantic choice creates a paradox which is only semantic.

If a future TOE provides that electrons can be further subdivided into strings, or something else, "conduction current" can still be defined as a flow of electrons (or other charge carriers) for simplicity. The possible fact that electrons are further composed of strings won't change the fact that "conduction current" is still a flow of electrons. And the unqualified word "current" can still be defined as:

[latex]\oint H \cdot dl[/latex]

It's just a matter of word choice; semantics in other words.

 

[ljcox]

The electrician,
I don't have a problem with the conduction versus displacement currents, but some of the others do.

And you're missing my point about the TOE.

What I'm implying is that the "real current" may be something we don't yet know about & it is driving the charge carriers & it is passing through the dielectric.

Hopefully this, if true, everyone will be happy with.

 

[The Electrician]

The electrician,



No, the inductive reactance is the primary inhibitor of current in this case.

Well, I think you know that doesn't do away with the alleged mystery. Of course, it's only a mystery at first glance, without thinking it through.

Let's say that the reactance of 2 turns through the core is 20 mΩ (approximately 50 uH @ 60Hz); then why don't we see a current of 50 amps between the ends of the 10 gauge wire? If a impedance of 20 mΩ exists between two contacts between which there exists 1 volt, wouldn't we expect a current of 50 amps in that impedance? I think you see what I'm getting at. The fact that a closed curve enclosing a changing magnetic field must see a potential around the curve is, of course, the explanation. We wouldn't suggest that the potential isn't real because we (electronics types) see and use transformers all the time. The fact that a voltage exists across the ends of a wire having no conduction current in it to provide a voltage drop is, or should be (in my opinion), just as mysterious as the "displacement current" in the vacuum capacitor, which isn't really a mystery at all. Ever since Maxwell explained it, the mystery is no more.

I think you are agreeing with what I said all along. In a vacuum capacitor, no conduction current is present. But changing electromagnetic fields are present in the free space between the plates. These changing fields are physical, but can be calculated and interpreted mathematically as a virtual current. Would you agree with that?

Ratch

Yes. As I said in my long post, except in perfect conductors (superconductors) and a perfect non-conductor (vacuum), the total "current" is always going to be a combination of:

"conduction current" (I)

and "displacement current" ([latex] \frac{\partial \Phi_D}{\partial t}[/latex]), or "virtual current".

the two parts of:

[latex]\oint H \cdot dl = I + \frac{\partial \Phi_D}{\partial t}[/latex]

 

[The Electrician]

And you're missing my point about the TOE.

What I'm implying is that the "real current" may be something we don't yet know about & it is driving the charge carriers & it is passing through the dielectric.

I didn't miss your point; I just don't think it's very useful.

How does this help us today? On could always postulate that some future increase in knowledge may explain something that seems paradoxical today. But is there really a paradox in the "definition" of "current" based on Maxwell?

There is no paradox when the modern point of view is used; it's the fields that count. The energy carried by an electric current is not carried by the charge carriers themselves; it's carried by the fields associated with the "current" as quantified by the Poynting vector. The relevant magnetic field can be due to a flow of charge carriers or due to a changing electric field. Either way, the energy resides in the fields.

Notice that the common D'Arsonval ammeter relies on the magnetic field to measure a current as does the clamp-on ammeter.

 

[ljcox]

I didn't miss your point; I just don't think it's very useful. I mentioned it originally as a possibility to see how readers would react. I certainly got a reaction.
How does this help us today? It doesn't. On could always postulate that some future increase in knowledge may explain something that seems paradoxical today. But is there really a paradox in the "definition" of "current" based on Maxwell?

There is no paradox when the modern point of view is used; it's the fields that count. The energy carried by an electric current is not carried by the charge carriers themselves; it's carried by the fields associated with the "current" as quantified by the Poynting vector. The relevant magnetic field can be due to a flow of charge carriers or due to a changing electric field. Either way, the energy resides in the fields.

Notice that the common D'Arsonval ammeter relies on the magnetic field to measure a current as does the clamp-on ammeter.

I don't find it a paradox.

As I said, I don't have a problem with conduction/displacement current, but some of the commenters do.