# AC flowing through a cap. What actually happens?

Discussion in 'General Electronics Chat' started by qtommer, Feb 25, 2011.

1. ### Mr RBWell-Known Member

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Now I think you are asking a much better question.

As for displacement current being a REAL current, I'm pretty much in your camp. I think the best definition of a "real current" has to be conduction current of electrons flowing through a conductor. Once we start to move away from that definition the current must become less "real".

Forget caps for a second and think of a transformer. As current goes into (loosely) the primary there is a physical field effect inside the transformer and causes a current to come out of (loosely) the secondary. (I say "loosely" in an attempt to ignore the fact the transformer has 4 wires not 2).

So if this case is analogous to what is happening in a cap, (and in some ways it is, as there is a current in, current out and an internal field effect) we would probably have no problem sayng "there is no REAL current flowing from primary to secondary".

The transformer may be a stretch of an analogy, but to my simple way of thinking any time you have 2 divided systems with no direct conduction, and current is only INDUCED across the divide, cannot be classed as a "REAL current".

But I still don't mind the term "displacement current" in a conceptual use. Then again I don't mind calling a "battery" "flat" even though it may be very round and it might even be a CELL.

If someone said we were not allowed to say the battery was flat I would argue because the term "flat" is conceptual and does the job quite nicely thank you. If nobody knew what a "flat battery" was, then that would be a different case. If you want to eliminate commonly used and well understood terms that are not 100% perfect you have to throw away half of the English language.

2. ### MrAlWell-Known MemberMost Helpful Member

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MrRB,

Well, well, well, if you read my reply in the other post about strain relief, you'll see me mentioning that you "nailed it" (meaning of course that it was a perfect description). Well, i think you just nailed it again! I was so stuck on the cap and inductor (inductor is interesting too because of the current in, current out thing) that i forgot all about the transformer.
The transformer operates (as you said) as current going into the primary and creating a B field, the B field couples to the secondary (copper usually, which is metal) and causes a current to flow in the secondary as well. Yet we dont ever say that the B field created current in the air inside the air core of the transformer, we only say that it causes a current to flow in the secondary WIRE. It's true that eddy currents flow in the metal of an iron core transformer, but that's still metal, and i dont have a problem with metal inside of a capacitor with a conduction current flow in it.
So now we get down to the field again, which can radiate out from a wire, and the field strength WOULD produce a current in a piece of wire in a loop IF there was really a wire there. If there is no wire, we say that the B field radiates outward until it does come to some metal or other conductor. Now if there was somehow real current flowing in the air, that energy would have dissipated and it could not reach the wire. In fact, it would short itself out. So how come the B field stops after meeting up with some metal, but it doesnt stop if there is no metal in the path. Simple, it doesnt dissipate until it can influence electrons to move, and we dont find electrons in a vacuum.

3. ### killivoltWell-Known Member

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I thought I would get something up if you want changes let me know.

Ok so, I thought I would take another crack at it. I'm not sure where you would make any changes the field changes on right angles churning through from Plate A to Plate B.

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5. ### MrAlWell-Known MemberMost Helpful Member

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Hello again,

Oh yes, nice picture.
I was also reading some other leading physics texts and every one of them says the same thing about the displacement current, and that is the main point is that the electric field can produce a magnetic field on it's own. They all get out of saying that it is a real current in every circumstance one way or another. Maxwell believed that some sort of true current would be required, so that's how he came up with the correction.
Also, i've also read that the term 'displacement' is not correct either, the way we normally think about displacement.

We've really got our hands full trying to explain this phenomenon in some sort of exact terms. The only way i see is through the fields themselves and forget about everything else. The electric field has sort of an assumed 'flow' to it, so that's it i guess.

6. ### BrownOutBanned

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Good grief man, I've been talking about this for weeks, do you never read anything before responding? If you don't know what I've said all along, then what is the point in repeating??? So you can go on disavowing any knowledge or understanding? Seems like everyone else has taken the time to read and understand my position, as no others seem to not know what I've said.

Last edited: Mar 28, 2011
7. ### BrownOutBanned

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HA! Didn't I say some days ago to think of the E-M wave as a chain? (Post #355)

Last edited: Mar 28, 2011
8. ### killivoltWell-Known Member

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Yes, you most certainly did. I completely read right over the top and highlighted another event.

Anything you wish to add. As far as I know they should be turning right or clockwise and should impulses and 90deg angles with forward momentum.

9. ### BrownOutBanned

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I would say a forward direction, or direction of propagation, as "momentum" would imply mass, and from a wave standpoint, no mass would be involved, though I'm sure a relativity theroist has proved otherwise.

10. ### killivoltWell-Known Member

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For some reason my mind also wants the rings to expand and contract, if the MF field has strength that begins at Plate A as intensity. Then it would be reasonable to assert that the field strength is formed at the source the field expands from Plate A or the source. Meaning the new rings are formed between Plate A these rings push out of Plate A and continue until the first field created comes into contact with Plate B.

I couldn't help but think that if this was stable enough the some kind of tunneling was occurring and that electrons would be permitted to flow over the field sort of like a skin effect. Each time the field contracts it has to build until your able to move electrons over the strengthened field.

Ok, nobody better be laughing. I guess it will be inevitable.

I know it's not correct but, a single conductor is how I want it. Moving electrons around on either Plates at the same time works as well for me. But, my mind wants it to be conventional in a lot of ways too!

11. ### MrAlWell-Known MemberMost Helpful Member

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Do you ever read anything before responding? I asked a simple question several times, and it takes three or four posts to get a straight answer out of you. Everyone else answers to the best of their knowledge at the time, while you simply complain about having to repeat a simple statement. It's starting to look like you dont want to really understand this at all, you're content with old knowledge part of which was never correct to begin with.

So i repeat, state what you are trying to prove here so we know exactly what it is you are arguing, unless of course you are afraid you will say something you'll regret
For example, do you agree with killivolt that there is an EM wave between the plates?
Do you agree that the so called displacement current in a vacuum cap is only a mathematical tool and will never be found to really exist?

One of our goals to to determine if the current really exits in a vacuum cap.

12. ### MrAlWell-Known MemberMost Helpful Member

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Hi again,

So you are saying that you think that an EM wave exists between the two plates? Is that what your drawing showed?

13. ### BrownOutBanned

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I've given straight answers for a couple weeks now, and you seem to be the only one who still doesn't get it. It doesn't matter how many times I state my position, you'll continue to pretend that I haven't given answers. It does't matter how many times I prove my position, you'll continue to pretend I've not shown any proof. If I haven't given straight answers, why is it you're the only one compaining? Why is it you're the only one pretending I haven't been forthright? Everyone else understands what I've been saying, looks like you're the only one still in denial!

I'm not afraid of anything. You can clearly see my position is on record here, and no amount of denial can change that. If you want to challenge what I am on record as presenting, then go ahead. If you want to continue your silly path of pretending that I've not honestly and clearly stated my position, then you're on your own. People here are way to smart to fall for that kind of distraction.

BTW,

In one post, you complain that I repeat my statements, in the next you complain that I refuse to repeat my statements. You don't seem to know if you're coming or going. Like I said, cheap distractions don't work in this forum.

Last edited: Mar 29, 2011
14. ### BrownOutBanned

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Sounds about right.

The beauty is that no electrons are necessary. The propagating fields are self supporting. It exapnds and contracts with the charge flowing into/out of the capacitor, but in the vacuum region, it supports itself.

The simple 'static' model is completely adequate for circuit analysis, and why so few understand what we've been discussing. The whole reason we use the "air" capacitor to illustrate the concept of displacement current is because it's a simple case that can be shown with easy math, relativel speaking. This concept of displacement current is very important to the field of electro-magnetics, and extends way beyond the simple capacitor case. So, it might hurt your brain to think in terms of EM waves in the cap, and you normally don't have to worry about it. If you want to go further into EM waves and such, then you might want to try to understand it.

15. ### MrAlWell-Known MemberMost Helpful Member

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Like i said, you'll continue to complain while wasting more replies. Just answer the question and save yourself some effort. Quit bickering.
Thanks

Last edited: Mar 29, 2011
16. ### BrownOutBanned

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Get a clue. You're the only one still complaining.

17. ### MrAlWell-Known MemberMost Helpful Member

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You know what, dont answer it then. Just go on your merry way as you have been since it's so hard for you to answer a simple question.
While you're at it, go back to school and sign up for "Conversation with humans 101".

Last edited: Mar 29, 2011
18. ### BrownOutBanned

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Don't worry about how I converse. All my answers are on the record, and you are the only one who doesn't know it.

Last edited: Mar 29, 2011
19. ### MrAlWell-Known MemberMost Helpful Member

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You dont have to worry about it anymore, im not asking you any more questions. Good luck with your task at hand.

20. ### BrownOutBanned

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I was never worried.

21. ### MrAlWell-Known MemberMost Helpful Member

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Yeah i can see from your myriad of posts