AC flowing through a cap. What actually happens?

[MrAl]

MrAl,
[In reference to asking real life people in various locations in the world if they know what happened to a battery that was 'energized']

Yes, I will. However, I don't get a chance to do that activity very much. I'll keep it in mind, however.

Ratch

Hi again,

Ok great, i think you'll be amused at the replies

Take care for now...

 

[Ratchit]

ljcox,

Thank you for pointing out where you think I have been wrong. The links I provided to you seem to say that charge flows across vacuum dielectrics and other strange stuff. They are remiss in not telling you that those are fictious artificial concepts devised by Maxwell to explain some contradictions and paradoxes in the application of his equations. Displacement current does not transport real charge. Look at the link below and find "displacement current". Then read the last sentence.

New Scientist - Google Books

That kind of explanation can be found elsewhere, too. Displacement Current Does Not Exist

Since you have been told a lot of things by several people, so I thought you would profit by a link to a MIT course that addresses that topic. Enjoy the explanation by a professor who has forgotten more about electromagnetics than we will ever learn.
YouTube - Displacement Current Theory

Ratch

 

[BrownOut]

You were wrong. Maxwell's mistake was to believe charge had to be involved, but it doesn't ( the article was not remiss about this). Displacement current exists without the presence of charge. From your last link:

∫β.dl= ∫(µoj + µoεodE/dt).dS

all terms are in units of current. The 2nd term on the right is the displacement current, and was defined in the video. This is an important quantity, and was given for the solution to the continuity equation for charge. Is are not fictional, as the crap you've made up. The displacement current is important to wave propagation, and is a very real quantity.

 

[ljcox]

ljcox,

Thank you for pointing out where you think I have been wrong. The links I provided to you seem to say that charge flows across vacuum dielectrics and other strange stuff. They are remiss in not telling you that those are fictious artificial concepts devised by Maxwell to explain some contradictions and paradoxes in the application of his equations. Displacement current does not transport real charge. That's what I told you. In essence, I said that there are no charge carriers in a vacuum, therefore it is not a conduction current - it is a displacement current. Look at the link below and find "displacement current". Then read the last sentence.

New Scientist - Google Books

That kind of explanation can be found elsewhere, too. Displacement Current Does Not Exist

Since you have been told a lot of things by several people, so I thought you would profit by a link to a MIT course that addresses that topic. Enjoy the explanation by a professor who has forgotten more about electromagnetics than we will ever learn.
YouTube - Displacement Current Theory

Ratch

Did you look at his Displacement Current Example 2?

He concludes that the conduction current I is equal to the Displacement current Id.

You are assuming that current has to be the movement of charge carriers.

As I said originally, we need a wider definition of current. See post #86.

 

[Ratchit]

ljcox,

Did you look at his Displacement Current Example 2?
He concludes that the conduction current I is equal to the Displacement current Id.

Yes, it is there in the mathematics. I played the lecture again and listened carefully. I did not hear him say that the displacement current was a conduction current. He did say that the displacement current was caused by a change in the electric flux density with respect to time, had units of current, and was very small. As far as I can see, it is a mathematical artifice used to make Maxwell's equations come out correct. That is similar to the conventional current method, where we assume all the charge carriers are positive, and calculate the currents/voltage mathematically that way, even though we know very well that the real physical charges are not positive. Other references I have read do distinguish between "conduction current" and "displacement current". They even mention different kinds of displacement current.

Now, you have heard of the the law called the Convervation of Charges, right? It basically says that all charge has to come from somewhere, and cannot just disappear. You and others think current can exist without any physical charge flow in a vacuum or a tight dielectric. Others accuse me of changing the definition of meanings, but that belief is a real mind snapper. Current is charge flow, and to have a physical current you need a physical charge to move. You should also notice in the second link that the author avers that the displacement current theory does not work well in antenna design.

One final thought. In your studies you have come across Kirchoff's Law, Gauss's Law, Lenz's Law, Newton's Law, Ampere's Law, etc. I have never heard of displacement current being referred to as a law. It is referred to as a theory, or just "displacement current". Look at the title of the lecture.

Ratch

 

[BrownOut]

If it's a mathematical artifice, then why can it be measured? Why is it necessary to solve the law of conservation of charge? Why does it give correct results in wave propagation? These equations have been successfully used for over 100 years, and have never been disproven. The author not only didn't say displacement current doesn't work for antennas ( he only mentioned a couple designs, cherry picked for the discussion, and not the many antenna designs that work perfectly well and that relies on the displacement current equations, neither did he mention wave propagation, which also has been proved to match the equations well, which in turn were based on displacement current) Also, the author, who doesn't seem to have any special credentials to be believed over the many other authors, didn't disprove the existence of displacement current as he promised. He basically did alot of hand waving, but not any proof at all.

 

[MrAl]

Hello again,


Ok, this is different now than trying to tell the difference between two different words. This is a different story, but i believe this is an old story.

We want to equate the current density to a changing electric field (nothing to argue about if the E field is not changing) and we find that we can do this through the introduction of the electric space constant. The electric space constant allows us to say that the right hand side of the equation has the same *units* as *current*. This happens because a long wire with a break in the center and charge moving through it has a B field around it and the B field does not break in the center even though the wire does break in the center. Thus it appears that we have a break in the wire but we have no break in the field.
Now the changing B field is present at the center, so we assume a current flow. But charge conservation tells us that there can be no charge flowing, so we've got a dilemma. What to do? We account for this 'current' by allowing the changing E field to produce a magnetic field just like we allow the magnetic field to produce a E field, and this shouldnt be too much of a problem. We end up with the term e0*dE/dt and although that is in units of Amperes, is that really a current comprised of charge flow? Lets take a look...

Set up a wire with a break in the center and attach two largish metal plates, one to each end of the wire at the center to form a largish capacitor. Separate the two plates by two inches or so. Connect a power supply able to take a 0 ohm load for a short period of time and get ready to turn on the switch. Now take a compass and hold it near the wire and turn the power supply on, and note the compass deflection. Now turn off, hold the compass near the center of the capacitor and turn on the switch again. Note that the deflection is the same as when the compass was held next to the wire.
Conclusion: The wire produces the same B field as the very center (between the plates) of the capacitor. That's all we can say so far.

Question: Does that mean that there was charge flowing through the center of the capacitor? Lets take a look...

Same setup, except this time we'll use an ammeter instead of a compass and later add a short length of wire.
Ok, so first we break the wire on each side of the center break and insert the ammeter and measure the current (needle deflection) when we turn on the power switch, and note how the reading jumps up (and then down), and we get the same response on both sides of the center break. We can also use the short wire and ammeter if we hold the short wire close to the long wire as the B field will couple to the short wire.

Next, we connect a short length of wire to the ammeter as if it was going to conduct current and we are to measure the current through that short piece of wire with the ammeter.
We then hold at least part of this short wire in between the capacitor plates holding part of the wire along the same axis as the long wire. We turn on the switch, and note the meter deflection. We note that the meter deflects again. How could this happen?
Well, if we assume that we can allow e0*dE/dt=i then we can also allow i=e0*dE/dt and so we see the current flow, so there is real current flow within the plates of the capacitor.
There's a catch here however, and that is that we inserted a (copper) wire with plenty of extra static charge, and that charge was moved because of the changing E field causing a changing B field, not because the current was there already. If the current was there already, we could use just an ammeter and would not need the extra static charge (short wire).
Thus the conclusion is that the current density due to dE/dt is not a real current, it's just something happening that happens to work out to the same units as current.

We could do other experiments, such as insert some solution between the two plates and try to electroplate some object. Im sure we could get it to work, but whatever real current causes that disappears as soon as we remove the solution.


Now this wouldnt be complete without the other side of the story.
An inductor conducts current through it and clearly no one would argue that it doesnt. Thus charge is conserved, yet it can still store energy.
Charge flows in one side and out the other, yet we still get energy storage. There's no 'loss' of charge just because we stored energy.
Also, when we power a light bulb charge flows in one side and out the other, yet much energy leaves the system as heat.

Obviously there must be something else happening here, and we note that in each case it involves the movement of charge, which is more or less the movement of electrons, and electrons are able to deliver energy just because they are moving. However, they cant move on their own, they need a forcing function to get them to move, so whatever field that acted in the deliverance ends up as the field in the final device, either stored or dissipated.
This field can be set up near 0.75 percent of the speed of light, so it would be very hard to detect it directly, but we can see how the energy gets transferred.
Thus, the field in the capacitor gets there because of a field somewhere else. It uses the electrons as the energy carriers.

We might also note that in order to change the direction of an electron we have to add energy to it. In AC current, the changing direction of the forcing field will add to the energy to all of the electron states in the wire, which can loose energy in the load, and then when the forcing field changes direction the electrons will acquire yet more energy, which can deliver more energy to the load. Thus the light bulb stays lit or the field in the capacitor keeps changing direction.

 

[BrownOut]

But your experiment doesn't tell us anything we didn't already know. If you had been able to measure displacement current dirctly, then we wouldn't need to make any distinction between it and plane old current. But because current can be defined by both a measure of charge passing a point, and by the resulting magnetic field, then at least by magnetic induction, a real current exists. It is distinct from charge moving along a wire, but that doesn't mean it's not real current.

 

[MrAl]

Hello there Brownout,

Well there are some interesting questions that come up, but if you read my whole post you would see that i had stated that charge passes through an inductor and creates a field and that charge actually gets through and doesnt have to change just because it created a field. I thought that would carry over to the capacitor for the counter point.

The interesting questions that come up are:


1. With a large plate capacitor and 1v of potential we can pass a 1000 amp current through a thick ceramic insulator (dielectric). The question is, as we increase the plate area we decrease the current density making it easier for charge to pass through, and as we decrease the thickness we increase the conductivity, yet charge does not pass for the entire time we apply the voltage, so how can we explain being able to pass current through an insulator for a short time period and not for the whole period? The changing field seem to be the only candidate, and that means it must have created a current in turn.
2. Starting with a small capacitor with some plate volume (total amount of metal in the plates) if we make the plates thinner and thinner we increase the capacitance without having to add any volume to the metal plates. This means more and more current gets passed 'through' the capacitor yet we havent added any static charge to work with on the side that supposedly looses charge. It cant loose charge it didnt already have to begin with, so we can ask if there is a limit to the process of thinning the plates. Does the charge build up? If the other side depletes then it had to have had enough charge to deplete to begin with or at some point it cant deplete anymore. Does this mean then the charge only builds up on one side and no longer depletes on the other side?
3. The magnetic field becomes static after a short time period (zero) so the current expires. Why doesnt it continue to exist? The only answer seems to be that charge accumulates, but we are not necessarily specifying WHERE it accumulates.
4. The current grows as we increase the insulating properties of the dielectric. Less conduction means better E field, but wait, we said the E field creates a B field creates a current. There must be a threshold, a tradeoff. The conduction current is much less than the generated current.

The way i understood it a long time ago was that the current creates a field and the field gets stored, then later the field creates a current. It would seem that this would have to happen within the capacitor but without specifying where.

If we want to say that charge accumulates on one side and depletes on the other, then we have to figure out where all that depletion side charge is coming from. It seems simpler to figure that it comes from the field or else eventually we run out of charge on the depletion side. If we run out of charge on the depletion side, then current no longer flows on that side. The question is then, are there enough valence electrons in the depletion side to account for the capacitance of every capacitor that can be constructed even if it is large value and it has very very thin plates? This gets interesting too because as we make the plates thinner we make it harder to conduct current also.
Anyone up for "Electrostatic Induction"?

 

[BrownOut]

if you read my whole post you would see that i had stated that charge passes through an inductor and creates a field and that charge actually gets through and doesnt have to change just because it created a field. I thought that would carry over to the capacitor for the counter point.

Hold on there, hoss! You were still editing your post after I made mine. I'll have to look over both a little later, when I have more time. Till then....

 

[Ratchit]

MrAl,

Thus the conclusion is that the current density due to dE/dt is not a real current, it's just something happening that happens to work out to the same units as current.

Good presentation Al, and the correct conclusion. I will add to that with a quote from a link below.

"The term is an important component of the Maxwell equations, and represents magnetic effects caused by varying electric fields that were unknown at Ampère's time. In a way, Jd is a fictitious quantity, since it does not correspond to an actual current, but inclusion of this term is mandatory to successfully explain observed physical phenomena accompanying time-varying electric fields."

Displacement Current -- from Eric Weisstein's World of Physics

Ratch

 

[BrownOut]

I've never seen a fictitious quantity produce a real effect.

 

[MrAl]

Hi,

Oh ok no problem. I often edit quite a bit while im thinking how to convey an idea.

By Electrostatic Induction, i mean maybe we can look at this as charges being coerced to reside more on one side of the atoms in the dielectric than the other. This would create a net charge across the dielectric. The charge does not have to move OUT of the atoms this way, and thus there are a lot of atoms to work with before we hit the limit point where it no longer behaves like a capacitor. This would certainly help explain why a better insulator allows more current to conduct during the 'charge' time.
Thus the energy of the moving electrons would be involved in unbalancing the charges in the dielectric, which would mean current flow out the other side. This would explain why the capacitor can have a voltage across it too without actually accumulating charge.

I couldnt help but think of another little pun here:
"I energized my battery and indeed it conserved charge".

 

[ljcox]

ljcox,



Yes, it is there in the mathematics. I played the lecture again and listened carefully. I did not hear him say that the displacement current was a conduction current. I did not say that he did! He did say that the displacement current was caused by a change in the electric flux density with respect to time, had units of current, and was very small. Wrong, he showed that Id = Ic where Ic is the conduction current in the wire. As far as I can see, it is a mathematical artifice used to make Maxwell's equations come out correct. That is similar to the conventional current method, where we assume all the charge carriers are positive, and calculate the currents/voltage mathematically that way, even though we know very well that the real physical charges are not positive. Have you ever heard of protons, + ions, positrons, etc.? Other references I have read do distinguish between "conduction current" and "displacement current". They even mention different kinds of displacement current.

Now, you have heard of the the law called the Convervation of Charges, right? It basically says that all charge has to come from somewhere, and cannot just disappear. You and others think current can exist without any physical charge flow in a vacuum or a tight dielectric. No, that shows your misunderstanding of Id. Id is not a flow of charge carriers. Others accuse me of changing the definition of meanings, but that belief is a real mind snapper. Current is charge flow, and to have a physical current you need a physical charge to move. That is your basic assumption. I don't agree with that assumption. You should also notice in the second link that the author avers that the displacement current theory does not work well in antenna design.

One final thought. In your studies you have come across Kirchoff's Law, Gauss's Law, Lenz's Law, Newton's Law, Ampere's Law, etc. I have never heard of displacement current being referred to as a law. It is referred to as a theory, or just "displacement current". Look at the title of the lecture.
By that reasoning, then theories such as Einstein's Theory of Relativity are not factual.

Ratch

It is a long time since I studied antenna theory so I don't know the context of his remark. Besides, it is beside the point.

We are talking about displacment current through a capacitor.

It has:-
1. the dimensions of current
2. has a magnetic field
3. is equal to the conduction current in the wire.

Your basic assumption is that a current can only be due to the movement of charge carriers. I have a wider view.

 

[Ratchit]

ljcox,

We are talking about displacment current through a capacitor.

It has:-
1. the dimensions of current

I would hope so.

2. has a magnetic field

As does all current

3. is equal to the conduction current in the wire.

No, the prof said it was much smaller than the conduction current, and not caused by the same thing. Did you read my link to Al's reply about it being a "fictious" entity. I did not come up with that, the physics website did.

Ratch

 

[ljcox]

ljcox,



I would hope so.



As does all current



No, the prof said it was much smaller than the conduction current, and not caused by the same thing. Did you read my link to Al's reply about it being a "fictious" entity. I did not come up with that, the physics website did. Wrong!
Ratch

He showed that Ic = 1.2 Amp & Id = 1.2 Amp.

 

[MrAl]

Hello again,

I'll say it again...Everything is an abstraction of reality, and mathematics is a huge abstraction. We cant do anything with math (logic) unless we establish some assumptions.

What we need is some sort of proof. Proof requires cause and effect. If anything we do causes an effect, that doesnt mean it's happening when we dont do anything. The very act of doing something changes the nature of the experiment so it's hard to determine if it is us or nature that is really acting.

A ship sails into a lock, a ship sails out of a lock, at a different water level. If we cant see what is happening inside it would be hard to figure out what was happening. If we could see the water, even without seeing the ship, we could figure it out. The idea then is to come up with an experiment that shows that the displacement current is either there or not there, and in such a way that we ourselves dont introduce a factor that would contribute to the success or lack of success of the experiment.

What happened to the Electrostatic Induction idea? Is that rejected entirely?

 

[BrownOut]

The proof rises over you every morning:

-d/dtcurlB = -µoεod²E/dT²
µoεod/dTcurlE = -µoεod²B/dT²

Let there be light!

 

[Ratchit]

ljcox,

Sorry to be so late in replying, but I got tied up with something. OK, I listened to all the lectures pertaining to displacement and took notes. The lecture you are referring to is the capacitor in series with a resistor. The prof shows a cap with a constant voltage applied. to the left side where the charge accumulates. The prof says by electrostatic induction, the right side shows a negative charge. The imbalance of charge causes a E-field to form from the left side to the right side. As the E-field changes, this causes a B-field to form. He says about the B-field, "Even though there is not current in the cap, it acts as if current is existing through it". Further into the lecture he says, "The cap has no current in it". Then he goes on to calculate the displacement current and conduction current which both come out to be 1.2 amps .

It seems pretty clear to me. The charge imbalance causes a E-field to form across the plates and through the dielectric. As long as the E-field is changing by charge imbalance, a B-field will form. This B-field will reach through whatever dielectric the cap has, and change the charge flow on both sides of the plates so as to make it appear as if current is existing through the capacitor. So no physical transport of charge through the dielectric happens. Notice the prof also said that there was no way to distinguish the displacement current from the conduction current because they are both the same.

I believe this is what MrAl was trying to show, and why the physics site I posted said that the displacement current was more or less ficticious. It is a calculated value of what appears to be current through a capacitor when a charge inbalance occurs. I call it a mathematical artifice.

Now to tie up some loose ends.

It is referred to as a theory, or just "displacement current". Look at the title of the lecture.
By that reasoning, then theories such as Einstein's Theory of Relativity are not factual.

No, I never said that Einstein's TOR was not factual. So far it has stood the test of events. It was a derived theory not obtained by physical measurements like the law of gravity was.

 

[Mr RB]

...
I believe I said that just because current can be measured on both sides of the capacitor was not proof that charge passes through the capacitor. If it did, the charge would not accumulate and it would not do what a capacitor does, store energy. Why didn't you discuss this earlier?
...

Now I'm going to get pedantic while you are busy backpedalling.

I said; "CURRENT flows though a cpacitor" and
You said; "No it doesn't"

I deliberately used the most simple example of where you were completely wrong about capacitor current rather than nitpicking the specifics of what happens inside a capacitor becasue frankly that doesn't matter.

If you still want to try to save your argument solve this;

1. you have 3 mystery comonents in series,
2. your instrument is one ammeter at the end of the series which registers 1 amp, a single reading at one point in time
3. deduce which one of the 3 components is the cap because "there is no current flowing through it"