AC flowing through a cap. What actually happens?

[BrownOut]

Hello again,

I just want to make sure im getting the same results as you are. There's lots of units named after someone so that doesnt help yet

Thanks.

I didn't say "someone" Ampère's Law

 

[MrAl]

Hello again,


Thanks for the link, interesting.
So you are saying that you are getting amperes on both sides then?

 

[BrownOut]

I didn't say it. Ampere did. Gotta go guys.

 

[ben7]

I didn't say it. Ampere did. Gotta go guys.

Well we are still going nowhere on this topic.

 

[MrAl]

Hello again,


Hello there neighbor (Ben)

Stick with us. That's about to change drastically.

Brownout:
Ok, so you say you agree with him so i assume you are getting amperes too then, unless you specify something else. Thanks.

 

[Mr RB]

...
Current does have an instantaneous value, a RMS value, and an average value, as does voltage.
...

More backpedalling. Now you are requiring that "current" includes a time component as this is the only way to try to maintain your point.

Current is an instantaneous value, like voltage. Neither requires a time component and the standard unit Amps does not include a time component although Amphours does. This is the most basic electronics, maybe you missed that class.

Now CURRENT flows equally through all parts of a series circuit, inlcuding the battery. More basic electronics. For CURRENT to flow through all parts of a series circuit is is not required for any particular ELECTRON to make it all the way around the entire circuit.

But CURRENT must flow all the way around, THROUGH every component, from battery+ to battery-. And THROUGH the battery too for that matter. Any particular electron may make it completely around the circuit or may not. CURRENT does not care.

If you are being electrocuted; any particular electron that goes in your body may or may not make it out the other side. You don't know. But the CURRENT flows THROUGH your body, no argument.

Current is one of the 2 most important concepts in electronics, and its most defining characteristic is that it goes THROUGH things. Theoretical current as we electronic people rely on goes THROUGH things. No electron movement is required. Just the concept.

I know you are not from an electronics background or you would have easily grasped this most fundamental concept that CURRENT goes THROUGH all components in a series circuit and can be measured with a single ammeter either real or theoretical.

You can focus on physics concepts as to what may happen to individual electrons in a capacitor and its plates but it is irrelevant to my original statement that you tried to correct. CURRENT goes THROUGH the capacitor, and the wire, and the battery.

When I called you out on it you backpedalled once by trying to change to the word "charge" instead of current, and the second time you backpedalled by trying to include definitions of current that also include time.

The fact is that CURRENT flows THROUGH the capacitor, end of story. Whether or not an individual electron does, you can argue out with others because my argument with you has concluded. If you are man enough, feel free to admit you were wrong without any juvenile "yeah.. BUT" disclaimers.

 

[killivolt]

With that we come to the table or throw the gauntlet into the mud. Because you have just been denied.

 

[Ratchit]

MrAl,

You might want to read this Physics Forum discussion on whether only a change in the E-field can make a B-field form. The thread drifts after a while, but the first part is interesting.

Ratch

Classical Electromagnetism: Question on Ampere's Law and Displacement Current [Archive] - Physics Forums

 

[killivolt]

I do believe he has left the building. Ratchit

And the Gauntlet goes to the Mud.

 

[MrAl]

Hi again,

MrRB:
Not everyone is in agreement that current goes through a vacuum capacitor though. In fact, the general answer to that question is "no" because what does, if anything, goes through a vacuum capacitor, may not be considered to be something that is actually physical.
Consider a B field acting on a charge. The B field is an influence that moves the charge and thus creates a current as defined by the standard dQ/dt. But what happens when there is no charge there to move, how can we possibly have a current like this? This answer is that we dont, we just have a field, and a field is not considered a current. A field can cause a current yes, but it needs something else to work with it or else it's just a field.
See there are two views here:
1. The internal view, and
2. The external view.
The external view is used in circuit analysis to simplify the equations. This views current as flowing through a capacitor. Although this view isnt necessarily as close to reality as the internal view, it helps to make circuit analysis a lot simpler and faster.
The internal view is of interest to physics itself, to understand the underlying concepts of how capacitors work, as close as possible to reality. Unfortunately this view brings in a LOT of extra complexity like field theory, lack of symmetry, and stuff like that, but if we want to understand nature as well as possible we have to consider things like this.

killivolt:
If you relax for a bit you'll find this thread will take a more well-defined route. We are going to pull the gauntlet (whatever you mean by that) out of the mud so to speak. This thread is about to get about 1000 percent more interesting very shortly.

Ratchit:
Im sure a changing E field can create a changing B field, i have no doubt about that. All i was asking of Brownout was what units he was getting, that's it. He says he agrees with Ampere and Ampere says amperes. So i ask you, when you solve the curl B equation what units do you get on both sides of the equation? Im trying to establish some ground rules so we can make some progress.
Let me write it out again in a different form:
LineIntegral B.dl = u0*i + u0*e0*dE/dt (i is current density and dE/dt is change of electric flux)
What units do you get on the left (and that has to be the same as on the right of course).
Thanks.
BTW, stick with me, it's been over 10 years since i've had to actually use this stuff in real life. I've used dv=i*dt/C a zillion times and all of it's ramifications over the years, but it's been a while for the internal definitions and theories for me. In other words, i've constantly used circuit theory but havent had to use the underlying physics theories for quite some time now.
Also, i now have it from expert authority that the displacement current is not physical. I think that word 'physical' is the operative word here. We think of something that is actually real to be something that is physical. That doesnt mean that some other form of the 'thing' is not something at all (such as a field) but it does mean that we are going to have a heck of a time measuring it directly, because everything we do means we have to make assumptions, and those assumptions may mean we'll never know for sure.
For example, it has been suggested that the displacement current is a traveling electric wave. This would put it in the category similar to light itself rather than a current of some type, yet it would still have direction, but then we might ask if everything that has direction is really a current? Current is quite well defined in electric theory and this definitely doesnt fit that definition, by any means. Im sure you agree.
Still a bit of ground to cover, but it will come to a head shortly.

 

[Ratchit]

MrRb,

More backpedalling. Now you are requiring that "current" includes a time component as this is the only way to try to maintain your point.

You keep talking about "back-pedaling". Where have I back-pedaled?

Current is an instantaneous value, like voltage. Neither requires a time component and the standard unit Amps does not include a time component although Amphours does. This is the most basic electronics, maybe you missed that class.

Your saying it's so, does not make it so. Current does imply a time relationship (coulombs/sec). Next you will aver that velocity (directional distance/sec) is disconnected from time. Amp-hours would not make sense unless amps did have a time component. I have never back-pedaled on this or any other point.

Now CURRENT flows equally through all parts of a series circuit, inlcuding the battery. More basic electronics. For CURRENT to flow through all parts of a series circuit is is not required for any particular ELECTRON to make it all the way around the entire circuit.

Yes, charges flow and current exists through all the conductive parts of the circuit. Just about everyone knows that the drift velocity of the charge carriers is very slow, and the charge carrier that enters the circuit at the beginning is not the same one which immediately leaves it at the circuit end. And the point is?

But CURRENT must flow all the way around, THROUGH every component, from battery+ to battery-. And THROUGH the battery too for that matter. Any particular electron may make it completely around the circuit or may not. CURRENT does not care.

Charge flows through all the conductive parts of the circuit. What you seem to have a hard time wrapping your mind around is that the dielectric of the capacitor is not conductive. If it were, it would be considered leaky and defective. So charges accumulate on one plate and deplete on the opposite plate. This allows a current to exists in a circuit as long as the accumulation/depletion can be maintained. This current existence does not mean that current is passing through the capacitor. You seem to have a hard time understanding that.

If you are being electrocuted; any particular electron that goes in your body may or may not make it out the other side. You don't know. But the CURRENT flows THROUGH your body, no argument.

Yes, my body is a conductor, not a capacitor.

Current is one of the 2 most important concepts in electronics, and its most defining characteristic is that it goes THROUGH things. Theoretical current as we electronic people rely on goes THROUGH things. No electron movement is required. Just the concept.

What you should mean to say is that current passes through conductors, and does not pass through dielectrics. If it did pass through dielectrics, then charges would not accumulate or deplete, and a capacitor would not work.

I know you are not from an electronics background or you would have easily grasped this most fundamental concept that CURRENT goes THROUGH all components in a series circuit and can be measured with a single ammeter either real or theoretical.

I learned at a very early age that current can exist in conductors. You have not learned yet that a dielectric is not a conductor.

You can focus on physics concepts as to what may happen to individual electrons in a capacitor and its plates but it is irrelevant to my original statement that you tried to correct. CURRENT goes THROUGH the capacitor, and the wire, and the battery.

Physics and mathematics are the foundation sciences of electronics. If electronics does not comply their principles, then it is wrong. I have never back-pedaled on my assertion that you are wrong to think that current passes through a capacitor.

When I called you out on it you backpedalled once by trying to change to the word "charge" instead of current, and the second time you backpedalled by trying to include definitions of current that also include time.

I stand on my statements that current is charge flow/per time, and current is intimately linked with time.

The fact is that CURRENT flows THROUGH the capacitor, end of story. Whether or not an individual electron does, you can argue out with others because my argument with you has concluded. If you are man enough, feel free to admit you were wrong without any juvenile "yeah.. BUT" disclaimers.

I have said no to that premise may times, and gave reasons why I think it is not true. I also explained why your reasons are incorrect. I do not admit I am wrong.

Ratch

 

[MrAl]

Hi Ratch,

Did you get to read my previous post?

 

[Ratchit]

MrAl,

Did you get to read my previous post?

Yes, I did. I was trying to get the ε*µ*dE/dt term to come out to something, but I could not make it happen. When you said "current density", didn't you mean just current? Anyway, as an attachment, I am enclosing the capacitor problem and solution from my book on electromagnetics. Notice the surface integral of the electric flux density has to be calculated to get the displacement current. Hope this helps.

Ratch

 

[killivolt]

Ratchit

I was just pointing out that "MR,RB" had concluded his statements. He was no longer interested in raising more points with "Ratchit". I think he may have thought it was more banter than discussion.

Although, there are some ramifications when you include your statements on the subject. He may want to conclude with you.

I think this is absolutely the best part of Physics though. When a Physical Phenomenon brings about idea's, to later be applied from a Mathematical extraction and put into a incubator to be hatched.


Turn up the heat.

 

[MrAl]

MrAl,



Yes, I did. I was trying to get the ε*µ*dE/dt term to come out to something, but I could not make it happen. When you said "current density", didn't you mean just current? Anyway, as an attachment, I am enclosing the capacitor problem and solution from my book on electromagnetics. Notice the surface integral of the electric flux density has to be calculated to get the displacement current. Hope this helps.

Ratch

Hello again Ratch,

Hey thanks for the scans. I'll take a closer look at this today. I have some things to do this morning but later i'll be free to look at this again. Thanks again. BTW, do you have the scan of the equation we have been talking about and if they go about a solution perhaps?
Do they give answers to odd numbered problems too Would be nice really

Yes the equation the way i always see it written is with current density J. Thus the first term on the right is u0*J. The second term is u0*e0*dE/dt, and dE/dt is the change of electric flux with time.
I dont want to point out how i arrived at 'my' solution because then if i made a mistake someone else will make the same mistake and that wont do us any good, so i was hoping for a "second opinion" here. Most of the sites (if not all) never seem to mention units, and that is strange because that is always a good sanity check when we want to check our results for something in physics.

 

[BrownOut]

Yeah, those scans are nice. They show that displacement current is equal to conduction current for the capacitor case ( and use the correct terminology ) just as jlcox, MrRB and I have been saying all along. It's nice to be validated.

 

[Ratchit]

MrAl,

BTW, do you have the scan of the equation we have been talking about and if they go about a solution perhaps?
Do they give answers to odd numbered problems too Would be nice really

I think I have just the thing you are looking for. These two pages are from a book called 2000 Solved Problems in Electromagnetics by Syed A. Nasar, published by McGraw-Hill, in connection with Schaum's Solved Problems Series. This book should be readily available in any technical or college bookstore. Although this is not a book for teaching, I have never seen any publisher who shows solutions as concisely as Schaums does. Look especially at solved problem 4.40 . It shows the derivative of the electric flux displacement (D) as the displacement current density. They are careful to differentiate it from conduction current density (J). Naturally, one will have to perform a surface integral around those two quantities to get the currents. In spite of showing how to calculate the displacement current, the book never says that displacement current is real or has a physical meaning. It does say more or less that it was conceived to "patch up" a contradiction of Maxwell's equations.

Ratch

 

[BrownOut]

MrAl,


In spite of showing how to calculate the displacement current, the book never says that displacement current is real or has a physical meaning.
Ratch

This is how you can tell someone is parsing language. Notice the false logic; "they don't say it's real." Of course they don't say it isn't real either, which is more important, because something that's real doesnt' need to be validated. If it's real, then it's real. If they thought it wasn't real, then they would say so, and not even publish all the details of it. And clearing up a contradiction proves that it's mathematically sound ( remembering that mathematics is the foundation of science ) and leans more towards confirmation than denial.

 

[Jony130]

Have you see Walter Lewin video lecture about displacement current?
**broken link removed**

 

[MrAl]

Hello again,

Jony:
Im sure Brownout didnt see that one
and i quote:
"Maxwell's solution was to say that a displacement current was flowing, but of course we now know that is not the case".
And now there we have an MIT professor stating out right that Maxwell was wrong, wrong about that ONE thing.
It doesnt mean that we ignore the idea of displacement current or that we necessarily have to apply that to every case, but it does mean we cant take it too seriously in the case of the vacuum capacitor.

Ratch:
Thanks again, i'll take a look.