I would only note that the current is rightly explained as a " fictitious displacement current ID "flows" in the vacuum equal to the "real" current in the wires

Hi,

After i posted my last reply i *knew* someone was going to drag this into the discussion

That's ok, but i must warn that the physical viewpoint is a separate argument from the purely electrical circuit analysis view and that it will lead to confusion. We see this kind of contradiction once in a while though.

The two views are:
1. The displacement current view is based on an internal view of the capacitor and how ti behaves as near to physical reality as possible, with measurements made from inside the capacitor body.
2. The circuit analysis view, which is an external view based on how it behaves in a purely electronic way, with measurements made from outside of the body of the cap.

Because the two different views are based on different premises, they usually can not be used to exactly explain anything outside of the view on which they were based. In other words, we cant use displacement current to explain how an AC coupling capacitor works in the electronic view because in the electronic view we view current as actually passing through the capacitor with no field effects. And when we try to explain the displacement current using the external electronic view it makes it look like the displacement current is an actual electron flow of current.

For these reasons i suggest that we always keep each view within it's intended reach and not try to mix the two.

In circuit analysis, it is acceptable that electron current flows though the cap and that charge accumulates. In the purely internal physical analysis neither of those two are acceptable because charge accumulates on one plate and diminishes on the other plate and electron current does not pass through the cap.

It is very interesting that we see such a difference, but that's how some things are. That's the state of physical science even in the current year. It sometimes separates ideas in order to form workable conclusions even if all the facts are not yet considered, and sometimes still has trouble finding a unifying theorem that really does include all the known facts.

I'm sure we could talk more about this and probably will, but in the interest of circuit analysis alone we should stick to the circuit analysis view.

Here's one immediate problem we run into with the electronic view if we try to analyze with the internal physical view:
"An AC coupling capacitor blocks DC current and passes AC current".

and in the physical view we have:
"Electron current does not pass through the capacitor".

So one argues that some types of current can pass through, while the other says that neither can.
That's what happens when we try to mix paradigms

 

Copied from Post 26:
So we cheat a little in the definition of the second case but once that current is flowing, it is a DC current and does not change until we shut off the circuit again, and we must turn off the circuit at some point, making it really a pulse of current not a DC current strictly speaking. or else the voltage would climb too high and thus blow out the cap. Theoretically we could leave it on indefinitely but then we would have to conclude that the circuit was unstable because of that infinite response.

Oh really???

Ok, I won't bother to pursue this any further since i see so many strange understandings of how a capacitor actually behaves in DC and AC circuits.

Hi,

Well i could ask how much formal training you had in this area, but i would also ask how much you studied on your own because i think that's great too. Books are meant to be read either in or out of a classroom

I am not entirely sure what issue you are asking possibly for a better explanation with your emphatic "oh really" question as that implies a certain amount of doubt as to the credibility of the target expression.

But hey, that's why we are all here right? To question what we think and see and try to find a better explanation. So anyone who has a problem with some statement that does not seem plausible should ask more about it and see if it is right or there is something wrong with it.

If your question was about the DC current being AC because it was turned on and then later turned off, the answer to that can be found in a Fourier analysis which will show there is some AC component. But even without that, surely you must agree that there must be something different about a regular DC current that has been flowing since t=minus infinity and will flow to t approaching plus infinity and a current that has been switched on at t=0 and will be switched off at t=1 second, and if we just extend that time a little we have it switched on at t=0 and maybe off at t=1 year. Either way, 1 second or 1 year, there is something different, and that difference shows up as an AC component making it, strictly speaking, not anymore DC but DC plus AC.

In your coupling cap example, the current charges the cap for a seemingly finite time, and that is how we take it to be true even though that is not entirely correct because of the way exponential responses are, but even if we take it to be completely charged at the end of say 5 seconds, that is still a different experiment than if we charge it with a CONSTANT CURRENT source. A constant current source keeps charging and charging a cap with the same current level as when it first started, and will continue to charge it until we shut it off or the cap blows up. There is nothing wrong with the cap, it's just that a constant current behaves differently than a current that charges a cap for a short while in an audio amplifier where we have an AC coupling capacitor.

But we can discuss this in PM's if you rather do that.

Check out the post just before this one if you care to think about the different views of an analysis of a capacitor in a circuit or just the internal view of a cap.

 

Analog Kid, AC(Ok I won't call it AC Current) is a time varying voltage as applied to a capacitor.

I know what AC is.

I never claimed caps pass current.

Neither did I.

ak

 

Further to the above. Have any of you come across a variable tuning capacitor as used in the days of analog radios.

Yes, back in the 50's. In the 60's I "repurposed" part of one from an All-American-Five to make a crystal set that tuned with a knob rather than with a slider on the antenna "coil". I still have that radio, and a small collection of air caps in my inventory.

The most common variable tuning caps are dual-gang. Do you know why the two caps are of different sizes?

ak

 

Put a capacitor across an ohmmeter. It reads near zero ohms at first, then builds to an open. Why? IT IS CONDUCTING A DC CURRENT.

No, it is conducting a *transient* current; or, if you must, a transient DC current. That is not the same. Transient and non-transient conditions are not the same, ever. Understanding this is critical to successful circuit analysis.

BTW, even the term "transient DC" is misleading. The transient actually consists of a series of sine waves of different frequencies, decreasing in amplitude as functions of both frequency and time. The peak amplitude of the lowest frequency sine wave is *greater* than the peak amplitude of the composit transient waveform.

ak

 

I'm pretty sure Mr Al is not confused with your examples.

*s*n*a*p*

 

Hi,

After i posted my last reply i *knew* someone was going to drag this into the discussion

That's ok, but i must warn that the physical viewpoint is a separate argument from the purely electrical circuit analysis view and that it will lead to confusion. We see this kind of contradiction once in a while though.
...
So one argues that some types of current can pass through, while the other says that neither can.
That's what happens when we try to mix paradigms

I concur exactly which is why I made this statement in post #33

Yes, it's true, a electronic capacitor because of the usual very close proximity of the plates acts much like a strange conductor so I really don't have a problem with 'current flows through a capacitor' but the actual physics are important too when you get beyond circuit theory in circuits.

 

No, it is conducting a *transient* current; or, if you must, a transient DC current. That is not the same. Transient and non-transient conditions are not the same, ever. Understanding this is critical to successful circuit analysis.

BTW, even the term "transient DC" is misleading. The transient actually consists of a series of sine waves of different frequencies, decreasing in amplitude as functions of both frequency and time. The peak amplitude of the lowest frequency sine wave is *greater* than the peak amplitude of the composit transient waveform.

ak

Yes. A DC current or voltage need not be constant but to be defined ad DC the direction of charge carrier flow must never reverse. By my description I am clearly talking about a transient impulse, though the duration can be very small to incredibly large.

Fourier analysis is just that, a method of analysis. It is no more true or fundamental that any other abstraction of the real world. However, in this case is it absurd to analysise an RC charging using Fourier; the problem is best described in the time domain where one sees a pulse of current always in the same direction, hence DC.

Should one do a Fourier analysis one would find a term with no frequency, or the DC component of this DC signal. Zero frequency, fixed value, though the value will change depending on the time span inspected.

The only way to get the DC term to disappear is to integrate from - infinity to + infinity, but then the entire event would also wash away, as would you and me and the entire universe too.

 

Fourier analysis is just that, a method of analysis. It is no more true or fundamental that any other abstraction of the real world.

I think the engines on an F-15 would disagree.

ak

 

I concur exactly which is why I made this statement in post #33

Hi there,

I had a feeling that could be a reason you brought that up. In my haste to explain things sometimes i forget to mention everything, so i think i should have at least said something about that. Note that i wasnt complaining though and i did indicate that a little when i said "That's ok"
So i was both agreeing with you and showing how each one has it's proper place. That's the way i understand it

 

Yes. A DC current or voltage need not be constant but to be defined ad DC the direction of charge carrier flow must never reverse. By my description I am clearly talking about a transient impulse, though the duration can be very small to incredibly large.

Fourier analysis is just that, a method of analysis. It is no more true or fundamental that any other abstraction of the real world. However, in this case is it absurd to analysise an RC charging using Fourier; the problem is best described in the time domain where one sees a pulse of current always in the same direction, hence DC.

Should one do a Fourier analysis one would find a term with no frequency, or the DC component of this DC signal. Zero frequency, fixed value, though the value will change depending on the time span inspected.

The only way to get the DC term to disappear is to integrate from - infinity to + infinity, but then the entire event would also wash away, as would you and me and the entire universe too.

Hi again,

You make a lot of sense of course, but you also should realize that the absolute truth is sometimes absurd but even so that is what we seek nonetheless. The reason we might do this is so that we can explain every single case that could ever occur, and we then always have the luxury to digress into an approximation of some sort. Having taken the approximation as truth first however, we can not go back to the absolute truth because we rounded some of the necessary information away.

In this case even if we assume a pulse that is a full 1 year long (365 days approximately) there is still an AC component. The fundamental frequency will be very very low, but still there, and of course there are the harmonics which go up towards infinity. That of course is the full analysis which we almost never use in the practical sense because most of our signals happen much faster or look like they are always present. The absolute truth of the matter however is that if the signal did not exist forever then there is some AC component, which we usually ignore but is still there. Does it matter? Most of the time probably not, but when we talk about charging a capacitor with a constant current we note that there are limits and those are the limits that make it either practical or not practical and lie mostly in the pure theoretical domain.

 

Hi there,

I had a feeling that could be a reason you brought that up. In my haste to explain things sometimes i forget to mention everything, so i think i should have at least said something about that. Note that i wasnt complaining though and i did indicate that a little when i said "That's ok"
So i was both agreeing with you and showing how each one has it's proper place. That's the way i understand it

Cool.

It's valuable to understand that not everything is a conduction current (usually moving slowly in wires). There are drift currents (particle accelerators, tubes, semiconductors, etc with some sort of velocity factor) and displacement 'currents' (changing electric flux over time behaves like a conduction current.)

 

Peanut gallery here:

And here's the funniest take on this you've ever seen. Agree or disagree with me, this is how I see capacitors in a DC blocking mode and in a DC filtering mode. The capacitor (like the diaphragm) fills to capacity or to current pressure level. Exceed the pressure capability of the diaphragm (or cap) and it will burst. But for the moment it is filling air is being displaced at the rate set by the valve (resistor). As the diaphragm fills to capacity the outflow (a.k.a current) diminishes till the pressure equalizes.

In the DC filtering, the constantly varying regulator simulates DC ripple. The diaphragm simulates a filtering capacitor and smoothes the output of air flow (a.k.a. current).

My drawing ignores the return circuit (ground) - obviously.

 

Tonyr1084, your soda bottle example is correct. Note that in the blocking example NO air from the "soda bottle" goes to the output through the diaphragm.
Also in the filtering example NO air from the "soda bottle" goes through the diaphragm

 

Note that in the blocking example NO air from the "soda bottle" goes to the output through the diaphragm.
Also in the filtering example NO air from the "soda bottle" goes through the diaphragm

AC or DC, no electrons pass through the capacitor. AC is just DC going in one direction at one moment then going in the other direction in the next.

As the diaphragm fills and distorts to one side (good analogy of what happens to electrons in orbit in the substrate between the plates) momentarily there IS a movement of some kind. Electrons still don't pass through, but there IS a current as the balloon fills. When the DC reverses (actually the AC) the stored pressure in the balloon adds its pressure to the applied pressure stream (electric pressure - a.k.a. voltage). AC appears to pass through but it's just the flexing of the membrane within the diaphragm that appears to pass current. In either case, AC or DC, no electrons pass through. It's electric pressure that either soaks up or adds back its potential.

 

Sorta like a trampoline for electrons.....huh.

Edit: kinda reactive too.

 

As for the challenge of proving electrons pass through a capacitor - take a 9 volt battery, a 12 volt lamp and a really really big capacitor. Ground one end of the capacitor to the negative lead on the battery and wire up the rest of it. Notice that the light bulb lights up temporarily. Eventually the light extinguishes because the capacitor reaches its capacity. HEY! Those two words come from the same latin root.

Now, reverse the capacitor. The light lights up and glows even brighter. WHY? It would appear current is passing though the capacitor. Really, it's not. It's just charging up and discharging then charging up in the opposite polarity. Yeah, exactly like a trampoline.

 

Tonyr1084, Take a bow. Your analogy and descriptions are spot on. I hope you won't mind if I use them if ever required in the future. Thanks!

 

Tonyr1084, Take a bow. Your analogy and descriptions are spot on. I hope you won't mind if I use them if ever required in the future. Thanks!

Whatever.

 

AC is just DC going in one direction at one moment then going in the other direction in the next.

True, but - I think Faraday (and Ampere (and Fourier (and Heavyside))) might think "just" is a bit misleading.

ak