EEVblog #486 - Does Current Flow Through A Capacitor?

[wbeaty]

And I learned that in every discipline, there's always *someone* who insists on splitting hairs in such a way that newbies will get hopelessly confused if they listen to that person.

Yeah, that's one class of internet troll.   They're trying to appear expert by playing the "oneupmanship game."

On the other hand, I frequently find myself in just that role, because often the conventional jargon needs to be avoided because it leads to beginners' misconceptions.  Or sometimes the beginners obviously learned some grade-school science which is simply wrong, and their unsuspected learning barriers could use some debunking.

In that case I must switch into pedantic nitpicking physics teacher mode, not because I'm trying to be a self-important troll, but because I've seen the harm that the bad language can do to students' concepts.  I can attack it and head it off early:  clarify with rigorously correct and clearly-defined words to shatter the learning barriers even before anyone runs up against them.

For example, terminology:  "electric current" is commonly defined the same as current.   Electric current isn't usually defined to mean electron flow except in this thread.   Perhaps "electron current" was meant?

But what we're really talking about here are currents ...versus "charge flows."     Currents include electron flows, ion flows, proton flows, charged particle beams, and displacement currents.  If something creates a closed loop of  magnetic field, then it's an electric current.    But when discussing these issues, the term "charge flow" is a bit better than the term "electric current" because charge-flow specifically excludes displacement current, while "electric current" doesn't  ...unless we SAY it does, and unless everyone reads the first messages and then adheres to the definitions there.

To cut through any BS and put it as clearly as possible, capacitor dielectrics do support "electric currents," even though there is no need to have any "charge-flows" in those currents.

Or simplicity from a non-theory standpoint: if a Rogowski probe (a clamp-on ammeter) says that there's a current through the center of its ring, then indeed there's a current, even if that current is entirely made of a vacuum with a changing radial pattern of e-fields.  Clamp-on ammeters measure charge-flows in wires and displacement-currents in empty space, and they can't tell the difference.   Displacement current is real.   But it's not charge-flow.

PS

Another issue:  are people curious about what REALLY happens inside capacitors?  Delve deeply?  Confront actual physics, remove the mysteries, yet try to do it without a pile of equations?   If so, that's a different thread than one about circuit design, or about the One True Meaning of some piece of jargon which is defined totally differently by scientists, engineers, and the public.

[wbeaty]

In order to derail the thread even further, let's take a minor detour into the subject of language.

Abbot and Costello attempt to clarify basic power-transmission issues:

WATT IS A VOLT?

[vk6zgo]

The point is current is not defined as a movement of electrons. That is best left to thick headed children or people with an MFA.
You mean as distinct from members of the Flat Earth society who still persist with Conventional Current Flow?

Consider a wet cell, lead acid battery, lemon with a nail and coin, etc. No electrons are passing, internally, between the terminals, yet a current is flowing.

Actually,positive ions which  still have electrons in their outer shell are passing internally,as part of the chemical reaction.

In an ideal electrochemical cell,there is no such flow except when electrons or pretend positive charge carriers are involved with the flow of current in an external circuit.

[NiHaoMike]

Oooh, i've a good question.

Why we can't let an ac 3kw line pass throught a capacitor?

I've seen it only for signal

In switching power supplies, it's pretty common to use series capacitors with half and full bridges.

[ftransform]

Did you know that you can actually break a circuit by feeding a unbalanced AC signal into it? If you send a binary signal into a capacitor, that is +1, -1, -1, -1, -1, +1, you create a electron vacuum in the capacitor, and you have to transmit several more positive pulses before you can unblock it?

[KedasProbe]

Nostalgia, in the time when they properly explained things
I looked for it on the internet and luckily I found it

In my time I recorded all episodes on VHS tapes (I treated it as my holy grail )

edit: I just realized I could have added questions
http://en.wikipedia.org/wiki/VHS

[EEVblog]

I looked for it on the internet and luckily I found it

Wow, awesome find! 

[John Coloccia]

So I'll ask the question again.  Does current flow from the radio station antenna to the receiver in your car?  Is current flowing from quasars to the radio antennas we use to study them?  Displacement current simply describes...or rather completes....the description of radiation in free space.  I've never heard anyone seriously consider this as "current" of any kind, especially since it doesn't follow Ohm's law.  It doesn't really have much in common with conventional current other than the name, so it just turns into a word game.  It's definitely unconventional to consider current flowing from a satellite to the dish on your roof.

That video is great, BTW.

[digsys]

And that was the end of that ...... :-)

[KedasProbe]

Wow, awesome find! 

You may also like all other full episodes:
http://www.learner.org/resources/series42.html

[John Coloccia]

I recall a series on Calculus that was presented in much the same style.  Very clear and very good.  I looked and I wasn't able to find it, but I wonder if these guys did that too.

[amyk]

But let's talk about current flowing through the dielectric.  What's the resistance of the dielectric?  What happens to the all the Watts you should be dissipating?  Now even more laws fall apart, including Ohm's law.

This reminds me of someone who told me that CCFL inverters can be tested using a capacitor as a dummy load, and he said it didn't get hot at all... didn't believe him at the time but now that I think about it, it must be related to this?

[IanB]

This reminds me of someone who told me that CCFL inverters can be tested using a capacitor as a dummy load, and he said it didn't get hot at all... didn't believe him at the time but now that I think about it, it must be related to this?

The lack of heat is because no power is being dissipated in the capacitor.

You figure out the power consumed by a device by summing up the product of voltage and current over every instant in time and looking at the average value of that product over a sufficiently long time period relative to the AC frequency. (Mathematically you could do it over one complete AC cycle.)

If you do this computation for a capacitor you find the current is out of phase with the voltage in such a way that the total power is zero. (The voltage wave and the current wave are 90 degrees apart, with the current leading the voltage.)

[Ericho]

Just thank you Dave.

Another useful tool for me     

[ashplant]

Great video!

I don't actually disagree. If you define current as, "anything that makes an Ammeter twitch.", current does flow through an ideal cap. But I define current as "flow of charge". 

I say, "Charge doesn't flow through an ideal capacitor, though it does flow through ideal capacitor leads."

In the dim mists of time, when I was a kid, I was really impressed when a teacher told me how you can get electromagnetism from relativity. Something about how, in a wire with Voltage across it, charge cancelled, but electric fields don't quite. He said that protons (which on average are stationary) have symmetric fields a' al Coulomb, but the moving electrons have asymmetric electric fields courtesy of Lorentz contraction, Then he did some algebra and got
results you usually get with fields here and fields there and right hand rules. I thought the advantage to his approach was it didn't make my head hurt.He said the advantage was you don't  need "ad hoc" notions like displacement current.

[wbeaty]

So I'll ask the question again.  Does current flow from the radio station antenna to the receiver in your car?

Obviously not.  Waves are involved, so displacement currents don't connect between distant antennas in the way that they between connect capacitor plates.

Instead you could have asked: are there any electric currents (displacement currents) associated with propagating EM waves in empty space?

[duskglow]

Ah, but couldn't the space between the radio station antenna and your car be seen as a very, very low value capacitor?  I mean, the fields would be so small as to be vanishing, and for all intents and purposes are zero, but wouldn't they actually be an extremely small (many zeros to the right of the decimal point) value?

Or past a certain distance does the dielectric become perfect?

[IanB]

So I'll ask the question again.  Does current flow from the radio station antenna to the receiver in your car?

Obviously not.  Waves are involved, so displacement currents don't connect between distant antennas in the way that they between connect capacitor plates.

Instead you could have asked: are there any electric currents (displacement currents) associated with propagating EM waves in empty space?

I think it is fair to say that the induction of current between the windings of a transformer and the induction of voltage between the plates of a capacitor are two dimensions of the same theory that allows electromagnetic waves to propagate through space. For instance, if we take the primary and secondary windings of a transformer and gradually move them further and further apart, at what point does the apparatus cease to be a transformer and start to be a transmitting/receiving pair of antennas?

The answer is that there is no defining distance as such. When the coils are close together they are very "transformery" and when they are far apart they are very "transmittery". Somewhere in between they are a bit of one and a bit of the other.

What is actually propagated through the air in electromagnetic waves is energy, and the energy waves/particles can interact with matter to induce voltages and currents in the things that they touch (e.g. a radio antenna or a solar cell).

[duskglow]

The answer is that there is no defining distance as such. When the coils are close together they are very "transformery" and when they are far apart they are very "transmittery". Somewhere in between they are a bit of one and a bit of the other.

I idly wonder what properties such a hybrid would have.

[IanB]

Ah, but couldn't the space between the radio station antenna and your car be seen as a very, very low value capacitor?  I mean, the fields would be so small as to be vanishing, and for all intents and purposes are zero, but wouldn't they actually be an extremely small (many zeros to the right of the decimal point) value?

Or past a certain distance does the dielectric become perfect?

Remember that "displacement current" is not a physical thing. It is the name of a term in a mathematical model that helps when using the model to understand and explain things.

There is of course a vanishingly small capacitance between two very distant objects, but this capacitance is not the explanation for how radio transmitters work. Once the distance is large enough the capacitance disappears in importance and electromagnetic waves grow in importance.

Consider an analogy with water.

If you take a small tub of water and you use a paddle to move the water at one end, you will see the water immediately move at the other end and overflow the tub. This is "capacitance". Any movement you make at one end of the tub immediately induces a similar movement in the water nearby.

Now try the same experiment in a lake. If you move the paddle in the water on one side of the lake, the water will not move at all on the other side. However, all is not lost. If you move the paddle back and forth in a vigorous manner you may make waves on the surface of the lake, and these waves may travel to the other side where they can be detected. No water has moved across the lake with these waves, only the energy of motion has been carried across the lake. This is like radio transmission.

[John Coloccia]

Exactly, Ian.  Displacement current is a necessary part of radiation propagating without the help of charge carriers scooting along.  That's the only point that I'm trying to make.  It's not a phenomenon that has to do with capacitors.  It's a phenomenon necessary to completely describe capacitors, but only because you need to consider the magnetic field in the situation when you have exactly NO real current.  That's the crux of it.  Without displacement current, you don't have radio.  If current flows through the capacitor, then current also flows from the radio station to your car.

That's the only point I'm trying to make.  I really hope I'm not ruffling feathers.  I don't mean to, but I want to link these concepts in a way that makes real sense and is consistent.  I know people have called me pedantic, but when we're talking about basic components and start invoking Maxwell, I think the point is to get a little pedantic and really have an interesting conversation about different implications each point of view has, and where each is useful...and were each breaks down.  I'm just probing the edges.

[JackOfVA]

The answer is that there is no defining distance as such. When the coils are close together they are very "transformery" and when they are far apart they are very "transmittery". Somewhere in between they are a bit of one and a bit of the other.

I idly wonder what properties such a hybrid would have.

If you study electromagnetic theory and antennas you will find that the coupling between two antennas is often expressed in a power series involving 1/rⁿ where r is the distance between the two antennas in terms of wavelengths. Antenna engineers usually describe the "far field" as the distance beginning where the field strength (in volts/meter or Amperes/meter) is dominated by the 1/r component of the power series (energy in this case is 1/r²) and the "near field" as the distance where this simplification no longer applies and the coupling between the two antennas is a combination of radiated, inductive coupled and capacitive coupled modes. 

You can also find  evanescent wave creation from the antenna to air interface discontinuity.

These relationships are incredibly complex to model with closed form solutions to Maxwell's equations in the general case and the numerical simulation tools developed in recent decades make life much easier for the antenna engineer.

[IanB]

If current flows through the capacitor, then current also flows from the radio station to your car.

Everything is OK until that sentence. The one does not imply the other.

We can say that current flows through a capacitor because as a two terminal device, any current flowing in or out of one of the terminals is exactly balanced by the same current flowing out or in at the other terminal. If the current flows in to one terminal and out of the other, then the current must have flowed "through" the capacitor.

To see why this is not true of radio transmission, consider a transformer, which is a four terminal device. When current flows through the primary winding, current can be induced to flow through the secondary winding. However, the two windings are isolated. The primary winding satisfies a current balance (in = out), and the secondary winding also satisfies a current balance (in = out). Consequently, no current flows between the primary and the secondary.

Radio transmission is more like a transformer than a capacitor. The transmitting antenna satisfies a current balance and the receiving antenna satisfies a current balance. However, no current flows from transmitter to receiver.

[John Coloccia]

Oh, now this is getting interesting.  What's really funny is that it's almost easier to show that displacement current implies a real current between transmitter and receiver than it is to show that there is none   You need to have displacement current to have waves propagating in free space.  It's a bit technical, but before Mawell you had an interesting situation where....damn, how do you say this in plane language?  Maybe you say that a jiggling electric field couldn't affect the magnetic field, and thus there was an inconsistency in the laws.  The displacement current is the term that allows the field itself to give rise to the magnetic field, and thus allows all of the laws to be consistent in free space even if you have no charge whatsoever.

Now we're getting to the nitty gritty.

edit:  and the whole thing about 1/r^2 is really just talking about when you're far enough away that you can consider something to be a point source and everything between you and it is a straight line.  That's my physical intuition about it, anyway.  You're so far away that that everything looks like a very simple, textbook, homogenous field.

[IanB]

It might be easier to go the other way and say that "real current" has no more actual existence than "displacement current".

What we have is a mathematical model of the physical world, and in this model we have terms which we give names like "voltage", "current", "magnetic flux" and so on. It turns out that this model accurately describes behavior we observe in the real world, so we say it is a good model.

But just because the model works, it does not mean that the things the model describes have any actual, real existence. We can't see electric current, we can only know of its effects. Because it has observable effects we assume it is there. This assumption is useful, so we hold onto it.

When it comes down to it, nobody really knows how electromagnetic fields work, or what they are made of. You don't need displacement current to enable electromagnetic waves to exist. Electromagnetic waves existed long before humans ever tried to model them. Displacement current is just an abstract mathematical quantity in a bunch of equations that happen to fit the data.