EEVblog #486 - Does Current Flow Through A Capacitor?

[JackOfVA]

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.

That's not quite right. In the near field, you have reactive coupling - due to capacitive and magnetic field coupling between the antennas - as well as some electromagnetic field.

The far field assumption is that the reactive coupling components of energy transfer are sufficiently attenuated to be negligible. Yes, this then becomes a homogenous field, at least over some small area, but a key element of the far field simplification is that you don't have to worry about the inductive and capacitive coupling between the two antennas.

 

[ejeffrey]

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

This.  Current is a mathematical concept, and as such we are free to define it however we like.  It turns out that 99% of the time, the only useful thing to call current is the total current (charge motion + displacement current).  Taken individually, those concepts are almost entirely useless for describing any sort of electrical behavior, except in the limit where one of them is zero.  In every equation you have ever seen, the symbol 'I' means the total current.

Radiation doesn't look like current flow because the displacement currents are transverse to the propagation direction.  The the current flux through, for example, a plane separating the emitter and receiver is zero.  But if you have a volume of otherwise empty space with radio waves traveling through it, then yes, the current density is non-zero.  That is the difference between near- and far-field.

Every physicist I know regards displacement current as 'just as real' as charge motion.  Electrical engineers probably don't think about it as much, but they implicitly agree because all of the equations they use to design and describe circuits treat the two as equivalent.

Finally, as a trivial point, the bulk of the current in most capacitors is actually due to microscopic charge motion.  The relative permitivity of most capacitor dielectrics is much greater than 1, so the majority of the current comes from reorienting microscopic dipoles withing the dielectric.  If you use the version of maxwell's equations for dielectrics (the one that includes H and D fields instead of just B and E), that would be a displacement current, but it is due to physical charge motion.

[NiHaoMike]

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?

Better known as DC balancing a signal. That is obviously required for channels that are inherently AC coupled, but even DC coupled channels sometimes have to use it as too much low frequency content can "charge up" the transmission line (remember, it's not ideal) and cause ISI.

[KedasProbe]

You shouldn't care too much on the naming of things, just understand what is happening.
You could say: If the magnetic field from the change of the electric field was discovered before they found the magnetic field around a current, then maybe Maxwell would have named that term (the current in a wire) a "Displacement Electric Field"

[Poe]

Just trying to see if I understand Dave's video and the reality of the situation.  I admit to being a neophyte and not yet reading the entire thread.  My take on it so far..

There are not two types of current.  Current is simply defined as the the flow of electric charge.

The equations used in Dave's video do not imply "through", they imply "into/out-of".

The phrase "Displacement current" appears to only be used as Maxwell's way of explaining why Amperes law doesn't discount the electric/magnetic field link.  Specifically, current isn't the only thing that is needed for a magnetic field to be created.  The change of flux in a capacitor is equally adequate.  This changing flux is "displacement current".  Just because it has the word 'current' does not mean it's a different type of current. 

Current flowing 'into and out-of' is not the same as "through" contrary to what Dave is saying in his videos. It appears he doesn't understand the concept and is using the "it's too complex for me to explain, trust me" technique to brush away the topic.  Not to be rude, this is just my interpretation of the information I have available and my own limited knowledge.  I'll edit this post after reading the posts from other more educated members.

Let me say thanks Dave.  I truly appreciate the videos, forum and the conversations they start.

[duskglow]

I think we were just trolled.

[PeteInTexas]

The FF videos are shaping up to be fun series.  Awesome work!

So, current flows through a capacitor.  Does Ohm's Law apply through a capacitor?

[digsys]

OHH NOOOO ... it's happening again :-)

[EEVblog]

I think we were just trolled.

Possibly!

I'll bite a bit and say that there are several things I wanted to point out with this video:
1) The term "through" is commonly used and accepted in not only the industry but in teaching as well.
2) There is another "type" of current that not many people know about, and as such there is more than one way to look at the problem, not just the flow of individual electrons.
3) The thought of current flowing "through" the capacitor is commonly how you explain circuit behaviour and design things.

Any of those alone is justification for viewing current as flowing "through" a capacitor.
You can debate the physics until the cows come home. The fact is it is right to say and think that current does flows "through" a capacitor.
And there is theory to back up assertion as well.

[EEVblog]

There are not two types of current.

Yes, there are.

  Current is simply defined as the the flow of electric charge.

As a one-sided simplistic definition, yes. Otherwise, no.

The equations used in Dave's video do not imply "through", they imply "into/out-of".

No, they don't. They do imply "through".
Where in those high level equations does it imply individual electron charge buildup on the plates etc?
The high level everyday equations most certainly do imply "through" and that is why is is thought of that way and why through is the most common industry term.

[AlfBaz]

If I pick up static in one room, walk into the kitchen and discharge it by touching the sink, did we just witness current flow from one room to the next?

[madires]

If I pick up static in one room, walk into the kitchen and discharge it by touching the sink, did we just witness current flow from one room to the next?

If that walk is caused by your wife watching her favoured TV show we may call that a displacement current :-)

[Robomeds]

Not entirely on topic but this was a general area which I covered a while back in a class on bond graphs. 
http://en.wikipedia.org/wiki/Bond_graph

One of the more interesting parts of the class what a discussion of how inductors work.  In EE we are taught to think of inductors as the electrical equivalent of a mass in a mechanical system.  Current through an inductor has inertia just as a mass has inertia.  This would mean we have electrical kinetic energy.
However it turns out this is not really true as there is no electrical kinetic energy.  Bond graphs are a way of explaining what is really happening.  Please note that I'm doing an abbreviated description here so I will probably miss a step.  Basically the electrical current is turned into a magnetic field in the inductor.  In that magnetic field we get a change in flux (the magnetic equivalent of capacitance).  It turns out the inductor is to the magnetic domain what a capacitor is to the electrical domain.  Thus an inductor is actually a FLUX CAPACITOR!  My professor seemed to think nothing of stating as much in the course of the lecture but I could help but snicker and think of 1985.

[IanB]

Current through an inductor has inertia just as a mass has inertia.  This would mean we have electrical kinetic energy. However it turns out this is not really true as there is no electrical kinetic energy.

There is stored energy though.

I mentioned earlier in this thread how Newton's second law may be simply expressed (for rectilinear motion) as:

F = m dv/dt

where F is force, m is mass and dv/dt is acceration; all being scalar quantities in the straight line motion case.

For a perfect inductor there is the similar equation:

V = L dI/dt

Here V is a "force", L is an "inertia", and I is a quantity that changes in response to an applied force.

The analogy continues. The stored kinetic energy in a moving body is given by:

E = ½mv²

And the stored magnetic energy in an inductor is:

E = ½LI²

Kinetic energy is to massive bodies as magnetic energy is to inductors. Note that it is not the velocity that holds the kinetic energy but the massive body, and it is not the current that holds the magnetic energy but the inductor.

[Robomeds]

Yes, energy is stored.  It is stored as magnetic potential energy rather than electrical kinetic energy (which doesn't actually exist). 

[merlinb]

The term "through" is commonly used and accepted in not only the industry but in teaching as well.

This is true, if a little unfortunate.
Strictly speaking, electric current is not 'made' of anything, and therefore is not a physical 'thing' that can flow anywhere.

Electric current is the rate of change of charge, or the rate of change of electric flux (flux and charge are the same thing to engineers; physicists use a slightly different definition).

i = dq/dt

Electric current is therefore an entirely abstract, mathematical concept. It is scalar. Nothing more than a number that exists wherever you choose to measure it, whether that is in a wire or between the pates of a capacitor.

Electric current cannot 'flow' from A to B any more than wind speed can be said to 'flow' from east to west. It is more correct to say the current in a component, rather than through it. Saying that current 'flows' 'through' something is just casual language that we get used to, as long as you're not thinking too hard about the reality of the thing.

[John Coloccia]

Well, if we're going to really get into the minutia, the Ampere is defined by the force between two parallel wires, and is not an abstract quantity at all.  It has definite units.  Something like the fine structure constant is a pure number.  It has no units.  Assuming the laws of physics apply everywhere in the Universe, anyone describing the fine structure constant will come up with precisely the same number.

So it's definitely not just a number.  In some sense, it's a way of considering the current density when J is uniform.

[tru]

Electric current is the rate of change of charge, or the rate of change of electric flux (flux and charge are the same thing to engineers; physicists use a slightly different definition).
i = dq/dt

I would have thought that electric current is the flow of charge (movement of charge propagated by a medium), and that is measured by it's flow rate by the unit called ampere?

Electric current cannot 'flow' from A to B any more than wind speed can be said to 'flow' from east to west. It is more correct to say the current in a component, rather than through it. Saying that current 'flows' 'through' something is just casual language that we get used to, as long as you're not thinking too hard about the reality of the thing.

You've cheated there!    Your analogy is more correct as wind current, but that can be said to 'flow' from east to west.

[IanB]

Electric current is the rate of change of charge, or the rate of change of electric flux (flux and charge are the same thing to engineers; physicists use a slightly different definition).
i = dq/dt

I would have thought that electric current is the flow of charge (movement of charge propagated by a medium), and that is measured by it's flow rate by the unit called ampere?

There's a subtle difference in physics or applied mathematics between a flow and a rate of change, even though both have the same dimensions.

In analysis, a rate of change is measured within a control volume (or at a point if expressed as a density), whereas a flow is measured across a control surface (which may enclose the control volume).

Both flows and rates of change occur together in equations when time is involved. It invariably happens that change in accumulation within a control volume occurs when there are flows in or out of that volume (or, for completeness, sources or sinks within the control volume).

[Eight8]

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

If you are in the USA. 

[KedasProbe]

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

If you are in the USA. 

or you can google PBS_The_Mechanical_Universe_and_Beyond.part1

[philaburns]

Found this and thought is summarised the answer quite elegantly :-)

[IanB]

Why did you put this at the end of a dead thread instead of starting a new thread?

[Brumby]

This thread was originally started to discuss Dave's video.

Your post was to present a particular application using a capacitor.  Certainly, capacitors and related math are common between the two - but thereafter the discussions diverge ... dramatically.

By that logic, once someone started a thread about an oscilloscope, then all other discussions about oscilloscopes can be tacked on.  This blends multiple topics and makes it harder to sift out what is of interest to a member and what is not.

You are starting a new discussion - not extending an old one - so it really belongs in its own thread.


The other, far more relevant point, is that by tacking onto the end of an old thread, people aren't going to pay as much attention to it - and you will lose the attention of a lot of people that may just have some very useful contributions to offer!