EEVblog #486 - Does Current Flow Through A Capacitor?

[firewalker]

It is Ohm. Thats why in Greek it is . If it was Om it would be O. About 4000 years ago, until the more recent past, Qmega had a different pronunciation from Omicron.

Alexander.

[John Coloccia]

Even worse than current flow in capacitors is the flow of holes in semiconductors. These behave like electrons with negative mass! Even more weird are phonons which knock electrons around and are quanta of crystal vibrations.

It's just convenient bookkeeping, though.  Charge comes in quanta of electrons, so lack of charge has to come in quantities of lack of electrons...holes.  It's like talking about negative money.  We all know that simply means "debt", but it's more convenient to handle it in the same units as regular money because then you can do straightforward math to get a straightforward answer.

It's convenient to consider the changing electric field in a capacitor as "displacement current", because it puts it into the same units as regular current, and the resulting field is something that needs to be considered because it can do work....but that's just book keeping too.  It's just a gimmic to get everything using the same units so that you can figure out where all of the energy goes.  He should have called it something like "equivalent current" to make clear that it's not a real current, namely charge flux through a surface.

I guess it doesn't matter too much.  It's useful to consider current flowing through a capacitor but thinking about it like that sets up an intuition that breaks down when you look too closely.  Better to consider the real process and consciously choose when to simplify it.  In my opinion...just my opinion.

Then again, I always do things "backwards".  Whenever I tutor for programming, I don't start with "Hello World".  I start with nuts and bolts explanations of gates, processors and things like that.  Then I work my way up through memory and stored programs.  By the time we get through that, assembly language seems rather tame.  Then we hit higher level language and it's like, "Hey, this is no big deal...what's all the fuss?".

[c4757p]

Then again, I always do things "backwards".  Whenever I tutor for programming, I don't start with "Hello World".  I start with nuts and bolts explanations of gates, processors and things like that.  Then I work my way up through memory and stored programs.  By the time we get through that, assembly language seems rather tame.  Then we hit higher level language and it's like, "Hey, this is no big deal...what's all the fuss?".

OT, I know - but if assembly is confusing and difficult for a programmer, that programmer does not understand computers, which is a bit pathetic. Tedious, perhaps, but not confusing. Kudos to you for starting in the right place, and shame on all these teachers and professors who think they need to start with the flashy stuff to get people interested (if they're not interested now, they'll lose interest again soon). You can't teach passion, why bother teaching something that requires it to someone who doesn't have it?

Not as OT as it sounds, by the way. The same applies to electronics - if you can make an RC filter but can't answer this question for yourself, somebody started you in the wrong place. As wishy-washy as a lot of EEs think lower-level physics is for their profession, I find myself relating things to it with surprising frequency. All you beginners out there - go get a physics textbook.

[IanB]

I guess it doesn't matter too much.  It's useful to consider current flowing through a capacitor but thinking about it like that sets up an intuition that breaks down when you look too closely.  Better to consider the real process and consciously choose when to simplify it.  In my opinion...just my opinion.

We do useful engineering by building abstractions that conceal the complexity of inner layers of detail behind concepts that address what we really care about. Computers are a prime example of this, with logic gates hiding transistors, and processors hiding logic gates, and assembly language hiding 1's and 0's, and compilers hiding assembly language, and so on and so on.

In a similar way, electric current is an abstraction. When we do calculations with it we don't care what the charge carriers are, we just have an abstract quantity measured in "amps" that obeys certain rules and results in good designs if we understand what the rules are. By every normal rule of circuit theory, a current flowing at one terminal of a two terminal capacitor must be balanced by an identical current flowing at the other terminal. If we (or Spice) do not obey that rule, our circuits won't work the way we expect them to.

[dr.diesel]

"Displacement Current Meter"        , good one Dave!

[Matje]

The only real problem with this is that it becomes very difficult to explain how an RC filter works if you simply consider a capacitor as a device that allows current to flow as a function of frequency, and you also loose the subtlety that current lags voltage by 90 degrees.

The differential equation Dave showed (the thing with the 'd's)  captures this perfectly well, for it shows that the instantaneous(!) values really don't depend on the frequency. One must choose the model to fit the application, as Dave said.

Aaahhhh, just thinking about differential equations makes my head hurt...

[John Coloccia]

Hi, Ian.

But you never develop the intuition why adding some other impedance to the mix affects frequency response.  All you can do is plug and chug through an equation to get an answer.  I'm pretty sure SPICE models concern themselves with time constants in order to get the right answer.  I'm not saying that there aren't simple ways to get to an answer without getting into the details, but it's a powerful thing to have that intuition in your back pocket.  That's all I'm saying.  It's really useful if a newbie approaches this by simplifying only after understanding the basic, real process and developing a real gut feeling about what's really going on.

[Matje]

That's why you use complex numbers for that.

Naaaa.

You use complex numbers to find out whether the other guy is a maths or physics person or a proper electronics person.

Because these crazy people use 'i' for the imaginary unit, while electronics people use 'j' as God intended.

Never let a user of 'i' touch your measuring equipment, let alone the soldering iron, for chaos and mayhem might ensue.

Hehehe. ;-)

[firewalker]

What about this capacitor? Does current flow through?

[ChadSeibert]

I guess it doesn't matter too much.  It's useful to consider current flowing through a capacitor but thinking about it like that sets up an intuition that breaks down when you look too closely.  Better to consider the real process and consciously choose when to simplify it.  In my opinion...just my opinion.

We do useful engineering by building abstractions that conceal the complexity of inner layers of detail behind concepts that address what we really care about. Computers are a prime example of this, with logic gates hiding transistors, and processors hiding logic gates, and assembly language hiding 1's and 0's, and compilers hiding assembly language, and so on and so on.

In a similar way, electric current is an abstraction. When we do calculations with it we don't care what the charge carriers are, we just have an abstract quantity measured in "amps" that obeys certain rules and results in good designs if we understand what the rules are. By every normal rule of circuit theory, a current flowing at one terminal of a two terminal capacitor must be balanced by an identical current flowing at the other terminal. If we (or Spice) do not obey that rule, our circuits won't work the way we expect them to.

I totally agree. Without meaningful abstractions in either profession, it's basically impossible to accomplish much. If that means being hopelessly confused some of the time, then so be it. It's the price we pay to work at that level. It doesn't mean that there isn't value in learning the lower level physics required for accurate interpretation of the phenomenon, it just means that depending on what you work on, it may have been better spent learning something else.

[Razor512]

Took apart a capacitor a while back. Had some weird chemical in it (didn't drink any so I am unsure of the flavor...)


Inside of a capacitor by Razor512, on Flickr

[Smithy]

Great video!

A good follow up to this would be the inrush current of a circuit. With maybe USB's inrush current limitations as an example.

[EEVblog]

OT: Love the whiteboard sessions, but please switch to manual exposure when recording. The brightness jumps all over the place.

I do.
The problem is that my camera switches out of manual exposure mode when I switch to playback mode to view a video, I then have to switch it back on and the camera my decide to select a slightly different exposure setting. It's a really annoying bug.

[EEVblog]

I guess it doesn't matter too much.  It's useful to consider current flowing through a capacitor but thinking about it like that sets up an intuition that breaks down when you look too closely.  Better to consider the real process and consciously choose when to simplify it.  In my opinion...just my opinion.

The whole point of the video, which many people still seem to miss (they still seem to think I'm talking about electric/conducted current, when I said half a dozen times I am not), is that I am pointing out there is a different type of current when you look and delve deeply enough into the mathematics of it. So rather than this idea "break down" when you look closer, it actually gets more valid mathematically the deeper down the rabbit hole you go.

[EEVblog]

Achievement unlocked: Inspired an EEVblog episode.

 

[EEVblog]

Don't know if someone already mentioned it, but apparently you can measure displacement current:
Measuring Maxwell's Displacement Current Inside a Capacitor

Sweet.
I was thinking it might be possible with the AIM-TTI current probe if you had a bracket to hold and move it accurately?
Collecting the data = fun
Working the math  to see if you are actually verifying any theory = yuck

[IanB]

Hi, Ian.

But you never develop the intuition why adding some other impedance to the mix affects frequency response.  All you can do is plug and chug through an equation to get an answer.  I'm pretty sure SPICE models concern themselves with time constants in order to get the right answer.  I'm not saying that there aren't simple ways to get to an answer without getting into the details, but it's a powerful thing to have that intuition in your back pocket.  That's all I'm saying.  It's really useful if a newbie approaches this by simplifying only after understanding the basic, real process and developing a real gut feeling about what's really going on.

Actually, instantaneous current balances and Kirchhoff's current law and differential equations really are the deep down low level details of what is really going on. I thought that was what you were advocating people to learn first. But I for one cannot develop much intuition about frequency response at that level.

If you want intuition about impedance and frequency response you have to go up a level and choose a higher level abstraction. That is exactly what AC circuit theory is. It is an abstraction built around frequency, phase angle, reactance, resistance, phase angle and so on. It allows you to develop intuition about how AC circuits work and hides all the gory details of differential equations.

AC theory is full of complex numbers, which is clearly bogus (  ) because real physical quantities are not imaginary and there is certainly no imaginary current or voltage in a real circuit.

[John Coloccia]

I guess it doesn't matter too much.  It's useful to consider current flowing through a capacitor but thinking about it like that sets up an intuition that breaks down when you look too closely.  Better to consider the real process and consciously choose when to simplify it.  In my opinion...just my opinion.

The whole point of the video, which many people still seem to miss (they still seem to think I'm talking about electric/conducted current, when I said half a dozen times I am not), is that I am pointing out there is a different type of current when you look and delve deeply enough into the mathematics of it. So rather than this idea "break down" when you look closer, it actually gets more valid mathematically the deeper down the rabbit hole you go.

I thought your video was good, Dave.

[juani_c]

Don't know if someone already mentioned it, but apparently you can measure displacement current:
Measuring Maxwell's Displacement Current Inside a Capacitor

Sweet.
I was thinking it might be possible with the AIM-TTI current probe if you had a bracket to hold and move it accurately?
Collecting the data = fun
Working the math  to see if you are actually verifying any theory = yuck

the researchers used a SQUID to measure the very low magnetic field, don't know if the AIM-TTI probe is that sensitive

[AlfBaz]

the problem is we never fully charge capacitors... a 2200uf 16 volt capacitor is not 'full' a t 16 volts... it flahses over beyond 16 volts. assuming there was no limit to its working voltage it would actually turn out to be a much larger capacitance.


youd have to figure out how many atoms there are in both plates , how many of those atoms can accept extra electrons to find out how many electrons you can actually ram in a plate. then you can work out what the voltage would be.

I always thought the voltage rating of a cap is simply the withstand voltage of the dielectric. If that's the case It would be interesting to see this "flow" thing in action.

We get a very high voltage capacitor with very low capacitance, I see digikey has some 1pF 6kV caps, or maybe some 100pF 50kV. Then we steadily ramp the voltage from 0V up toward the caps voltage limit.

I would expect to see the current rise from zero and level out at C*(dV/dT). Since the only way to get current to "flow" is to have voltage change with time we continue to raise the voltage at a steady rate. At this stage it would be interesting to see if the current continue to flow at the above mentioned value and what. if anything, happens toward the end.

I can't help but think that it becomes a chicken and the egg thing in that to move enough charge to make this interesting you have to increase the voltages rate of change, which means you'll get to the caps withstand voltage to quickly and if you decrease the rate of change then the current becomes to small. Either way we may not get to that "critical" electron mass 

Might have to get in touch with that photonic induction guy see if he can use his high voltage gear for science

[AlfBaz]

Don't know if someone already mentioned it, but apparently you can measure displacement current:
Measuring Maxwell's Displacement Current Inside a Capacitor

Hey Juani, that link seems to be broken

[c4757p]

It's just an extra letter on the end of the URL.

Measuring Maxwell's Displacement Current Inside a Capacitor

[AlfBaz]

It's just an extra letter on the end of the URL.

Measuring Maxwell's Displacement Current Inside a Capacitor

Ahh... I did look at that but didn't have my glasses on
Thanks

[IanB]

I would expect to see the current rise from zero and level out at C*(dV/dT). Since the only way to get current to "flow" is to have voltage change with time we continue to raise the voltage at a steady rate. At this stage it would be interesting to see if the current continue to flow at the above mentioned value and what. if anything, happens toward the end.

I can't help but think that it becomes a chicken and the egg thing in that to move enough charge to make this interesting you have to increase the voltages rate of change, which means you'll get to the caps withstand voltage to quickly and if you decrease the rate of change then the current becomes too small. Either way we may not get to that "critical" electron mass 

You seem to misunderstand the capacitor equation. It is a linear equation:

I = C dV/dt

This equation is of the form:

y = Cx

It says "y is a constant times x".

Therefore, as long as dV/dt remains constant, then I remains constant.

This is exactly how the op amp integrator circuit works. For as long as there is a constant current flowing at the input, then the output voltage continues to change at a steady rate. There is no doubt at all that this works exactly as the formula predicts.

[AlfBaz]

I would expect to see the current rise from zero and level out at C*(dV/dT). Since the only way to get current to "flow" is to have voltage change with time we continue to raise the voltage at a steady rate. At this stage it would be interesting to see if the current continue to flow at the above mentioned value and what. if anything, happens toward the end.

I can't help but think that it becomes a chicken and the egg thing in that to move enough charge to make this interesting you have to increase the voltages rate of change, which means you'll get to the caps withstand voltage to quickly and if you decrease the rate of change then the current becomes too small. Either way we may not get to that "critical" electron mass 

You seem to misunderstand the capacitor equation. It is a linear equation:

I = C dV/dt

This equation is of the form:

y = Cx

It says "y is a constant times x".

Therefore, as long as dV/dt remains constant, then I remains constant.

This is exactly how the op amp integrator circuit works. For as long as there is a constant current flowing at the input, then the output voltage continues to change at a steady rate. There is no doubt at all that this works exactly as the formula predicts.

Hi Ian
Not sure if I understand what it is your are trying to point out. The more likely scenario is that I'm not explaining myself properly.

The current is directly proportional to the rate of change in voltage, yes? If so I can't clearly see where in the highlighted text I seem to go against this, except in that I am curious what would happen to his linear relationship as we approach charge saturation/depletion of the capacitor plates, or if even charge/saturation can occur