EEVblog #486 - Does Current Flow Through A Capacitor?

Dave proves he has no fear by opening this can of electronic worms by posing the question - "Does Current Flow Through A Capacitor?"
The answer may surprise you, or drive you into a physics induced rage...

Turns out you can measure the displacement current:

NOTE: before commenting, please watch and understand that there are TWO types of currents.

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[guillaumeerard]

I have no interest whatsoever in EE but I find your videos really compelling! I am so glad to live in a time when people with your enthusiasm can have the means reach out to so many people!

[EEVblog]

I did not slip up on the term. The term "through" is defacto standard in the industry and used in many aspects of teaching.

[DjVortex-w]

So the simplistic answer is:
Do electrons flow through a capacitor? No.
Does current flow through a capacitor? Yes.

One circuit can affect the electron flow of another circuit even if they are not physically connected. That's how eg. transformers work.

[russelltennis]

Why am I paying $20, 000 a year to learn electrical engineering at a university when you explain it better?

[LuckeWent]

I would also describe James Clerk Maxwell as an incredibly smart dude

[mmichaeldonavon]

About 50 years ago, I was in a class that discussed, "Capacitors, does current flow through them?"    The answer:  "A capacitor BLOCKS DC, and passes the "EFFECTS" of AC."   Simple as that.   As AC is applied to a capacitor, electrons LEAVE one plate, and ACCUMULATES  on the other  (via the AC power supply) - when the AC reverses, electronics again leave one plate and accumulates on the other - back and forth.    It will APPEAR that the current is going THROUGH the cap, but it is not.    Yes, there will be DC current, momentarily, in a series DC circuit with a cap and resistor, as the current from one plate leaves that plate, and heads for the battery - then EVERYTHING stops!   The DC is NOW blocked.   Apply AC, to that same circuit, then it will APPEAR that current passes through the cap but, again,  it really does not.

[EEVblog]

Bingo, you said it yourself, the displacement current does go "through" the capacitor. At no time did I ever claim that electric conducted current flows through the capacitor, it of course does not. I did not "misspeak" because the term "through" is the accepted term in the industry and in many aspects of teaching.

[Airbag]

Dipole excitation is how displacement current works. When you apply an electric field to a dielectric it realigns (rotates the dipoles). This rotation of dipoles in response to a sinusoidal voltage is how a perceived current propagates through an insulator/dielectric. A good analogy is doing the "Wave" at a sporting event. Even though the wave moves in series, no one actually physically touches each other. This is also why capacitors block DC current, because there is no "force" to rotate the dipoles.

[weylin]

How was there any doubt of this??
Take this comparison, using a hydraulic model:
You have two pre-filled pipes leading into a chamber, in the middle of the chamber is a flexible diaphragm separating both pipes. normally, both sides are in balance, but when a pressure is applied to one side, a voltage, it pushes the diaphragm, inducing an equal current on the other side until the inflow has stabilized.

Correct me if that is an incorrect way to look at it, though.

[DragonHawk]

I see it this way: current flows into a cap, and back out rather than through. In the demonstrated DC circuit with a switch on one side of the cap and the other side grounded, (conventional) current flows into the + side of the cap. Hook up a resistor across the cap and the current will flow back out of the + side of the cap. Presumably in the ammeter demo at the end of the video, a second ammeter on the grounded side of the cap would show no current (since that side always stays at the same voltage, ie: zero), hence the terminology of "into" rather than "through". Of course here we are talking about an ideal cap with no leakage etc.

[joeyoest]

Quote of the day:
"Does current flow through a capacitor?"
*touches lead to battery - ammeter spikes*
"Yep! See you next time!"

Made me lol!

[proyectosledar]

I'm sure we are learning something. thanks dave

[EEVblog]

When did I say it was electric current flowing through? Yes, the practical demo was meant as a joke.

[EEVblog]

Yes, that simple explanation is indeed very useful for understanding how a cap works in terms of charge build up, and if you want, how current flows "in and out" but not "through". And I would have used that in a basic how how capacitance video and nothing else. But with this video I wanted to show there is a deeper mathematical theory and a different "type" of current many people may not be aware of.

[hellnawnaw]

Thank you so much for all of your videos Dave.

I'm an electrical engineering student in Germany, and I'm in my second semester right now. Even though I've been doing electronics as a hobby for 3 years now, and even designed a board for money already, I always learn so much great stuff from you, which helps me very often. For example, your soft switch tutorial inspired me to create a RS-flipflop softswitch that can be turned on by a button and turned off by an AVR which can also read the button.

[eqlipse]

From a physics standpoint, there's a very easy way of looking at this. "Energy" flows through the capacitor; the first plate, while charging, produces a magnetic field, which in turn generates an electric current on the other plate, thereby allowing energy to flow from one plate onto the other, even if the electrons themselves are not.

If you think of it from the abstracted view of "energy, " which doesn't necessarily have to be held by the electrons themselves, but any combination of the electrons and their electromagnetic fields, then it actually becomes rather simple.

[WhoWantsToKnow]

Fantastic, now every time I read a circuit, I do so in an Aussie accent. :D

[LeeMaitland]

REVELATION! When you did the dQ/dt and explained it as 'change in charge over change in time', suddenly six months of calc just made sense! THANK YOU!

[EEVblog]

Agreed. Nothing short of pure genius. Probably one of the greatest discoveries of all time.

[DavidSprings]

You'd be more credible if you sat next to a workbench with an o-scope showing a sine wave. Throw in a brand new soldering iron and a power supply for extra gravitas.