EEVblog #486 - Does Current Flow Through A Capacitor?

[IanB]

Forget the subject title, how about this:

Does current flow through a resistor?

I mean, suppose my resistor is 5 mm long and the electron drift velocity is (say) 0.1 mm/s, then any electrons flowing in at one terminal are going to take 50 seconds before they appear at the other terminal    At any instant in time the electrons entering the resistor are not the same electrons that are leaving the other end. The resistor is a giant electron storage container...

[c4757p]

Forget the subject title, how about this:

Does current flow through a resistor?

Elaborating on this would make a great sequel to this video IMHO. Movement of electrons vs. movement of charge vs. current, macroscopic vs. microscopic, etc. I think a failure to establish the relevant definitions along those lines was responsible for most of the last one's confusion. Plus, it's just damn interesting sometimes. But Dave doesn't seem to go for physics much, so maybe someone else would have more fun making this kind of video.

[don.r]

Well done, Dave. Put that puppy to bed and move on... next... the transformer... and Ed, Edd and Eddy currents...

[c4757p]

I don't think they're very current.

[don.r]

I don't think they're very current.

LOL! Very good... 

[sagdahl]

Does Current Flow Through A Capacitor? should maybe be refrased to:
Does Current Flow To A Capacitor? (And From?)
Excellent infotainment by the master of joyful electronic videos!
Does anyone know where Jeri Ellisworth gone?

[madires]

I have not seen the video yet however. Current flow through a Capacitor in a DC circuit will not flow. IE the current is blocked or stopped. It will charge but that is about all that will happen.

Let's see, you switch your device on or off, the DC power could have ripple or maybe there's some switching or oscillation. In any case that involves a voltage change you'll have a current through the cap.

[beaker353]

Had you been teaching my AC theory class last semester, there would have been much less confusion amongst my classmates.  College professors could learn a lot by watching your teaching methods...

-EM

[c4757p]

College professors could learn a lot by watching your teaching methods...

The problem with college professors is that when you know 100 times above what you're teaching, you forget that it's not as obvious to everyone else as it looks to you. This is why Maxwell's equations confuse so many people - a physics professor is so steeped in calculus that he can take one glance at the equation and know what it means. A student looks at the equation and at best sees a sequence of steps to be performed to get the result, not the true, physical relationship it implies.

College professors out there, remember this - whatever it is, it's only 1/100 as obvious as you think it is.

[IanB]

It's been touched on above, but when you consider questions like this you need to be concerned with the system parameters. When analyzing any system you may perform a static (or steady state) analysis, or you may perform a transient (or dynamic) analysis.

It is very appealing, intuitively, to consider systems as static and unchanging, as this reduces the complexity of the analysis. (This is why AC circuits and control systems are often studied in the frequency domain rather than the time domain. You can represent them using time-independent parameter values like frequency, phase angle and impedance and not include time in the calculations.)

So in the capacitor question, we can say that in a static system where nothing is changing with time, then current does not flow through an ideal capacitor.

However, in the real world, most systems of interest are dynamic (or they wouldn't do anything). In a dynamic system, current most certainly can flow through a capacitor.

[firewalker]

Alexander.

[chicken]

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

Doesn't make the actual content of the video worse, just bugging the photographer in me    (probably like me doing it all wrong when messing with electronics  )

[staxquad]

next week's Fundamentals Friday: "does current flow through a transformer?"

in both cases (capacitors and transformers), they are black voodoo boxes that do magical things, and the lay person sees current flow through both

actual current only flows through both by leakage

and in both cases, voltage is manufactured by one isolated side affecting the other isolated side by their electrowhatever properties, and with a load on the effected other side, there's current flowing 

to use them, you don't need Maxwell

to make them you do need Maxwell

edit: premature blah blah
now that I've seen the video, good one!

[c4757p]

Yes, I removed a stupid brainfart comment so as not to confuse anyone. Sorry if you were replying to it 

[free_electron]

Yeah that's the question I was alluding to in my post's last paragraph, even had your name in it.

According to komet however we should either stop taking about this or move it to a chemistry forum 

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.

[sagdahl]

and in both cases, voltage is manufactured by one isolated side affecting the other isolated side by their electromechanical properties, and with a load on the effected other side, there's current flowing 

I presume you mean "electromagnetic properties"?

[Dave]

Achievement unlocked: Inspired an EEVblog episode.

[free_electron]

A really interesting question would be : what amount of electrons do i need to place on the left plate so that the right plate is completely void of electrons ? For a given plate size and plate distance , what voltage would that yield ? Anyone care to work that out ?

here is a back of the envelope calculation:

The atomic volume of copper is 7.1cc/mole

if we take capacitor plates of 10cmx10cmx1mm then this is 10 cc which at 1 electron per atom is (10/7.1)x6.022E23 electrons

the charge density is then this number times the electronic charge (1.602E-19) divided by the area of 1E-2 square meters,

if we make the very drastic assumption that all the field is within the capacitor (clearly wrong if the copper is completely depleted!) then
we can calculate the field by dividing the charge density by epsilon zero (8.854E-12),

the answer I get is E = 1.53E18 V/m which for a 1mm gap would be around 1.53E15 V !!!

I suspect that the air or dielectric would break down a bit before that point was reached.

If my maths is right, the energy stored would be the equivalent of around 2,480 million atomic bombs (of the 84TJ variety) so it would be
a rather dangerous capacitor to be near.

now imagine a technology suffiecinetly advanced enoug to pull of 1/100 of this. a small 10cmx10cm capacitor could power an electric car for millenia !

[free_electron]

Forget the subject title, how about this:

Does current flow through a resistor?

I mean, suppose my resistor is 5 mm long and the electron drift velocity is (say) 0.1 mm/s, then any electrons flowing in at one terminal are going to take 50 seconds before they appear at the other terminal    At any instant in time the electrons entering the resistor are not the same electrons that are leaving the other end. The resistor is a giant electron storage container...

that raises another quesion.. why we need to pay for electrical power .. after all every electron coming out of that wall sockets left prong , goes into the wall sockets right prong ! i should be charging the electricity company for the wear and tear these electroncs cause on my equipment ! it's not like i consume electrons , they get each and every one back.

and at a rate of 50hz or 60hz they don;t travel that far into my equipment either !
electricity is waay overpriced.

far better of with batteries. at leas tthere you know what you get for your money. once the electrons have run from one pole to another and potential differential is zero the cell is empty. not this wishy washy ' we'll jiggle the electrons a bit and you need to pay for that !

[XynxNet]

I like your video, Dave. It hits a sweet point between the point of view of a physicist and an EE.

The funy thing is, we physicists label this problem as experimental physics.  Go figure...

[Ketturi]

This was awesome. They really should show Dave's videos in schools, they would make so much more sense! I was once wondering pretty much same topic at one physics course. Needless to say that my practical opinions differed somewhat to more traditional theoretical ones teachers and books had. And like one instance I used "ohm" in my baccalaureate/final exam instead of "?", which of course is insignificant which one to use in real life, but nah, it's fastidious in school.
Fundamentals Friday is third best segment, or second best. Or what the smeg, they all are as good! Keep up Dave!

[c4757p]

"ohm" in my baccalaureate/final exam instead of "?"

What kind of electronics forum doesn't recognize ? 

Seriously, that symbol needs to die. How impractical to use a symbol that not only isn't supported by a number of computing systems, but also doesn't have a practically useful similar-looking symbol that can be used in its stead ( vs. u, for instance). Just use R...

[jpb]

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.

[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


EDIT: link fixed.

[IanB]

What kind of electronics forum doesn't recognize ? 

Seriously, that symbol needs to die. How impractical to use a symbol that not only isn't supported by a number of computing systems, but also doesn't have a practically useful similar-looking symbol that can be used in its stead ( vs. u, for instance). Just use R...

'Tis an amusing question. Now of course "" is a Greek capital letter "O", the first letter of "ohm". I suspect the reason we don't use "O" for ohms is because of the confusion with zero. So some bright spark thought, "Hey, let's use a Greek 'O' instead!"