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Thread: Current "Through" Capacitors??

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    Junior Member Ah-Those-Old-Days!'s Avatar
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    Default Current "Through" Capacitors??

    Elsewhere, I had quite a 'debate' about whether current actually flows internally, through a Capacitor... Sigh...
    Try as I might, I could not get others to understand what I was trying to say!! They thought I was talking about +ve to -Ve, or in
    reverse, -ve to +ve, but i was NOT! They talked about 'ions', and 'chemical structures' etc etc, but never responded to what I was saying!

    I 'simply' stated that such circuits/current flows, with what ever nomenclature you would like to label it as, is EXTERNAL to the capacitor!?
    It ;flows', (by what ever one determines current to be other than 'electron' flow?), from ONE side of the capacitor, and THROUGH the external
    circuit, and BACK to the other side of the capacitor, due to the potential differences... but does NOT flow through the Dielectric?????
    I'm of course not talking about 'losses' within such a capacitor, but the fundamentality of what I am saying????

    However, they KEPT coming back with, (no ref to what I said), talks about ions and chemistry??? AAARRRGGGHHH....
    One small step for man... (but it wasn't me!!)



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    Typically holds true for DC circuits -- when AC is involved the discussion moves onto capacitive reactance and the complex impedance (of the capacitor used)...ie; a mains 'dropper' capacitor does in fact pass current through it (relative to freq/voltage)

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    Quote Originally Posted by wotnot View Post
    Typically holds true for DC circuits -- when AC is involved the discussion moves onto capacitive reactance and the complex impedance (of the capacitor used)...ie; a mains 'dropper' capacitor does in fact pass current through it (relative to freq/voltage)
    Thank you for your reply. But even on A/C, one may have a series, or parallel tank circuit, that is 'resonating', but that involves current
    flowing back & forth between the capacitor & inductor in this case, as to AFFECT the external circuit, but is not through the actual Dialectric??
    One small step for man... (but it wasn't me!!)

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    Tell you what -- it's late, and maybe other folks here might like to chime in before I show you the math, but for poops & giggles if you ever want to see current cross the dielectric of a capacitor, watch your next local lighting storm...always awesome stuff.

    Parallel aside, it is -only- in an AC circuit wherein current flows through a capacitor, and I've tossed you a couple of keyword phrases you can hit google up with, to discover the components needed to flesh out the equation...but if you want to chat about it here instead, all good =) Like I say, your statement holds true for a DC circuit...but for AC circuits, not so, and then it depends on what class of capacitor you're talking about, and which dielectric material/technology the cap in question fits into...class rating.

    ...I think the easiest example is ceramic capacitors made with ferroelectric dielectric materials ; as I say, often found taking a cameo role in the guise of AC dropping capacitors..."Ferroelectric materials are typically insulators, but the boundaries that separate regions of differing orientations of spontaneous polarization namely, domain walls can act as metallic conductors. ... As the domain walls deviate from their neutral position, they become partially charged and electrically conducting."

    M'kay?...because that is so, you can get an AC voltage....errg...240vac...define the frequency...humm..50Hz...and calculate what capacitance class2 cap value you need, to pass whatever current you require...and I haven't even started on the properties of bipolar/non-polarized capacitors in AC circuits. "Would you like to know more?" ...I really like those movies, damn bugs... =)

    Next place I can think of, is in snubber circuits...you use the ceramic cap to block DC currents, but when the polarity is reversed the cap ostensibly looks like a short circuit, and you guessed it...current flows (through the cap).

    This has much to do with materials technology, and it's kind of 'magic' in a sense that these materials display these kinds of properties...a bit like magnetism - easy to make, use, do all sorts of things with...what is it? Nobody knows.. =)

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    No, electrons do not flow through the insulator between the plates or though the dielectric. If they do then that is considered a loss or leakage.
    Blocking the flow of electrons is the whole point of an insulator.


    Quote Originally Posted by wotnot View Post
    Tell you what -- it's late, and maybe other folks here might like to chime in before I show you the math, but for poops & giggles if you ever want to see current cross the dielectric of a capacitor, watch your next local lighting storm...always awesome stuff.
    That is considered a catastrophic loss when the electric field is so high that it can break through.

    Otherwise the flow of the electrons is determined by the charge of the plates until the potential between the plates is equal to the potential applied to them.
    Then you have zero, assuming no electrons are sneaking through with leakage.

    With AC the plates are just charging and discharging the whole time it is applied. Reactance is just a mathematical model to deal with this 'imaginary' current flow. There are still NO physical electrons passing between the plates.
    Last edited by Uncle Fester; 25-11-21 at 12:50 PM.
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    That is considered a catastrophic loss when the electric field is so high that it can break through.
    LoL...I realize that -- this'd be my way of pointing the original premise/topic is so generalistic and undefined, I can use a 'primitive' example like that, and still be 'on point' as it were...ie; moist air makes for a leaky, limited dielectric, subject to the 'catastropic breakdown' you/I point out here, but it's most impressive when you're out on the water in a tinny with a storm rolling in, and the tips of your fishing rods start crackling and hissing..'inside' that leaky capacitor =)

    There are still NO physical electrons passing between the plates.
    Of course not ~ once you leave the conductor medium, and enter the dielectric/electrolyte medium, you move out of electrical theory domain, and into the realm of chemistry/physics...I mean, duh, that's pretty obvious...but like I say, topic line and premise there-under are so poorly defined, it's little wonder (to me) that folks started citing chemistry and ions etc etc -- this is roughly the difference between these two questions -- "how do you use a capacitor in a circuit?" ; "how does a specific capacitor type actually work?"

    I was hoping someone else had done a decent treatise on this (to save me reciting stuff I grappled with at Uni =), and it turns out Dave from EEVBlog did a vid several years that does a damn good job of explaining it;


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    He did a great job of explaining it. Thanks Wotnot.
    I'm out of my mind, but feel free to leave a message...

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    That video wasted 17 minutes to say almost exactly what I said above in a few sentences.
    Only difference was that I called it imaginary current but both of us did not bother to actually explain it.

    As always Wikipedia does it best:


    But I will try to explain this with an analogy.
    I stand by the fact that electrical current is solely defined by electrons moving collectively.
    Of course when electrons flow they create an electromagnetic effect, which will form electromagnetic waves when this flow alternates.
    These electromagnetic waves/energy will radiate from the plates as the electrons move in and out of them.
    Now imagine you tear the capacitor apart, unroll it and spread the plates wide open 180˚ apart. You could also stick one in the ground and leave one poking up in the air.
    Now the electromagnetic energy will freely radiate to the world and beyond.
    Electrons are still not flowing through the air but current is flowing to power that capacitor that has become an antenna.

    So just think of a capacitor as a closed rolled up antenna that practically does not radiate out to the open.
    Last edited by Uncle Fester; 29-11-21 at 04:26 PM.
    I am an arrogant, irritating RSole.

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