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  • EEVblog #486 – Does Current Flow Through A Capacitor?

    Posted on June 21st, 2013 EEVblog 33 comments


    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…

    Vent your rage HERE

    Turns out you can measure the displacement current:

    http://personal.rhul.ac.uk/uhap/027/PH2420/PH2420_files/displacement.pdf

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

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    33 responses to “EEVblog #486 – Does Current Flow Through A Capacitor?” RSS icon

    • Cheers Dave, these fundemental Friday vids are really enjoyable. Kepp up the good work!

    • FYI Dave, it doesn’t help to spoil the answer in the preview of the video :).

      (The entry goes “Does current flow through a capacitor?” and the YouTube still image preview for embeds has a big “YES” on it with the proof…).

    • I didn’t know this can of worms even existed. :) Because from electronics standpoint… who cares?

    • Excellent job explaining the “magic”. I find it funny how big the gab between Practical Electronics and Theoretical Physics is for many engineers and physicists.
      I was lucky to receive top notch theoretical education by formal Soviet physicists, who at the same time were great masters in practical application. I find it very disturbing how fundamental knowledge is missing for many Electrical Engineers in USA. Here are couple of examples for future episodes of “Fundamental Friday”:
      1. Why and How grounding works in the desert.
      2. Why incandescent light bulb can’t have more then 5% efficiency and why florescent light bulbs start from 30% or better.

      • I do get nearly 100% efficiency from my incandescent bulbs as they reduce the amount of electricity I spend on heating :)

        • Incandescent bulbs can be even more than 100% efficient if you consider that bugs sometimes get too near them and die, thus saving on pest control costs.

          • The gas furnace is cheaper to run than an electric heater.

            • Perhaps, but that’s only because gas is less expensive than electricity on a cost-per-joule basis. In terms of energy conversion, electric resistance heating is more efficient than burning gas – at least when considering only that final conversion step.

              Now, if you take into account the energy used to produce and deliver the fuels, that may flip back in gas’s favor, depending on how your electricity is produced and how much transmission loss there is from the plant to you.

      • I didn’t even scratch the surface here though!
        But I’m happy if people go away and look up what displacement current is, and then get interested enough to dig deeper.

        • And this is what I liked. You don’t have to teach physic lesson to show the link between practical engineering and theoretical physics. Both are important, but what is more important to understand the models used to simplify are day-to-day job. Each model is build on basic assumptions and understanding those assumptions are crucial to correct application of the models and avoiding way too common problem when engineer designs something that “should work on paper, but doesn’t work in real life”
          Thanks for the great work!

    • I think it’s practical to think of capacitors as blocking DC current, but allowing AC current.

      Capacitors are like your lungs. You cannot keep breathing either in or out forever. You run out of space (breathing in) or air (breathing out). This is like DC current “not going through” a capacitor.

      But you can alternate directions, and continue breathing forever (assuming an ideal human body :D ).

      This is like AC current, and we use capacitors in circuits in the same manner. Current goes in and out through both of the capacitor’s terminals, which can be thought of as the current going “through” the capacitor.

    • That’s the different between a true Electical Engineer and hobbist. True EE has been blasted by at least 3 quarters of E&M theory class.

    • To relinquish the physics to an asterisk and say that for all practical purposes we can ignore it is harmful in my opinion. Its easy to dismiss the importance of actually understanding what is going on under the hood once you know it since at that point, your right, it no longer effects how you work on practical electronics. When you are first learning the theory though dismissing these details as being to complex to worry about can lead to major mental road blocks that many people like myself just cant let go. I need to understand why something works and not just accept the formulas.

      I have my undergraduate degree in electronics engineering but never worked in the field (I’m a system administrator). It was not until years later when I found William Beaty’s webpage ( http://amasci.com/ele-edu.html ) that I actually started to understand electronics. He has many people accuse him of being nit picky but at least for me his nit picking lead to a lot of major “AH HA” moments and really changed my understanding of electronics. Admittedly I never had any intention of going into a cube farm and working on practical real world electronics. I always considered myself more of an academic and scientist albeit armature one. I think Bob Iannini of Information Unlimited has my Dream Job.

      I hope this does not come off as elitist or nit picky or anything like that. I really appreciate the effort you put into the blog. I just know first hand that it really does matter. But maybe I am just weird.

      • Actually, I don’t think I relinquished the physics to an asterisk. Yes, I did not go into Maxwells and the detailed physics etc (that was not my intention), but I at least was trying to point out that displacement current exists. Most treatments of capacitors ignore that physics aspect of it and treat is as no current flowing through and the traditional charge build-up etc.
        So I like to think this video is a small step forward in terms of revealing the physics, not a small step backward by hiding it behind an asterisk.
        But of course, in the end it all depends on how you want to view the problem.

        • I agree. You started out by saying you where going to discuss the asterisk and that is what you did. I just got the overall feeling that you felt it did not mater and where explaining it to keep the nit pickers at bay while standing by your origional assertion that current does flow through a capacitor. My comment was about the importance of the information (atleast to me) not your coverage of it.

          • My “original assertion” in the previous video was not an assertion as such, it was simply use of the industry standard terminology. Engineers say “current through” when talking about capacitors.

      • Heh, teachers and textbook authors are *supposed* to be extreme nitpickers. After all, any fuzzy/wrong concepts might screw up their students permanently. If they’re not constantly trying to correct their own understanding, something is seriously wrong. In the same way, if the one teaching your English grammar class isn’t a grammar-nazi, something is bad wrong.

        And when detecting concealed Trolls in physics discussions, remember this…

        “I know that most men, including those at ease with problems of the greatest complexity, can seldom accept even the simplest and most obvious truth if it be such as would oblige them to admit the falsity of conclusions which they have delighted in explaining to colleagues, which they have proudly taught to others, and which they have woven, thread by thread, into the fabric of their lives.” -Tolstoy

    • In summary, current flows… until it doesn’t (the displacement current being finite).

    • When I saw this video I asked myself
      “Do really exist electronic practicers who don’t know about this argument?”
      And I was shocked to find out that the answer was yes :|

      • Then better not ask which direction electric current really flows!

        Seriously. There are entire online groups dedicated to the false belief that all electric currents are flows of electrons, and that electric currents should flow backwards.

        coughProtonConductorscough

    • Hello David,

      I would opt for a democratic scientific vote to get rid of disturbing facts that undermine social scientific acceptance and keep paying my bill’s contrary to evidence in the real world. I love communis opinio as introduced by the IPCC and wait for a resolution on the subject of negative gravity :=0.

      As to capacitors in this video:

      http://video.mit.edu/watch/dissectible-capacitor-3540/

      it is clear that big money has dished out a large sum to alter physics in their favor. According to youre explanation this a trick? A smart guy will now go in to the stock market buying shares in glass as it is evident that energy is stored in glass :-)

      Uuuhmm, you can measure (V/m) between to capacitor plates just before the lightning strikes you and you get wings :-)

      • The copper cupsaren’t perfectly smooth and the electric field exceeds the breakdown voltage of air at imperfections in the plate. Charge therefore gets sprayed onto the glass. Being an insulator, this charge is trapped on the glass.

        The cups are removed, discharged and replaced.

        Now, the charge on the glass induces charge in the cups, hence the spark.

        You can try to remove the charge from the glass, but short of giving it a bath in a conductive liquid, you won’t succeed and can repeat the second half of the experiment – remove the cups, discharge them, replace them and draw a spark.

    • First off, generally your videos are great. That said, there’s no way you can say that current flows “through” a capacitor. The wording is just wrong. However, I agree that it might help in some circuit design/analysis to think of current as flowing through a capacitor. Current is the rate of flow of charge, so the question is does charge flow through a capacitor. The answer is definitely no.

      • If you see the equations of current of the capacitor as long the derivate(dv/dt) voltage change in time there will be a current it´s just math and physic.

        • The current is used to charge and discharge the capacitor. The charge never flows “through” the capacitor.

    • Absolutely excellent video & explaination.

    • Really?

      I knew caps conducted current during their charging cycle when I was 12 years old and put them in series with relays to make little one-shots. I don’t know how one could even claim the title “electronics hobbyist” and not know that, all the mathematics aside.

      Dave’s word should be good enough, on component theory, anyways. I mean, he IS a real engineer.

      Anyway, good video, Dave. You are great at turning a challenge to your knowledge into learning experiences for all. I’m waiting for the one where you prove ohms law.

      • My word on technical things should never be good enough!
        There is actually some very deep theoretical physics involved here, but ultimately it all comes down to convention. And practical electronics circuit theory (+ maxwell’s) says current flows “through” a capacitor. But then, when talking about just caps on their own, it’s usually described as charge buildup instead.

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    • Yes current does flow through capacitors… but electrons don’t.

      Electrons, instead, in the presence of capacitors, prefer to flow only through the rest of the circuit and they do so just to fool us, so that we can see them in our galvanometers.

      They’ve been deceiving us for over three hundred years, since the invention of the Leyden jar in the eighteenth century.

      But now we know the truth :-)

    • Dave this time thumbs down! they way yo explain it wil confuse the young generation that just have finished first yeat learning D.C. cause its a contradict with the material that they have just learnd.
      A good explanation is to be found on You Tube under the following link:
      http://www.youtube.com/watch?v=NInt1Ss3vQ4

      greeting and thanks for the great job of EEVBlog

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