Proof theres no current flow through a capacitor! Has our comments blocked? - Page 1
Forum section: electronics / beginners

author: accesscontrolforum
author_rank: Contributor
timestamp: May 02, 2015, 12:26:22 am
content: This Youtuber had our comments blocked because we pointed him to Dave's video about current flow through a capacitor. His videos are great and we like his channel. But , are our comments wrong? Oh, that's right you can't see them. Here's a picture of the comments you can't see!

author: codeboy2k
author_rank: Super Contributor
timestamp: May 02, 2015, 11:21:52 am
content: More likely he chose not to allow you to post links to other youtube videos. Dave's video is correct and sums it up well, presenting both sides but concluding that for practical purposes current does flow through a capacitor.It's pointless arguing this with someone who has already made their mind up to look at it from a purely physical viewpoint and choosing to ignore the bigger picture. Simply put, from a purely physical point of view, ELECTRONS DO NOT FLOW through a capacitor. That's what his YouTube video shows you, and it is correct. There is nothing wrong in his video. However, from a practical point of view, for an EE, there is a current flow both into and out from a capacitor. The electric current flow into the capacitor causes an electric field to be built-up inside the capacitor, which causes a displacement current across the plates of the capacitor, and this displacement current in turn causes an electric current to flow out of the capacitor.So for a capacitor, we have: electric_current -> displacement_current -> electric_currentThis displacement current is caused by energy stored in the electric field between the plates of the capacitor.Similarly, current flow into a primary side of a transformer (inductor) causes a magnetic field to be created, which induces a current to flow out of the secondary side. Current can flow "through" a transformer, but doesn't really flow "through" it directly. No one seems to have a problem with this.For an inductor, we have: electric_current -> induced_current -> electric_currentThe induced current is caused by energy stored in the magnetic field of the inductor.And finally, I'll leave this mind-bender: with a diode (or any forward biased PN junction, for that matter)... we always say that current flows through a forward biased PN junction, because it's a simplification that works for us when designing circuits. However, if you get down into the physics of it, there is a depletion region at the PN junction that has no charge carriers whatsoever, and the majority current that flows across that depletion region is adisplacementdiffusion current. And yet again, no one ever argues that current cannot flow "through" a PN junction.What makes capacitors so special that people have to take sides on this?

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 02, 2015, 11:27:22 am
content: Quote from: accesscontrolforum on May 02, 2015, 12:26:22 amYou can enable the comment system to filter out comments with a link.Like a automaic spam filter.Maybe he dit not remove you comment intentionally, maybe it got caugt in the spam filter.But maybe not... who knows

author: jay
author_rank: Regular Contributor
timestamp: May 02, 2015, 11:34:12 am
content: Well said 2k. People just love arguing about stuff like this. As far as I'm concerned there's current flowing

author: Excavatoree
author_rank: Frequent Contributor
timestamp: May 02, 2015, 12:42:01 pm
content: Quote from: PeterFW on May 02, 2015, 11:27:22 amMy comment with a link to Dave's video is still there, 10 hours later. I'm not sure if that means that the other comment was manually deleted, or if the filtering doesn't happen continuously.

author: G0HZU
author_rank: Super Contributor
timestamp: May 02, 2015, 01:40:35 pm
content: I enjoy watching the AA5 videos and this one seems OK to me because he is correct in that no electrons flow through the insulator between the plates. I don't think anyone would argue with that.However, AA5 does have a track record of deleting any comments that point out errors in his videos. He has done it to me after I pointed out he had got a resistor value (or two) wrong in a tutorial schematic. All he had to do was acknowledge this and correct the video but instead the comment was removed. The video is still wrong years later.I tried to add the same comment a few minutes ago and it was blocked straight away. Maybe he only allows comments to appear after he filters them first?

author: Excavatoree
author_rank: Frequent Contributor
timestamp: May 02, 2015, 01:44:31 pm
content: Probably the wrong place to ask, but I see Dave using annotations frequently for the inevitable "oops" remark. Does the whole video have to be re-uploaded to youtube to add those, or does youtube have a process to add them to existing videos?

author: robrenz
author_rank: Super Contributor
timestamp: May 02, 2015, 01:49:47 pm
content: You can add text comments to your existing videos with the YT editor

author: IanB
author_rank: Super Contributor
timestamp: May 02, 2015, 03:56:37 pm
content: Quote from: codeboy2k on May 02, 2015, 11:21:52 amI would have to disagree with this statement. A flow of current is a net movement of charge. Now a diode can conduct in steady state conditions continuously for an unlimited time. The conditions for steady state of a system are that there is no accumulation occurring anywhere in the system (no rates of change of concentration with time). For this condition to hold, it must follow that there is a steady state charge balance, i.e. flow of charge in at one end = flow of charge out at the other. Since charge can neither be created nor destroyed, it follows that there must a be a flow of actual, physical charge across the junction in the middle of the diode. Electrons that enter on one side of the diode will eventually leave on the other side.

author: w2aew
author_rank: Super Contributor
timestamp: May 02, 2015, 05:10:36 pm
content: Quote from: Excavatoree on May 02, 2015, 01:44:31 pmYou can add them later, right within YouTube. I do it all the time on my videos, after I discover (or a viewer discovers) the inevitable error or two that creeps into my videos. You'll find these "oops" moments in most of my videos.

author: accesscontrolforum
author_rank: Contributor
timestamp: May 02, 2015, 09:11:04 pm
content: 2k for me its not about sides it about truth. good to know G0HZU

author: bson
author_rank: Unknown
timestamp: May 02, 2015, 10:40:23 pm
content: Electric current != electron movement.Even in a pure electron conductor the electron movement is trivial - it's the charge that moves. Imagine a pipe full off billiard balls. Put in 1 ball/sec and 1 ball/sec comes out - but not the same ball as was input. The inserted ball creates a pressure wave which pushes out a ball at the egress point; this wave is the current analogy. Now, if there's 10^30 balls in the pipe and additional ones are inserted at one per second, well the ingress balls will never make it to the egress (for some finite life estimate of the universe). Yet the ball at the exit pops out almost immediately. The current propagates almost immediately even though the actual ball movement is barely measurable.Devices like capacitors and transformers forward charge without permitting electrons to cross. They mediate the electromagnetic wave betweendifferentelectrons on either side. Hence, current flows across.

author: IanB
author_rank: Super Contributor
timestamp: May 02, 2015, 11:46:40 pm
content: Quote from: bson on May 02, 2015, 10:40:23 pmActually, exactly the opposite is true. Electric current is the rate of movement of electric charge, and electric charge is not an abstract concept, it is a physical property of charge carrying particles. For all practical purposes the charge carrying particles that give rise to the phenomenon of electric current are electrons. Therefore it is true to say that electric current is measured as the movement of electrons.(This is true even of ionic solutions, plasmas and p-type semiconductors. Moving electrons make them conduct electricity.)QuoteIn fact, the electron drift velocity in a wire can be a significant fraction of a millimeter per second, so it really doesn't take very long for an electron that enters one end of a wire to come out the other end. Hours, maybe. Certainly far less than the age of the universe.QuoteNo charge crosses from the primary side to the secondary side of a transformer. This is how transformers provide electrical isolation. In an ideal transformer the windings are perfectly insulated from each other.Similarly, no charge crosses between the plates of an ideal capacitor. They are also perfectly insulated from each other. When a differential charge appears across the plates of a capacitor being charged, that differential charge doesn't flow through the capacitor, it actually flows from one plate to the other by going round the external circuit. (A bit like going out the back door of a house, going round to the front, and coming in the front door.)

author: codeboy2k
author_rank: Super Contributor
timestamp: May 03, 2015, 08:54:55 am
content: Quote from: IanB on May 02, 2015, 03:56:37 pmI think that many texts simplify this concept. I had to re-read it myself when you objected, and yes, I presented it wrong and over-simplified it myself too. Thanks, Ian. Inside the semiconductor, there are actually 3 currents that contribute to total current flow: the drift current, the diffusion current and the displacement current.Except in the case of a PiN diode, the displacement current is short lived and only serves to charge up the semiconductor. When excess charge carriers are continually injected (i.e. current in) then the current flow across the junction is made up of drift currents and diffusion currents. Under normal conditions (non-breakdown) the majority carriers flow mostly by drift and the minority carriers flow mostly by diffusion. Forward biasing the diode causes the depletion zone to shrink making it easier for these currents to flow. The net of these currents is what we see as a net flow of charge in one direction or the other.PiN diodes, on the other hand, have an intrinsic region (un-doped semiconductor) between the PN junction, and this region creates a depletion zone much larger than a normal PN junction, and of almost constant width. This causes a continual displacement current across the intrinsic region. So PiN diodes have a total current that is the sum of the drift currents + diffusion currents + displacement currents.I'm not a physics expert, but the point I was trying to make, perhaps badly, was: why is a capacitor treated so special by people trying to argue that current doesn't flow through a capacitor? At the silicon level, there are many different types of current flows that contribute to a net movement of charge "through" the device, as you said, yet everyone accepts it and no one argues that current doesn't flow "through" a semiconductor, even though the nature of that current flow in a semiconductor device is never as simple as mere electron flow. Yet when someone wants to argue that current doesn't flow through a capacitor, they conveniently want to ignore all other types of current flows known to exist, if only to argue their point, and wrongly so.Cheers!

author: AKM
author_rank: Contributor
timestamp: May 03, 2015, 09:08:23 am
content: And then there is leakage current.

author: IanB
author_rank: Super Contributor
timestamp: May 03, 2015, 02:45:28 pm
content: Quote from: codeboy2k on May 03, 2015, 08:54:55 amDisplacement current is a transient quantity that depends of the rate of change of an electric field with time. So we cannot talk of a "continual displacement current" in a non-time-varying context. If we consider steady DC conditions (i.e. no frequency content above zero) then the displacement current will become zero and will vanish from the system.QuoteWe can see why a capacitor is different from other things by separating steady, non-transient conditions from transient, time-varying conditions. Under steady conditions with an unvarying applied DC potential difference then an ideal capacitor has no current flow of any form and behaves as a perfect insulator.However, under the same steady, DC conditions a semi-conductor device like a diode does have current flow, even if it is a minuscule reverse leakage current. Inside the device this current does consist of a net migration of electrons across any chosen control surface bisecting the device. Drift currents and diffusion currents are just different mechanisms by which electrons can move.

author: 22swg
author_rank: Frequent Contributor
timestamp: May 03, 2015, 03:26:30 pm
content: My Wizard electronics instructor described a capacitor as such,,, Capacitor , two insulated plates , now connect one plate to a potential a source of electrons they quicklyfill that plate to same voltage ... Electrons don't like each other ! so the electrons sitting quietly on the nearby plate find a way out via the second wire you now connect to a lesser voltage source , these are not the same electrons , did current flow yes .

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 03, 2015, 03:39:05 pm
content: Quote from: bson on May 02, 2015, 10:40:23 pmIf you took this literally there would be no current involved in a beam of positive ions or positrons. It takes a lot of viewpoints to understand the world. Those who get hung up on whether there is current through a capacitor (or not) are missing chances at a broader understanding. Same for those who stop with one explanation for how things work.

author: c4757p
author_rank: Super Contributor
timestamp: May 03, 2015, 03:43:53 pm
content: Is there a purpose to endless arguments over the semantics of current, or is it just a bunch of stubborn nerds mentally masturbating?Seriously guys, we've had this one before. Search the history.

author: G0HZU
author_rank: Super Contributor
timestamp: May 04, 2015, 04:14:47 pm
content: Here's a link to one of the AA5 videos that has obvious errors in it.Biasing An Audio TransistorObviously, he's got more than one resistor value wrong here. What is amazing is that this video has stood for many years with all kinds of positive comments. But I suspect there is a reason that the thumb up/down rating is hiddenI wonder how many people have tried to point out the design/schematic errors and had their comments blocked/removed in the last 5 years or so?

author: c4757p
author_rank: Super Contributor
timestamp: May 04, 2015, 04:23:48 pm
content: What a wanker. People do this with concepts all the time, butresistor values?

author: G0HZU
author_rank: Super Contributor
timestamp: May 04, 2015, 04:54:04 pm
content: It's supposed to be a tutorial on how to bias a transistor and yet he's managed to choose resistors that will just bias it into saturation. Even if he changed the input biasing the circuit would still be awful over temperature because there is 33K in the collector and 150R in the emitter. The operating point would change a lot over temperature. eg a change of just 30degC would make a significant impact.I think he probably just marked up the schematic wrongly. Maybe the 33K should be 3.3K. But he states it as 33K in the video. Maybe his actual test circuit was different to what was in the schematic. I think the 150R resistor was probably wrong too...I've tried twice to point this out and the comments just get blocked.