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.
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!
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.
Transformers do work as you explained, but it's important not to confuse magnetic and electric fields, inductors and capacitors etc. transformers have no (intended) capacitor and have practically nothing to do with this video.
I would say that much more accurate is to say that "electric field travels" through capacitor, not a current. Current is dQ/dt and right in the middle of capacitor, where dielectric is, there are no charge carriers. If there are no charge carriers, there can't be a current. In other words, the dQ=0 so dQ/dt = 0. Current is not flowing inside capacitor, but electric field is pushing charge carriers outside of the capacitor. What Maxwell did with displacement currents is just a modeling trick to balance out the equations. For all the practical purposes it does the job.
or rather more simply - it's an electric field that does the work of a capacitor but NOT a magnetic field as in a transformer. With a transformer - we have electricity converted to an INDUCED current via coils which generate magnetic fields (hence the ability to change voltages across the gap by induction. But with a capacitor, there is no associated magnetic field created beyond what you get traveling through the connected traces or wires. Otherwise you would see capacitors exhibiting magnetic behaviours when you turn a circuit on.
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.
I agree, Michael. The way I think - it's like electric field crosses the capacitor to push those electrons from another plate, to leave wholes, instead of electrons. Up to a point where there is nothing to push (no free electrons in the capacitor's plate), and thus current stops. So, in AC, when the electromagnetic field is reversed, the capacitor on one side has too many electrons, and on the other side, too few. So, electrons don't pass capacitor (in an ideal capacitor), but the electromagnetic field does pass through to affect electrons on another plate of the capacitor thus making electrons move, thus creating a current. Another point - any broken wire is a capacitor. Except that capacitance is so small, that it's not being considered as a capacitor. So, just blindly saying that current goes through a capacitor is like saying that current flows through a broken wire. so, the answer is a bit complicated. So, technically, current doesn't flow through a capacitor, electric fields does, affecting nearby electrons up to a certain distance).
@@hcetink Current is electrons flowing doesn't mean it has to flow through! Using your analogy, if there is a way that controls the water on one side of a barrier and controls the water on the other side. The current/information is essentially flowing through. But not the same water flowing through. Understand?
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.
+weylin6 This is the explanation that finally made me intuitively understand what a capacitor is after four years of college. The flexible diaphragm constantly moving back and forth provide the appearance that the current is flowing through barrier when it is just an equal response to the change in movement. No change means that stable, steady, not moving and basically current less.
+weylin6 When "pressure is applied" in your analogy infers a changing pressure which is alternating current. We know that a capacitor does not block AC of adequate frequency but completely blocks a continuous level of DC current since this DC current immediately starts to drop as the capacitor charges. Electric Current will stop flowing but the displacement current is another subject. Any device needs that Electric Current even though this thing called displacement current is existing. It really depends on what we are talking about.
When I was in electronics classes in college (I've really not used it since), I built circuits that used in series caps to disentangle DC from AC. The DC was effectively blocked while the AC flowed straight through. Obviously I did the choke to do the opposite and allow AC to be disentangled from DC and effectively blocked the AC. It was really while learning about chokes that magnetic fields came into play. So why did my circuit work, presumably?
I understand that that AC is not truly passing through, but is due to the reversal on AC. What I don't understand is why my circuit supposedly worked (disentangling DC from AC) if caps pass DC current through an electromagnetic field in the cap (which is what he appears to be saying). Is the AC collapsing the DC EMFs?Also, I noticed his current meter went up, then went down and stayed down. I know it can take 5 seconds or so before a cap fully charges, but it was at least a couple of seconds on 0 after that initial sharp rise. I double checked, the circuit was still closed. If you offer any assistance in this, I'd greatly appreciate it..
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.
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.
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.
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.
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.
Come on, press the envelop. Let go the concept of a fundamental particle and embrace the reality that everything is an electric field, just balanced in different ways that limit and control the interactions with other fields. You just might come to realize that magnetic fields are like shadows... but that's for the advanced class. Meanwhile you can begin to distinguish the actual field attributes and how to take full advantage, unlimited by the artificial restrictions of particle physics.
@BMan18, I agree most of fundamental physics is still wrong. . . I've come to the conclusion that our "reality" (energy/forces and matter) is fundamentally the movement (waves) of aether, which is everywhere, even in "empty" space. . .We don't really have physical substance, the aether does. . .Nikola Tesla know decades ago.
Sure am glad Christmas is just around the corner, every time I watch your videos, these BELLS ring in my head as something sort of comes around and flows properly. Thanks for teaching me a bit of electronics every day.
It can, as the switch in fact is a (small value) capacitor. But just like the capacitor you need a changing electric field in order for displacement current to flow. No more changing electric field (the switch capacitance is charged up), no more displacement current flow.
Why is it so hard to explain? You have electrons building up on one plate and their electric field repels the electrons of the other plate. Moving charge, and thats current. Isn't it... it?
Yes, "it depends". It doesn't matter how you define your boundry conditions in fact, displacement current can still flow cap "through" the cap in either case. If you don't want to talk in terms of displacement current, and only electric "conducted" current, then no current flows through the cap.
You don't actually even need to understand how a capacitor works to answer this question. If you replace the capacitor with a mystery component X which attracts electrons on the other side and repels them on the other side the current still flows. What's happening inside this component X is irrelevant.
actually...current does not physically flow through the capacitor. The whole circuit is set up by an electric field, there is an electric field induced inside of the capacitor, not actual current.
The *electrons* don't physically cross the gap. But what about the current? (Does the word "current" really just mean "moving electrons?" Always?) For example, is there a current just in front of each flowing electron? And just behind? A clamp-on ammeter says yes. A clamp-on ammeter thinks that electric currents are a particular type of magnetic field: the relative change in each electron's moving e-fields. The ammeter doesn't have to detect each electron. Instead it just has to detect the motion of the radial e-field surrounding each electron. If we look at things in this way, then whenever a cloud of electrons is moving, the "current" isn't concentrated inside each moving particle, but instead fills the space between them. If we look at capacitors from this viewpoint, then the current fills the capacitor's dielectric gap, since those changing fields extend out ahead of the electrons entering the negative plate, and also extend behind the electrons leaving the positive plate. Get a capacitor with a wide gap between its plates. Apply a high-freq AC current. Slide a clamp-on ammeter along the capacitor terminals, and across the gap. The ammeter cannot see the gap. It thinks the current is continuous.
Hey it's wbeaty! I remember your website from nearly 20 years ago! Your pretty much right we can pretty much consider current to be where ever we have a magnetic field, which includes the displacement current in the gap in a capacitor. There *really* is a magnetic field in the gap in a capacitor because of the changing electric field. if you were to put a wire there, 'electron' current would flow (imagine putting two extra plates inbetween connected to give you effectively two capacitors in series. charge moves from one plate to the other as it charges. there really is current in a capacitor!
wbeaty Yes current is the amount of moving charge per cross sectional area, if you look a certain distance in the capacitor you will not see current on the phyiscal level. What you will see is an electric field in the gap wether it be air or a di-electric with ions. Also it doesn't matter if the ammeter cannot see the gap, what do you think a battery is? Batteries induce current via electric field, those electrons are not in the battery, they are in the wire itsself.
You will find Maxwell's Displacement Current, the fundamental discovery which led to all modern EM technology. Displacement current is the same as all others in that a clamp-on ammeter measures it as current. It's different in that it's part of the fields around moving charges, and not described by the charges themselves. In SI definition of Ampere, the definition involves forces between currents, *not* amounts of coulombs transferred over time. So as we define "amperes," displacement current produces real forces just like any electric current. Of course these forces originate in the increasing charge on the capacitor plates. When a capacitor plate has an increasing charge, we find displacement current in the plate, in the adjacent dielectric gap, and in the capacitor lead wires. But no moving charges in the gap, if a vacuum capacitor.) > Batteries induce current by electric fields Wrong. They induce current by charge-transport, same as any power supply. The moving charges are in the battery electrolyte, as well as in the connecting wires. A battery is inherently a conductor, with a short circuit between its plates (conductive salt water or acid, etc.) Don't be misled by erroneous grade-school textbooks which erase the current between the battery plates. To learn correct battery physics you have to go to college-level texts (and un-learn the incorrect garbage we were taught about batteries in earlier grades.)
The answer is simply, as it turns out no actual current (or electron flow)makes it across the gap, at least in an ideal capacitor.Nevertheless clerk maxwell noted that even if no real current passed from one capacitor plate to the other,there was a changing electric flux through the gap of the capacitor that he believed (and proved) that it permeated the empty space between the capacitors and induced a current in the other plate.this curren is known as displacement current (Id)
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.
Dang. Dave, I wish I lived across the street from you. What thrust me into my obsession with electronics fundamentals was my attempt at using a dynamic microphone on a hardwire telephone. I ran into a problem until a docent at the Pacific Bell building museum on 2nd street in San Francisco informed me that the telephone volts were clobbering my dynamic microphone and that I should use capacitors to prevent it. I ended up using one 22 microfarad capacitor on each pole and the result was magic. I still had problems with the receiving end of my phone calls but I'm sure that I could have worked it out. But I had a lot of fun and I got to gain knowledge about microphones.
I ADORE your videos! EE used to bore the crap out of me, in school it even intimidated me, although the actual building and soldering in itself always seemed interesting. Then I got into repairing game consoles and such, I became more interested in the actual theory behind all of this thanks to your channel and this is one of those videos where I can proudly say: Learned and keeping more from this short video than 1-2 lessons in Physics. Go figure. I'd have loved to have you as a teacher and some more motivation and goals back in high school. :)
Maybe, but that wasn't the point. Because the term "current though" is so commonly used and accepted in the industry, that I wanted to show that there is indeed a fundamental principle in physics and maxwell's equations that effectively allows (a different type of) current to "flow through" the dielectric.
Another gem from Dave - displacement current - brilliant! I was on the 'No' side of this question (and still am to the extent that electrons don't physically pass through a cap) but also understood that the practical answer was 'Yes'. Now I have a name for the current that flows while a cap is building its static charge.
A dissectible capacitor is a beautiful thing to behold, and further convolutes this issue and leans away from electromagnatism and how charges are stored and where... There's a wonderfully WEIRD video from MIT right here on youtube if you use search term "MIT Physics Demo -- Dissectible Capacitor". Take a peek for fun!
fred brooks Are you saying the video was a fake? I've read about this experiment enough places to have taken it for fact. But, I do have all the gear here to put it to the test if you're telling me these videos and accounts are hoaxes...
It's not fake or a hoax. It's a running joke that hides the fact that the surface charge (on a thin film of water from the air as the conductor) on the glass inner and outer surfaces receive the charges from the metal plates when they are removed (via high voltage corona discharge) and hold the energy in a electric field with the glass as a insulator (not a dielectric) and is transfered back to the metal plates when reassembled.
fred brooks That is absolutely fascinating! Thanks for that explanation. I had heard that Franklin fooled himself in doing this experiment, but never heard the reason why. I'm currently reading 3 books on electrostatics as it's something I've had a long term hobby interest in (I also do a kids electrical show with tons of HV gear). I do see claims, in several places in the books I'm reading that charges are in fact stored in the dielectric and not in the plates. I love to experiment. If I were to reproduce the MIT experiment, but heat everything up as hot as possible with a heat gun to avoid moisture, would that be a good control to eliminate the possibility of moisture? I do also have a vacuum chamber where everything could be placed after heating, but it would be more of a pain to rig... Would I be wasting my time?
A very good description Dave. A good analogy I once came accross was that of a water pipe with a flexible diaphram inside. The diaphram would stop actual water flow but any variations in flow would still get transmitted from one side of the water pipe to the other. Since water movement is the analogy to electric current, there you have it. Simplistic but easy to understand.
"and you can spend years and years of your life trying to understand how, or if, current flows through a capacitor." No thanks, I'll just trust I_in -> I_out. I learned my lesson on the difference between practical circuits and physics because when I was a kid and thought current was backwards because electron flow, and then later I learned about holes as charge carriers and realized oh, current doesn't actually mean electron flow after all, and those engineers aren't a bunch of idiots. Who knew?! LOL I really liked the brute force proof at the end. The video would have been less fun with that at the start.
Great video. Thanks Dave. All the theoretical "stuff" and fancy formulas aside, I heard it like this: in this series circuit, when SW1 is closed, electrons from B1 flow into plate 1 of C1 and "displace", by the electromagnetic repulsion of like charges, electrons from C1's other plate. Essentially, 3 electrons from B1 moving to plate 1 of C1 means 3 electrons leave plate 2 of C1. Current doesn't flow "through", but it does flow into and out of C1 until plate 1 of C1 is charged to "capacity". Thx
2:24 "The current starts off incredibly high because the capacitor's effectively a short circuit when you close that switch." This isn't quite correct. The maximum current is limited by the resistance of the resistor that lies in between the capacitor and the battery (and its voltage). If we remove all resistors and other components and simply connect a capacitor directly to a voltage source, then we can say we're charging a capacitor on a short circuit. But even in that case, while current flows through the circuit (i.e. through the wires), no current flows through the capacitor (i.e. from one terminal of the capacitor to the other).
When the left plate gets charged, the charge will need to be compensated on the right plate so therefore there is also charge transport (current) to the right plate.
Good video Dave but I always teach that inductors work by electromagnetics and capacitors work by electrostatics. You said, quite correctly, that electric current is the flow of electrons. If the electrons are not moving then there is no electric current. Consider now the following analogy which I use for teaching how capacitors work. Imagine a shopping Mall filled only with heterosexual men, who represent the electrons on one plate of the capacitor. All of a sudden a naked lady appears (+ charge) in a shop window. All the men rush to the window and because they move there is an electric current. Quite large at first but tails off as the furthest men get to the window. The glass window (insulating dielectric) stops the men touching the girl but because the Mall is now devoid of men (negative electrons) you can say that the Mall has attained a positive charge because of this.Now you change the polarity, we can do this in the analogy by replacing the naked lady with a naked man! The men zoom away from the glass window, creating a reverse current flow, back into the Mall, so making it negative. So with this simple analogy, one can see how a + and then - charge on a capacitor plate can make the electrons go back and forth, creating a current even though non of the electrons actually go through the dielectric. It's not a perfect analogy but always causes a laugh in the classroom and gets the students attention. :-)
Les Carpenter, the problem with your analogy is that if you hold the (+) charged side of the capacitor to the (+) charged battery terminal voltage by attaching them together, when you connect the (-) charged terminal of the battery to the (-) charged terminal of the capacitor according to your theory none of the particles of electricity from the (-) battery terminal should make it through the capacitor, yet they certainly will. When they changed Ben Franklin’s markings on a battery they really messed things up nicely, that is what caused all the confusion.
Its not flowing through the capacitor.its charging the surface of the positive side of the cap. Then when its fully charged no more current builds up until its discharged again.
If the current is not flowing through the capacitor we should be able to disconnect one side of the capacitor from the circuit and it should work the same way. BUT in that case the capacitor dosen't charge, why: because the freaking current has to flow through the capacitor!!
Well, if you want to be specific, if you put n electrons into a capacitor, n electrons will come out the other end, but they will not be the same electrons that you put in. So yes, one could say that no current flows through a capacitor because no electrons go all the way through a capacitor. However, seeing one can't id individual electrons (and even if you could it would be a waste of time), there is no effective way to tell the difference. Electrons go in, electrons come out, current flows.
If you will take LED, resistor, battery and big capacitor and connect it all in series you will see that led will turn on for a short time. So current is flowing. Same thing like at the end of video, but LEd instead of meter. What else you need to know to convince you that yes current flowing thru cap. Current flow from battery, LED, resistor to capacitor and than out from capacitor back to battery. So what is going on in capacitor ? Current using teleportation from one electrode to another ? Doesn't matter, same current goes in as goes out. So You can tell that it flows thru. And If you wil take compensating capacitor for example from linear fluorescent luminare (classic one with magnetic ballast) and connect it in series with light bulb (not powerfull) and connect it to 230/110V AC 50Hz bulb wil glow. In AC circuits you can calculate Z or Xc of capacitor.
Current does not flow. Current is a name that means flowing. And is a term that is used to represent the charge flow rate. In Coulombs per second. So to say "a current flows" is the same as saying "The rate charges flow flow" which does not make any sense. But amazingly everyone keeps saying sentences like "current is flowing". Which will be totally confusing for beginners. It is only charges that flow, never ever current. Its as wrong as saying "a river current flows". Also displacement current is the term used to describe the "rate of change" of the electric field between the plates of a capacitor. For example in an air gap capacitor there is no polarization current (dielectric charge effects) at all. So its all just dE/dt only, which itself comes about from the changing PD between the plates. That is all that displacement current means.
Shamel Sanders Current means 'charge flow'. So if you see a sentence in a text book like so "In the circuit there is a current of 4 amps" That is perfectly correct. And is just another way of saying "In the circuit there is a 'charge flow' of 4 amps" which is also perfectly correct. But if the sentence in your text book (sadly many of them make the same mistake) is like so "In the circuit there is a current flow of 4 amps" That is incorrect use of the English language, as it translates to the following. "In the circuit there is a 'charge flow' flow of 4 amps" It is not that the person who writes or says this does not understand about circuits etc. Its purely a semantic error only, but one that is very difficult if English is not your first language. It might seem pedantic to pick up on this, but I think it is important for many who will be confused by this wrong use of the English language. So the following examples below, are all incorrect uses of the word current. I have added a correct usage afterwards. (Looks like many comments on this video is making same or similar errors) In every correction below, if you just substitute the word "Current" with "charge flow" you will see they still read correctly, where as the ones I am correcting you will see very clearly (if you make the same substitution), that they are all grammatically incorrect. here is an example where the same error has been done twice. "Displacement current flows but electric current does not flow" A proper way to describe the above is as follows: "A Displacement current exists but an electric current does not" Here is another wrong sentence "Does current flow through a capacitor" Here is one correct way to describe that. "Do capacitors plates have a current between them" here is another wrong sentence "current never flows through the dielectric of the capacitor" And the correct way to express that meaning. "Current does not exist within the dielectric of the capacitor" etc etc.
Dave is absolutely correct. It requires the addition of charge density to really describe it. Schematic diagrams don't consider that current flows differently over different surfaces.
I've gone through semesters of electronics in middle school, high school and university and nobody was able to explain which is the actual direction of current flow. Until I looked for an answer on youtube and kind of got it. Having diplomas and degrees only means you know formulas and how to work with electronics, not that you have the absolute understanding of what's happening.
That's a shame you had that experience. When I was doing electronic engineering at the university of kent it was one of the first things taught and explained very clearly. For circuit anyysos and design we used conventional current flow but were taught about actual current flow in solid state physics.
Having diplomas or degrees means you have diplomas or degrees. It means you are SUPPOSED to have an understanding of what's happening. Whether you do or not depends on the quality of your teachers and the quality of your effort (which is largely dependent on your interest).
We learned electric current flow the 1st day of introduction to electric circuits 101. The electrons, the charge carriers in an electrical circuit, flow in the opposite direction of the conventional electric current.
"Actual" charge flow is too complicated: electrons flowing in metals, protons flowing in battery acid and in proton-conductors. In salt water it's flowing +Na and -Cl ions. In alkaline electrolyte it's -OH ions. In wet dirt and in human bodies it's +Na and -Cl ions, same as in salt water. In sparks and gas discharge it's electrons flowing one way and positive ions flowing the other, going past each other in opposite directions. Cont... So, which is the REAL direction of electric currents? (The answer must apply to conductors, not just metal wires.) Think carefully: when you're getting electrocuted, no bare electrons are flowing in your body, but opposite-charged ions are flowing in two opposite directions. Physics has the answer: we hide the complexity behind a simplified concept called "Conventional Current." To do this, we add up all the flows to form a single current. For example, in wet dirt, if pos and neg ions are flowing in opposite directions, that means the negative charges that flow backwards must add together with the positive charges flowing forwards. (A double-negative makes a positive current, so negative charges, flowing backwards, will give the same amperes as positive charges flowing forward.) Wet salty ground has a single amperage made of conventional current. (We don't need to think about the many charge flows in the dirt: the +Na and -Cl and +K and +Mg and +H and -OH.) For those who want the REAL currents, and not the simplified version, it's not enough to just say that "current equals electrons." Current in copper wires is actually electrons flowing past positive copper ions ...so if we move the wires backwards at just the right speed, then the electrons stop moving, and the electric current is now composed of positive copper ions going backwards! (See, the REAL current was never just a flow of electrons. It had always been a *relative motion* between the copper atoms' nuclei and the copper electron clouds.) Conclusion: if we want simplicity, where we just deal with a wire's volts and amps, then we use Conventional Current to hide the actual charge motions. But if we want reality, then we'll end up with Ben Franklin's Kite String, the famous twine which is an acid-based conductor. Ben Franklin's kite string used +H ions as its charge carriers. Only the protons were flowing along his twine, not electrons. (Heh, so when Franklin assumed that the charge-carriers within conductors were positive, HE WAS RIGHT?!)
Do you think that learning from the internet will make you an electrical engineer (EE) in time? Have you seen the curriculum of (EE) from a major university?
Beginner question :) If it is flowing through it, why does the current go down? If it could flow through, it should not go down? I imagine it as the buildup of a potential between the two plates, like two magnets where the two norths are facing each other. Maybe both are correct as you said "how you look at it"?
I'm soon to begin studying a HNC in Electronic Engineering :D And having these videos available so freely is incredible! I love that we, in this day and age, can hear from passionate, knowledgeable people, the world over!!! It's such a huge benefit over having to hope that you have a tutor who is passionate enough to not kill your fledgling interest in a subject! Even bad teachers can now be offset by the passion of those who wish to share theirs with everyone else! :D
Dear Professor.... What is the difference when 1) Discharge a capacitor with some load connected 2) Charge with reverse polarization (already charged) I want to get some more grasp undertanding about this difference. Thank you!!
Hi Dave :) Love your blog! This is the frist time you have dissepointed me, there is too much wrong in this episode.. :( It is the physics that is the rule, but how you concider it in practial electronic is up to you. So as long as you don't have a dielectric breakdown, there will be next to no current flowing through the capacitor. You can only change the electric potencial over the capacitor, and that involve a current going in and out of the capacitor.. Keep up the good work :)
Can you clarify for me: My understanding in a series circuit: Electrons flow into the more negative side of the capacitor plate, charging it essentially with static electricity, and electrons repel one another. The higher concentration of negative charge on the negative plate 'displaces' the electrons on the more positive plate through the conductor back to the current source. No?
Touch a cap that is in a high amperage circuit, that is fully charged, like the one that is hooked up to the magnatron in your microwave, and your corpse will prove that current flowed through that cap into your body and killed you.
All your proving is that current did at one time flow into the capacitor and at one time flowed out of it into your body and killed you ;) this video should explane it all, maybe watch it again?
OK, connect one side of the capacitor to your wall socket, and hold the other side with one hand. Now, ground your other hand and you'll know for sure if current flows through the capacitor, or not, because if the current flows it will roast you. (it will roast you, don't try it; use a meter like in the video)
Really, no current "flows through" the capacitor, in an AC circuit. It just "appears" to do so. Unless a capacitor's dielectric is "ruptured," there is no current actually "through" the cap. It just "appears" that currect is flowing "through" the cap. The cap just "charges, discharges, charges (in the opposite direction) then discharges. Current looks like it is moving "through" the cap, but really is not. (yes, you would feel the effects of that AC :) An electrolytic cap, in a DC circuit, will pass "the affects" of AC, but will block DC. No current will actually "pass through" the cap. If there is an "AC ripple" riding on the DC value, the DC will be blocked, but you would see that ripple on both "sides" of the cap. Or, am I wrong? I
I don't understand why you've overcomplicated something so much that's clearly for people that are fairly new to the subject. Why you'd bring up displacement current without just mentioning electrons repelling is beyond me. (Especially when you don't even slightly mention what displacement current is. You just say displacement current flows therefore current flows without giving any reason whatsoever) The absolute exact same thing happens in a wire as a capacitor. Electrons repel other electrons, in a wire they move forward and since they repel other electrons those electrons move forward by the same amount. They move forward onto one plate of a capacitor and since they repel other electrons they repel the electrons forward off the other plate of the capacitor.
quadcatfly A capacitor is a conductor, electric current flows across it. Once the plate becomes charged enough, it requires too much energy to put more electrons onto the plate, due to the electrons on the plate repelling them away, so it stops conducting.
You should brush up on electric conductivity of dielectric materials. Especially the part where it says "dielectrics are insulator". All those electrons being piled up on one side of dielectric would immediately clot and prevent (e.g. repel) any new electrons from coming in, let alone push electrons from the other side of the plate (btw since when we have free electrons in dielectrics?). While you are contemplating this try to come up with explanation of how many capacitors IN SERIES could possible work based on your theory. If I place 20 caps in series the last in line would NEVER be charged because the first one (closest to -) will have charged and cut off the supply long before. But that is not how it works in a real world.
"All those electrons being piled up on one side of dielectric would immediately clot and prevent (e.g. repel) any new electrons from coming in, " Yes, that is exactly what I just said. "let alone push electrons from the other side of the plate (btw since when we have free electrons in dielectrics?)." A capacitor is in its simplest form just two plates. Put one electron on one plate, it will push the electron off the other plate. Continue to put more on, you'll keep pushing them off the other plate, until eventually it takes too much energy to put an electron on the first plate. "While you are contemplating this try to come up with explanation of how many capacitors IN SERIES could possible work based on your theory." Completely trivially? There is nothing magic about putting two capacitors in series. The electron that is pushes off the first capacitor goes and charges up the plate on the second capacitor and so on. "If I place 20 caps in series the last in line would NEVER be charged because the first one (closest to -) will have charged and cut off the supply long before." That is nonsense. You put one electron on the first plate of the first capacitor, one electron comes off the second plate. The time it takes is irrelevant.
The way i see it: is the Voltage applied to the capacitor Pushes or Pulls electrons onto the plates of the capacitor causing the field to change . While the electric field is changin, current is flowing to/from the capacitor. Once the field stops changing current stops flowing in a practical sense.
Perhaps will it necessary to make the difference between Alternating Current and Direct Current. They both behave very differently when charging/discharging a capacitor.
Let's send him all our dried out old capacitors. You know the ones that let current go TROUGH them ;) Maybe he likes to use those ones. I prefer the new ones, you know the ones that DON'T let current go through them ;)
Dave, thanks a million for this and your other videos! This was helpful and made perfect sense to me. I have been a hobbyist in electronics for 20+ years and hate to admit it but you have forced me to rethink what I thought I knew about such a basic component. Cheers
you must establish what current is first before you even discuss it...the real simple discussion should be '' DOES ELECTRONS FLOW THRU THE DIELECTRIC OF THE CAPACITOR'' the answer is NO...
Old post, I know...if the current changes medium it's still current. I.E. Current in a wire being turned into radio waves, transmitted, received and transmitted back through a wire. Did the current actually flow "through" the air? Yes, yes it did. Just because it wasn't some visible plasma arc doesn't mean it didn't flow. Same idea here.
My mind is blown by this. I had trouble trying to wrap my head around why/how caps worked in series despite creating open circuits. Kudos to you. :) Could you do a fundamental fridays video about inductors, and one about RC, RL, and RLC circuits?
Caps in SERIES is like ONE cap with a GREAT distance between the plates (the first cap's plate, and the last cap's plate) In PARALLEL it's like ONE giant cap with HUGE plates. So, in series, the capacitance will be smaller than the smallest cap. In paralled, they are all additive (their cap values)
Pull your head outta your asterisk Dave, You're confusing TO (not two or too) with THROUGH (not threw). TO! not through. 8+ minutes to get to Displacement Current. Charge flows TO the plates of the capacitor. Not through. OMG. Couldn't a cut a long story short... much shorter. Thank you for the effort anyway.
Dave said voltage charges up in a linear fashion. From my understanding voltage doesn't charge up in a linear fashion in a capacitor, it charges exponentially depending upon the time constant (t=RC) This can be calculated in increments of 5t. It is also possible to find the instantaneous voltage at any point with this formula if increasing and beginning at 0v: v=Vf(1-e^-1/RC) with RC being one time constant.
When he said that he was showing a circuit with an 'ideal' current source connected to the capacitor. In this case, the voltage would charge linearly since the current is linear. However, if you were to look at the voltage across the current source it would not be linear, it would be exponential.
(commenting to you Travis) The voltage across the current source is the same as the cap that it is across, as they are in parallel. However, you are correct that a constant current source would ideally (if it had infinite voltage available and the circuit could take it) drive the voltage across the cap LINEARLY, not exponentially, described by the eq. v = i*t/c. This formula also shows that the voltage rises linearly with both current and time, and that the larger the cap, the slower it'll go. Also, even if a series resistor were inserted, the voltage on the cap would be identical. The exponential formula applies when it's a voltage source and there is a resistance for that voltage to divide across as the cap charges.
Neat treatment of the topic. I graduated 4th in my class (book smart *only*) from Stevens Institute 35 years ago as an engineer, and I can't recall us learning about displacement current. I loved physics and math and of course we learned Maxwell. Doesn't mean we didn't learn about displacement current, but I've never paused to think about the fact that the dielectric does not conduct electrons. As you say, what is the practical use of that fact? Nice job!
So where is the second plate getting the electrons to send on down the circuit? Is it the electrons in the plate itself are stimulated by the electric field to flow?
So are you saying that if you had an ammeter connected between the capacitor and the negative side of the battery, you would get a reading when you closed the circuit switch?
I do like the explanation which means for me that the answer is in between "the current does flow through a capacitor and it doesn't at the same time." This way we can make use of the capacitor accordingly. :) However, I look at it as the positive charges of the battery goes to one plate, while the negative goes to another plate. And the two plates are in communication because of the electric magnetic. But, it still an issue to understand whether the current pass through the capacitor or not, and this can be seen if we put three capacitors in series, how is it possible for the middle capacitor to be charged? Therefore, we just force ourselves to believe that the current does pass the capacitor in order to resolve the problem. Perhaps, you got a better answer for this puzzle of three capacitors in series. And I do appreciate it! I like the way you teach. Thank you so much!
Nice video, Dave! I'd love to hear your thoughts on conventional current vs. electron flow current. Never did understand why they decided to go the opposite route of the obvious!
So in a series circuit, the circuit won't have any current after the capacitor is fully charged? So a light bulb added to the circuit will stop glowing when the capacitor is fully charged?
So when you build a radio as a kid that works without a power source, but can power a little earpiece, is that displacement current flowing from the radio tower to the oscillating piece of quartz?
Eletrolytic caps forms an oxide coating on one of the plate materials when exposed to the right polarity. This serves as an insulator. When reversed connected, it does the opposite by unforming the oxide which allow current to flow at a high rate. This produces heat and preasure which must be releaved through the rupture of the cap.
Does the graph look like that no matter the capacitance of the capacitor, or does that represent a 1 farad unit, or , would like a 15 micro farad cap , charge a whole lot faster with the volts, compared to the current passing through and coming out it., so the volt line would be a whole lot steeper?
The way I understand it, current enters one side of the capacitor increasing the electrical charge on that plate (+Q becomes more positive) while simultaneously, the electrical charge on the opposing plate decreases (-Q becomes more negative). This matched but opposite change causes an apparent current to be measured through the capacitor (I = dQ/dt). Thus, it appears that current flows in one terminal and out the other. See 6:11 for the diagram which Dave explains the charged plates.
This presentation is useful for me. However It can be concluded that electric current can flow through a capacitor like a bulb. But it can't do continuously, electric current give up ( I equal to zero) when maximum voltage of a capacitor reached (showed in the graphic). Is my conclusion true ? Thank.
Hasan Yahya You are correct. Capacitors are by definition an open circuit and incapable of passing any voltage. When they are in a DC condition, be that charged or discharged, they do not conduct power of any kind. But when the charge is changing, they will increase the power they hold with the change in charge. So store power when charging up, and dump power when charging down. The guys who first investigated electricity called this kind of stored power dielectricity. Modern dudes call it displacement current. Once a dielectric material is charged fully, it will conduct that displacement current just like a conductor. Capacitors break down and short out before they get to this state. Displacement current is real, but does not generate an electric field when it moves, like charge current when it moves. The present theory that is the most simplified is that dielectricity is the source of magnetic field lines. It flows with charge movement, orients with charge orthogonally and repels other dielectric fields orthogonally when capacity is filled. This is the cause of the magnetic effects people see and measure. It is motivated to move by capacity differentials in circuits, and will surround a conductor which typically would attract more dielectricity than the conductor has capacity for, so it hangs out at the surface, in the insulation and air surrounding a conductor (or magnet). A conductor in this space of another charged conductor can pull it away by activating the movement of it's charge field, or with circuits that have more capacitance. There are ways to configure circuits to operate with dielectric energy controlling the propagation of the electric field, and these propagate signals faster than light speed. It should be obvious that electricity is not a particle phenomenon and the fields are not fully contained in the physical structures of circuits. So, the systems are open and external conditions can influence the circuits beyond the typical analysis used that focuses just on components. Electric fields are everywhere and energy can be harvested simply with the proper activation of electric fields.that draw dielectricity to conductors and then transfer it to the proper mix of charge based on the application. The real mystery is in how smaller fields will combine to form larger fields that affect other fields without using up the smaller fields. No one knows how this happens, but its there for the taking. It happens all the time, no one notices. Permanent magnets don't wear our by usage any more than a light will wear out by what it illuminates. Electric induction generators do not require more energy based on how much energy you use unless you force the used energy back into the generators. Or do I digress? Knowing what is really happening leads to more effective designs.
I haven’t watched the video, but in brief: • Conduction current flows through both leads/wires of the capacitor, but not through the dielectric/gap between the plates of the capacitor. • The conduction current in both leads is the same. • You don’t ever charge (add net charge to) or discharge (remove net charge from) a capacitor as a whole (i.e. considering both plates); the net charge of a capacitor is always constant. But you do charge or discharge the individual plates; their net charge does change with time. • Displacement current “flows” through the dielectric/gap between the plates of the capacitor, but not through the leads/wires of the capacitor. • The numerical value of both types of currents is the same in a capacitor.
Sir what will happen after the connection. If I connect a finite current carrying inductor(say 5A) across a inductor having no current initially. (We assume small series resistance of inductor.) Will the current just after connection be 5A or 0 A. As we say that inductor resist sudden current change.
To stir up more trouble: ignore vacuum capacitors. Instead... in modern ceramic caps about 99.99% is a real electron current in the dielectric. Those electrons in the dielectric can move a bit, but aren't totally free like in metals. Look up ferroelectrics and PZT ceramics. Not "vacuum polarization," but polarized ceramic. So, Maxwell's displacement current actually plays an insignificant role, at least in ceramic caps. That's why their dielectric const is 2,000, or 5,000, or higher.
You sound like the real thing, my question on the condenser fan motor instructions says the two brown wires goes on either side of capacitor is there a power wire or black that should go on one side of the capacitor to charge it ? I have two brown wires one brown with white stripe but I forgot if one of the black wire is connected too capacitor?
Reminds me of the old conundrum: Capacitor C charges to voltage V has energy of 0.5*C*(V)^2 and a charge of Q=CV. Connect a capacitor of the same capacitance in parallel. Voltage on each capacitor is now the same charge divided by twice the capacitance or V/2 Energy of each capacitor is now E=0.5*C*(V/2)^2 or (0.5*C*(V)^2)/4. Energy in the two capacitors combined is (0.5*C*(V)^2)/2. Where has half the energy gone? cheers
I remember this lesson from university. The point was that current is not only movement of charged particles, but also a change in electric field is also a current. I think that was on forth year. Another argument was that magnetic effect this current has are the same as those of moving charged particles. Can't remember any math or details nowdays, don't really use it in practice. Amazing that, at some point in time, I had some understanding of that stuff.
A beginner question, Dave, regarding the matter: if the current flowing through the capacitor isn't coming from the main electron source (i,e, the battery) because, as you explained, its electrons are building up on the capacitor plate, where does the exact same amount of electrons come from? I mean, they most come from somewhere, right? And if it's not from the battery where the main current is flowing from, where is it flowing from actually? Thanks :]
You still have net electron movement to build the potential (in the form of a magnetic field) between the two plates. It is also practical to define it as such, because that is what you will physically see in your circuit.
The concept of dipoles that develop in the dielectric more easily explain the concept of current flow through the cap. Also, it is important to remember that the cap will only have current flow during times of a CHANGE in voltage. Current will flow foe AC, but not DC (since DC does not change). This also explains Xc, the phase shift, and why caps block DC (Xc is infinite when F=0)
so there is no stopping the current flow after the cap is charged? or does the cap act like a battery and stop current when it reaches max V? until the polarity is reversed?
But when there is no change in charge density across the plates, how will a change in magnetic flux take place to ultimately give rise to displacement current ? Someone clarify please. Thanks.
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.
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!
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.
Wow, that's such an obvious way of saying it, but I never thought about it like that. Thanks!
Transformers do work as you explained, but it's important not to confuse magnetic and electric fields, inductors and capacitors etc. transformers have no (intended) capacitor and have practically nothing to do with this video.
I would say that much more accurate is to say that "electric field travels" through capacitor, not a current. Current is dQ/dt and right in the middle of capacitor, where dielectric is, there are no charge carriers. If there are no charge carriers, there can't be a current. In other words, the dQ=0 so dQ/dt = 0.
Current is not flowing inside capacitor, but electric field is pushing charge carriers outside of the capacitor.
What Maxwell did with displacement currents is just a modeling trick to balance out the equations. For all the practical purposes it does the job.
or rather more simply - it's an electric field that does the work of a capacitor but NOT a magnetic field as in a transformer. With a transformer - we have electricity converted to an INDUCED current via coils which generate magnetic fields (hence the ability to change voltages across the gap by induction. But with a capacitor, there is no associated magnetic field created beyond what you get traveling through the connected traces or wires. Otherwise you would see capacitors exhibiting magnetic behaviours when you turn a circuit on.
HitAndMissLab actually - i wonder if it would be provocative to suggest that YES - current flows through a capacitor - but VOLTAGE does not (?)
I did not slip up on the term. The term "through" is defacto standard in the industry and used in many aspects of teaching.
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.
This answer Michael is more correct to me. Dave is obviously a sharp cat, but his answer sounds more philosophical.
I agree, Michael. The way I think - it's like electric field crosses the capacitor to push those electrons from another plate, to leave wholes, instead of electrons. Up to a point where there is nothing to push (no free electrons in the capacitor's plate), and thus current stops. So, in AC, when the electromagnetic field is reversed, the capacitor on one side has too many electrons, and on the other side, too few. So, electrons don't pass capacitor (in an ideal capacitor), but the electromagnetic field does pass through to affect electrons on another plate of the capacitor thus making electrons move, thus creating a current.
Another point - any broken wire is a capacitor. Except that capacitance is so small, that it's not being considered as a capacitor. So, just blindly saying that current goes through a capacitor is like saying that current flows through a broken wire. so, the answer is a bit complicated.
So, technically, current doesn't flow through a capacitor, electric fields does, affecting nearby electrons up to a certain distance).
@@hcetink He explained the same thing. No difference.
In my words, current yes, electrons no. And an animation will be a better explanation.
@@johnyang799 uhhh ..... have you ever swam in a current without water?
@@hcetink Current is electrons flowing doesn't mean it has to flow through! Using your analogy, if there is a way that controls the water on one side of a barrier and controls the water on the other side. The current/information is essentially flowing through. But not the same water flowing through. Understand?
I would also describe James Clerk Maxwell as an incredibly smart dude
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.
+weylin6 This is the explanation that finally made me intuitively understand what a capacitor is after four years of college. The flexible diaphragm constantly moving back and forth provide the appearance that the current is flowing through barrier when it is just an equal response to the change in movement. No change means that stable, steady, not moving and basically current less.
+weylin6 When "pressure is applied" in your analogy infers a changing pressure which is alternating current. We know that a capacitor does not block AC of adequate frequency but completely blocks a continuous level of DC current since this DC current immediately starts to drop as the capacitor charges. Electric Current will stop flowing but the displacement current is another subject. Any device needs that Electric Current even though this thing called displacement current is existing. It really depends on what we are talking about.
+Steve Seifer buy today pay tomorrow
When I was in electronics classes in college (I've really not used it since), I built circuits that used in series caps to disentangle DC from AC. The DC was effectively blocked while the AC flowed straight through. Obviously I did the choke to do the opposite and allow AC to be disentangled from DC and effectively blocked the AC. It was really while learning about chokes that magnetic fields came into play. So why did my circuit work, presumably?
I understand that that AC is not truly passing through, but is due to the reversal on AC. What I don't understand is why my circuit supposedly worked (disentangling DC from AC) if caps pass DC current through an electromagnetic field in the cap (which is what he appears to be saying). Is the AC collapsing the DC EMFs?Also, I noticed his current meter went up, then went down and stayed down. I know it can take 5 seconds or so before a cap fully charges, but it was at least a couple of seconds on 0 after that initial sharp rise. I double checked, the circuit was still closed. If you offer any assistance in this, I'd greatly appreciate it..
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.
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.
Quote of the day:
"Does current flow through a capacitor?"
*touches lead to battery - ammeter spikes*
"Yep! See you next time!"
Made me lol!
The best ending for this video! he made me laugh. excellent !!!
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.
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!
I'm sure we are learning something. thanks dave
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.
If you want to see a good argument, put a physicist and an electrical engineer in the same room and ask them their viewpoint on this topic!
you explain it so much better....much better than sitting in a 2 hour lesson...
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.
interesting concept
Come on, press the envelop. Let go the concept of a fundamental particle and embrace the reality that everything is an electric field, just balanced in different ways that limit and control the interactions with other fields. You just might come to realize that magnetic fields are like shadows... but that's for the advanced class. Meanwhile you can begin to distinguish the actual field attributes and how to take full advantage, unlimited by the artificial restrictions of particle physics.
Yes! It's good to keep the goal in mind: why are we moving charge through the circuit? Just to move energy to do some work elsewhere.
@BMan18, I agree most of fundamental physics is still wrong. . . I've come to the conclusion that our "reality" (energy/forces and matter) is fundamentally the movement (waves) of aether, which is everywhere, even in "empty" space. . .We don't really have physical substance, the aether does. . .Nikola Tesla know decades ago.
+eqlipse333 No magnetic fields are involved.
Applying a voltage across the plates allows electrons to be added to one and subtracted from the other.
Sure am glad Christmas is just around the corner, every time I watch your videos, these BELLS ring in my head as something sort of comes around and flows properly. Thanks for teaching me a bit of electronics every day.
Fantastic, now every time I read a circuit, I do so in an Aussie accent. :D
I love the ending. Throwing the marker in the air. Then actually connecting the circuit asking the sarcastic question. Priceless.
Love your enthusiasm.
It can, as the switch in fact is a (small value) capacitor. But just like the capacitor you need a changing electric field in order for displacement current to flow. No more changing electric field (the switch capacitance is charged up), no more displacement current flow.
so how come you do not specify current ratings when buying a capacitor?
Current ratings are specified in terms of temperature raise...
Also, you CAN buy a displacement current meter. That's what clamp-on meters measure. Fields, not counting any moving charges passing through the hole.
Why is it so hard to explain? You have electrons building up on one plate and their electric field repels the electrons of the other plate. Moving charge, and thats current. Isn't it... it?
moyrml: Thats a chicken and egg situation, its hard to imagine 'spaces' moving, so its better to use electrons in the description.
@@suzesiviter6083Space isn't "moving"
Dave is a sweet person. I'm glad to share the world with wonderful people like him. Thanks for all the knowledge and memories
Yes, a lot of repetition in this one is obvious when I rewatch it. Wasn't that obvious in editing.
Yes, "it depends". It doesn't matter how you define your boundry conditions in fact, displacement current can still flow cap "through" the cap in either case. If you don't want to talk in terms of displacement current, and only electric "conducted" current, then no current flows through the cap.
You don't actually even need to understand how a capacitor works to answer this question. If you replace the capacitor with a mystery component X which attracts electrons on the other side and repels them on the other side the current still flows. What's happening inside this component X is irrelevant.
actually...current does not physically flow through the capacitor. The whole circuit is set up by an electric field, there is an electric field induced inside of the capacitor, not actual current.
The *electrons* don't physically cross the gap. But what about the current? (Does the word "current" really just mean "moving electrons?" Always?)
For example, is there a current just in front of each flowing electron? And just behind? A clamp-on ammeter says yes. A clamp-on ammeter thinks that electric currents are a particular type of magnetic field: the relative change in each electron's moving e-fields. The ammeter doesn't have to detect each electron. Instead it just has to detect the motion of the radial e-field surrounding each electron.
If we look at things in this way, then whenever a cloud of electrons is moving, the "current" isn't concentrated inside each moving particle, but instead fills the space between them.
If we look at capacitors from this viewpoint, then the current fills the capacitor's dielectric gap, since those changing fields extend out ahead of the electrons entering the negative plate, and also extend behind the electrons leaving the positive plate.
Get a capacitor with a wide gap between its plates. Apply a high-freq AC current. Slide a clamp-on ammeter along the capacitor terminals, and across the gap. The ammeter cannot see the gap. It thinks the current is continuous.
Hey it's wbeaty! I remember your website from nearly 20 years ago! Your pretty much right we can pretty much consider current to be where ever we have a magnetic field, which includes the displacement current in the gap in a capacitor. There *really* is a magnetic field in the gap in a capacitor because of the changing electric field. if you were to put a wire there, 'electron' current would flow (imagine putting two extra plates inbetween connected to give you effectively two capacitors in series. charge moves from one plate to the other as it charges. there really is current in a capacitor!
wbeaty Yes current is the amount of moving charge per cross sectional area, if you look a certain distance in the capacitor you will not see current on the phyiscal level. What you will see is an electric field in the gap wether it be air or a di-electric with ions. Also it doesn't matter if the ammeter cannot see the gap, what do you think a battery is? Batteries induce current via electric field, those electrons are not in the battery, they are in the wire itsself.
You will find Maxwell's Displacement Current, the fundamental discovery which led to all modern EM technology. Displacement current is the same as all others in that a clamp-on ammeter measures it as current. It's different in that it's part of the fields around moving charges, and not described by the charges themselves.
In SI definition of Ampere, the definition involves forces between currents, *not* amounts of coulombs transferred over time. So as we define "amperes," displacement current produces real forces just like any electric current. Of course these forces originate in the increasing charge on the capacitor plates.
When a capacitor plate has an increasing charge, we find displacement current in the plate, in the adjacent dielectric gap, and in the capacitor lead wires. But no moving charges in the gap, if a vacuum capacitor.)
> Batteries induce current by electric fields
Wrong. They induce current by charge-transport, same as any power supply. The moving charges are in the battery electrolyte, as well as in the connecting wires. A battery is inherently a conductor, with a short circuit between its plates (conductive salt water or acid, etc.) Don't be misled by erroneous grade-school textbooks which erase the current between the battery plates. To learn correct battery physics you have to go to college-level texts (and un-learn the incorrect garbage we were taught about batteries in earlier grades.)
Love the enthusiasm that you approach these fundamentals with! You have a real talent for explaining concepts of EE.
The answer is simply, as it turns out no actual current (or electron flow)makes it across the gap, at least in an ideal capacitor.Nevertheless clerk maxwell noted that even if no real current passed from one capacitor plate to the other,there was a changing electric flux through the gap of the capacitor that he believed (and proved) that it permeated the empty space between the capacitors and induced a current in the other plate.this curren is known as displacement current (Id)
You just did in 15min what my teacher spent 3 hours on. Nicely done. Thank you Dave.
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.
Lols. +1 for batteriser ref
Dang. Dave, I wish I lived across the street from you. What thrust me into my obsession with electronics fundamentals was my attempt at using a dynamic microphone on a hardwire telephone. I ran into a problem until a docent at the Pacific Bell building museum on 2nd street in San Francisco informed me that the telephone volts were clobbering my dynamic microphone and that I should use capacitors to prevent it. I ended up using one 22 microfarad capacitor on each pole and the result was magic. I still had problems with the receiving end of my phone calls but I'm sure that I could have worked it out. But I had a lot of fun and I got to gain knowledge about microphones.
I ADORE your videos! EE used to bore the crap out of me, in school it even intimidated me, although the actual building and soldering in itself always seemed interesting.
Then I got into repairing game consoles and such, I became more interested in the actual theory behind all of this thanks to your channel and this is one of those videos where I can proudly say: Learned and keeping more from this short video than 1-2 lessons in Physics.
Go figure.
I'd have loved to have you as a teacher and some more motivation and goals back in high school. :)
Maybe, but that wasn't the point. Because the term "current though" is so commonly used and accepted in the industry, that I wanted to show that there is indeed a fundamental principle in physics and maxwell's equations that effectively allows (a different type of) current to "flow through" the dielectric.
Asterisk is the little ASCII character and Asterix is the cartoon character ;-)
Thank you very much, Dave. One EEVblog video is worth more than 3 hours of school lessons.
When did I say it was electric current flowing through? Yes, the practical demo was meant as a joke.
Another gem from Dave - displacement current - brilliant! I was on the 'No' side of this question (and still am to the extent that electrons don't physically pass through a cap) but also understood that the practical answer was 'Yes'. Now I have a name for the current that flows while a cap is building its static charge.
A dissectible capacitor is a beautiful thing to behold, and further convolutes this issue and leans away from electromagnatism and how charges are stored and where... There's a wonderfully WEIRD video from MIT right here on youtube if you use search term "MIT Physics Demo -- Dissectible Capacitor". Take a peek for fun!
It's only WEIRD if you don't understand how it's easily explained by simple physics. The MIT guys were having a bit of fun with the naive.
fred brooks
Are you saying the video was a fake? I've read about this experiment enough places to have taken it for fact. But, I do have all the gear here to put it to the test if you're telling me these videos and accounts are hoaxes...
It's not fake or a hoax. It's a running joke that hides the fact that the surface charge (on a thin film of water from the air as the conductor) on the glass inner and outer surfaces receive the charges from the metal plates when they are removed (via high voltage corona discharge) and hold the energy in a electric field with the glass as a insulator (not a dielectric) and is transfered back to the metal plates when reassembled.
fred brooks
That is absolutely fascinating! Thanks for that explanation. I had heard that Franklin fooled himself in doing this experiment, but never heard the reason why. I'm currently reading 3 books on electrostatics as it's something I've had a long term hobby interest in (I also do a kids electrical show with tons of HV gear). I do see claims, in several places in the books I'm reading that charges are in fact stored in the dielectric and not in the plates. I love to experiment. If I were to reproduce the MIT experiment, but heat everything up as hot as possible with a heat gun to avoid moisture, would that be a good control to eliminate the possibility of moisture? I do also have a vacuum chamber where everything could be placed after heating, but it would be more of a pain to rig... Would I be wasting my time?
A very good description Dave. A good analogy I once came accross was that of a water pipe with a flexible diaphram inside. The diaphram would stop actual water flow but any variations in flow would still get transmitted from one side of the water pipe to the other. Since water movement is the analogy to electric current, there you have it. Simplistic but easy to understand.
"and you can spend years and years of your life trying to understand how, or if, current flows through a capacitor." No thanks, I'll just trust I_in -> I_out.
I learned my lesson on the difference between practical circuits and physics because when I was a kid and thought current was backwards because electron flow, and then later I learned about holes as charge carriers and realized oh, current doesn't actually mean electron flow after all, and those engineers aren't a bunch of idiots. Who knew?! LOL
I really liked the brute force proof at the end. The video would have been less fun with that at the start.
Great video. Thanks Dave.
All the theoretical "stuff" and fancy formulas aside, I heard it like this: in this series circuit, when SW1 is closed, electrons from B1 flow into plate 1 of C1 and "displace", by the electromagnetic repulsion of like charges, electrons from C1's other plate. Essentially, 3 electrons from B1 moving to plate 1 of C1 means 3 electrons leave plate 2 of C1. Current doesn't flow "through", but it does flow into and out of C1 until plate 1 of C1 is charged to "capacity". Thx
2:24 "The current starts off incredibly high because the capacitor's effectively a short circuit when you close that switch."
This isn't quite correct. The maximum current is limited by the resistance of the resistor that lies in between the capacitor and the battery (and its voltage). If we remove all resistors and other components and simply connect a capacitor directly to a voltage source, then we can say we're charging a capacitor on a short circuit. But even in that case, while current flows through the circuit (i.e. through the wires), no current flows through the capacitor (i.e. from one terminal of the capacitor to the other).
When the left plate gets charged, the charge will need to be compensated on the right plate so therefore there is also charge transport (current) to the right plate.
Good video Dave but I always teach that inductors work by electromagnetics and capacitors work by electrostatics. You said, quite correctly, that electric current is the flow of electrons. If the electrons are not moving then there is no electric current.
Consider now the following analogy which I use for teaching how capacitors work.
Imagine a shopping Mall filled only with heterosexual men, who represent the electrons on one plate of the capacitor. All of a sudden a naked lady appears (+ charge) in a shop window. All the men rush to the window and because they move there is an electric current. Quite large at first but tails off as the furthest men get to the window. The glass window (insulating dielectric) stops the men touching the girl but because the Mall is now devoid of men (negative electrons) you can say that the Mall has attained a positive charge because of this.Now you change the polarity, we can do this in the analogy by replacing the naked lady with a naked man! The men zoom away from the glass window, creating a reverse current flow, back into the Mall, so making it negative. So with this simple analogy, one can see how a + and then - charge on a capacitor plate can make the electrons go back and forth, creating a current even though non of the electrons actually go through the dielectric. It's not a perfect analogy but always causes a laugh in the classroom and gets the students attention. :-)
Woow, I had never read any such explanation.
Les Carpenter, the problem with your analogy is that if you hold the (+) charged side of the capacitor to the (+) charged battery terminal voltage by attaching them together, when you connect the (-) charged terminal of the battery to the (-) charged terminal of the capacitor according to your theory none of the particles of electricity from the (-) battery terminal should make it through the capacitor, yet they certainly will. When they changed Ben Franklin’s markings on a battery they really messed things up nicely, that is what caused all the confusion.
Agreed. Nothing short of pure genius. Probably one of the greatest discoveries of all time.
Its not flowing through the capacitor.its charging the surface of the positive side of the cap. Then when its fully charged no more current builds up until its discharged again.
MrHydroguy sure the electrons don't accumulate on the negative? Conventional current is wrong.
It's charging both surfaces (one subtracting and the other adding electrons).
+johnDon No electron accumulation was mentioned though.
If the current is not flowing through the capacitor we should be able to disconnect one side of the capacitor from the circuit and it should work the same way. BUT in that case the capacitor dosen't charge, why: because the freaking current has to flow through the capacitor!!
totally agree
That was the quickest practical lesson I've ever seen. thumbs up
Displacement current flows but electric current does not flow through a capacitor.
Well, if you want to be specific, if you put n electrons into a capacitor, n electrons will come out the other end, but they will not be the same electrons that you put in. So yes, one could say that no current flows through a capacitor because no electrons go all the way through a capacitor. However, seeing one can't id individual electrons (and even if you could it would be a waste of time), there is no effective way to tell the difference. Electrons go in, electrons come out, current flows.
If you will take LED, resistor, battery and big capacitor and connect it all in series you will see that led will turn on for a short time. So current is flowing. Same thing like at the end of video, but LEd instead of meter. What else you need to know to convince you that yes current flowing thru cap. Current flow from battery, LED, resistor to capacitor and than out from capacitor back to battery. So what is going on in capacitor ? Current using teleportation from one electrode to another ? Doesn't matter, same current goes in as goes out. So You can tell that it flows thru. And If you wil take compensating capacitor for example from linear fluorescent luminare (classic one with magnetic ballast) and connect it in series with light bulb (not powerfull) and connect it to 230/110V AC 50Hz bulb wil glow. In AC circuits you can calculate Z or Xc of capacitor.
Current does not flow. Current is a name that means flowing. And is a term that is used to represent the charge flow rate. In Coulombs per second.
So to say "a current flows" is the same as saying "The rate charges flow flow" which does not make any sense. But amazingly everyone keeps saying sentences like "current is flowing". Which will be totally confusing for beginners. It is only charges that flow, never ever current. Its as wrong as saying "a river current flows".
Also displacement current is the term used to describe the "rate of change" of the electric field between the plates of a capacitor. For example in an air gap capacitor there is no polarization current (dielectric charge effects) at all. So its all just dE/dt only, which itself comes about from the changing PD between the plates. That is all that displacement current means.
Kevin O'Brien so if current doesn't flow what does it do?
Shamel Sanders Current means 'charge flow'.
So if you see a sentence in a text book like so
"In the circuit there is a current of 4 amps" That is perfectly correct. And is just another way of saying
"In the circuit there is a 'charge flow' of 4 amps" which is also perfectly correct.
But if the sentence in your text book (sadly many of them make the same mistake) is like so
"In the circuit there is a current flow of 4 amps" That is incorrect use of the English language, as it translates to the following.
"In the circuit there is a 'charge flow' flow of 4 amps"
It is not that the person who writes or says this does not understand about circuits etc. Its purely a semantic error only, but one that is very difficult if English is not your first language. It might seem pedantic to pick up on this, but I think it is important for many who will be confused by this wrong use of the English language.
So the following examples below, are all incorrect uses of the word current. I have added a correct usage afterwards. (Looks like many comments on this video is making same or similar errors)
In every correction below, if you just substitute the word "Current" with "charge flow" you will see they still read correctly, where as the ones I am correcting you will see very clearly (if you make the same substitution), that they are all grammatically incorrect.
here is an example where the same error has been done twice.
"Displacement current flows but electric current does not flow"
A proper way to describe the above is as follows:
"A Displacement current exists but an electric current does not"
Here is another wrong sentence
"Does current flow through a capacitor"
Here is one correct way to describe that.
"Do capacitors plates have a current between them"
here is another wrong sentence
"current never flows through the dielectric of the capacitor"
And the correct way to express that meaning.
"Current does not exist within the dielectric of the capacitor"
etc etc.
Dave is absolutely correct. It requires the addition of charge density to really describe it. Schematic diagrams don't consider that current flows differently over different surfaces.
I've gone through semesters of electronics in middle school, high school and university and nobody was able to explain which is the actual direction of current flow. Until I looked for an answer on youtube and kind of got it. Having diplomas and degrees only means you know formulas and how to work with electronics, not that you have the absolute understanding of what's happening.
That's a shame you had that experience. When I was doing electronic engineering at the university of kent it was one of the first things taught and explained very clearly. For circuit anyysos and design we used conventional current flow but were taught about actual current flow in solid state physics.
Having diplomas or degrees means you have diplomas or degrees. It means you are SUPPOSED to have an understanding of what's happening. Whether you do or not depends on the quality of your teachers and the quality of your effort (which is largely dependent on your interest).
We learned electric current flow the 1st day of introduction to electric circuits 101. The electrons, the charge carriers in an electrical circuit, flow in the opposite direction of the conventional electric current.
"Actual" charge flow is too complicated: electrons flowing in metals, protons flowing in battery acid and in proton-conductors. In salt water it's flowing +Na and -Cl ions. In alkaline electrolyte it's -OH ions. In wet dirt and in human bodies it's +Na and -Cl ions, same as in salt water. In sparks and gas discharge it's electrons flowing one way and positive ions flowing the other, going past each other in opposite directions. Cont...
So, which is the REAL direction of electric currents? (The answer must apply to conductors, not just metal wires.) Think carefully: when you're getting electrocuted, no bare electrons are flowing in your body, but opposite-charged ions are flowing in two opposite directions.
Physics has the answer: we hide the complexity behind a simplified concept called "Conventional Current."
To do this, we add up all the flows to form a single current. For example, in wet dirt, if pos and neg ions are flowing in opposite directions, that means the negative charges that flow backwards must add together with the positive charges flowing forwards. (A double-negative makes a positive current, so negative charges, flowing backwards, will give the same amperes as positive charges flowing forward.) Wet salty ground has a single amperage made of conventional current. (We don't need to think about the many charge flows in the dirt: the +Na and -Cl and +K and +Mg and +H and -OH.)
For those who want the REAL currents, and not the simplified version, it's not enough to just say that "current equals electrons." Current in copper wires is actually electrons flowing past positive copper ions ...so if we move the wires backwards at just the right speed, then the electrons stop moving, and the electric current is now composed of positive copper ions going backwards! (See, the REAL current was never just a flow of electrons. It had always been a *relative motion* between the copper atoms' nuclei and the copper electron clouds.)
Conclusion: if we want simplicity, where we just deal with a wire's volts and amps, then we use Conventional Current to hide the actual charge motions. But if we want reality, then we'll end up with Ben Franklin's Kite String, the famous twine which is an acid-based conductor. Ben Franklin's kite string used +H ions as its charge carriers. Only the protons were flowing along his twine, not electrons. (Heh, so when Franklin assumed that the charge-carriers within conductors were positive, HE WAS RIGHT?!)
Do you think that learning from the internet will make you an electrical engineer (EE) in time? Have you seen the curriculum of (EE) from a major university?
Beginner question :) If it is flowing through it, why does the current go down? If it could flow through, it should not go down? I imagine it as the buildup of a potential between the two plates, like two magnets where the two norths are facing each other. Maybe both are correct as you said "how you look at it"?
Yes, I can. And yes, I did.
I'm soon to begin studying a HNC in Electronic Engineering :D And having these videos available so freely is incredible! I love that we, in this day and age, can hear from passionate, knowledgeable people, the world over!!! It's such a huge benefit over having to hope that you have a tutor who is passionate enough to not kill your fledgling interest in a subject! Even bad teachers can now be offset by the passion of those who wish to share theirs with everyone else! :D
I appreciate all your efforts Dave! :D I look forward to being able to increasingly understand what the hell your talking about :P
Dear Professor....
What is the difference when
1) Discharge a capacitor with some load connected
2) Charge with reverse polarization (already charged)
I want to get some more grasp undertanding about this difference.
Thank you!!
Hi Dave :) Love your blog!
This is the frist time you have dissepointed me, there is too much wrong in this episode.. :( It is the physics that is the rule, but how you concider it in practial electronic is up to you. So as long as you don't have a dielectric breakdown, there will be next to no current flowing through the capacitor. You can only change the electric potencial over the capacitor, and that involve a current going in and out of the capacitor..
Keep up the good work :)
I agree with you
Can you clarify for me:
My understanding in a series circuit: Electrons flow into the more negative side of the capacitor plate, charging it essentially with static electricity, and electrons repel one another. The higher concentration of negative charge on the negative plate 'displaces' the electrons on the more positive plate through the conductor back to the current source.
No?
Touch a cap that is in a high amperage circuit, that is fully charged, like the one that is hooked up to the magnatron in your microwave, and your corpse will prove that current flowed through that cap into your body and killed you.
I liked my humans fried, double fried like my chicken! hahhahaha! ;-)
All your proving is that current did at one time flow into the capacitor and at one time flowed out of it into your body and killed you ;) this video should explane it all, maybe watch it again?
***** Har, har, har.
OK, connect one side of the capacitor to your wall socket, and hold the other side with one hand. Now, ground your other hand and you'll know for sure if current flows through the capacitor, or not, because if the current flows it will roast you. (it will roast you, don't try it; use a meter like in the video)
Really, no current "flows through" the capacitor, in an AC circuit. It just "appears" to do so.
Unless a capacitor's dielectric is "ruptured," there is no current actually "through" the cap. It just "appears" that currect is flowing "through" the cap. The cap just "charges, discharges, charges (in the opposite direction) then discharges. Current looks like it is moving "through" the cap, but really is not. (yes, you would feel the effects of that AC :)
An electrolytic cap, in a DC circuit, will pass "the affects" of AC, but will block DC. No current will actually "pass through" the cap. If there is an "AC ripple" riding on the DC value, the DC will be blocked, but you would see that ripple on both "sides" of the cap.
Or, am I wrong?
I
Never a dull moment here. Thank you for a enthusiastic overview of caps.
I don't understand why you've overcomplicated something so much that's clearly for people that are fairly new to the subject. Why you'd bring up displacement current without just mentioning electrons repelling is beyond me. (Especially when you don't even slightly mention what displacement current is. You just say displacement current flows therefore current flows without giving any reason whatsoever)
The absolute exact same thing happens in a wire as a capacitor. Electrons repel other electrons, in a wire they move forward and since they repel other electrons those electrons move forward by the same amount.
They move forward onto one plate of a capacitor and since they repel other electrons they repel the electrons forward off the other plate of the capacitor.
+BlueCosmology That is exactly what I was thinking !
+BlueCosmology If that were the case, capacitor would be conductor no? Why does it stop repelling electrons on the other side?
quadcatfly A capacitor is a conductor, electric current flows across it.
Once the plate becomes charged enough, it requires too much energy to put more electrons onto the plate, due to the electrons on the plate repelling them away, so it stops conducting.
You should brush up on electric conductivity of dielectric materials. Especially the part where it says "dielectrics are insulator". All those electrons being piled up on one side of dielectric would immediately clot and prevent (e.g. repel) any new electrons from coming in, let alone push electrons from the other side of the plate (btw since when we have free electrons in dielectrics?). While you are contemplating this try to come up with explanation of how many capacitors IN SERIES could possible work based on your theory. If I place 20 caps in series the last in line would NEVER be charged because the first one (closest to -) will have charged and cut off the supply long before. But that is not how it works in a real world.
"All those electrons being piled up on one side of dielectric would immediately clot and prevent (e.g. repel) any new electrons from coming in, "
Yes, that is exactly what I just said.
"let alone push electrons from the other side of the plate (btw since when we have free electrons in dielectrics?)."
A capacitor is in its simplest form just two plates. Put one electron on one plate, it will push the electron off the other plate. Continue to put more on, you'll keep pushing them off the other plate, until eventually it takes too much energy to put an electron on the first plate.
"While you are contemplating this try to come up with explanation of how many capacitors IN SERIES could possible work based on your theory."
Completely trivially? There is nothing magic about putting two capacitors in series. The electron that is pushes off the first capacitor goes and charges up the plate on the second capacitor and so on.
"If I place 20 caps in series the last in line would NEVER be charged because the first one (closest to -) will have charged and cut off the supply long before."
That is nonsense.
You put one electron on the first plate of the first capacitor, one electron comes off the second plate. The time it takes is irrelevant.
The way i see it: is the Voltage applied to the capacitor Pushes or Pulls electrons onto the plates of the capacitor causing the field to change . While the electric field is changin, current is flowing to/from the capacitor. Once the field stops changing current stops flowing in a practical sense.
He could have said that magnetic fields are responsible. That would be a 5 sec long video.
Perhaps will it necessary to make the difference between Alternating Current and Direct Current. They both behave very differently when charging/discharging a capacitor.
Let's send him all our dried out old capacitors. You know the ones that let current go TROUGH them ;) Maybe he likes to use those ones. I prefer the new ones, you know the ones that DON'T let current go through them ;)
Dave, thanks a million for this and your other videos! This was helpful and made perfect sense to me. I have been a hobbyist in electronics for 20+ years and hate to admit it but you have forced me to rethink what I thought I knew about such a basic component. Cheers
you must establish what current is first before you even discuss it...the real simple discussion should be '' DOES ELECTRONS FLOW THRU THE DIELECTRIC OF THE CAPACITOR'' the answer is NO...
Old post, I know...if the current changes medium it's still current. I.E. Current in a wire being turned into radio waves, transmitted, received and transmitted back through a wire. Did the current actually flow "through" the air? Yes, yes it did. Just because it wasn't some visible plasma arc doesn't mean it didn't flow. Same idea here.
hattt be chutiye..
My mind is blown by this. I had trouble trying to wrap my head around why/how caps worked in series despite creating open circuits. Kudos to you. :)
Could you do a fundamental fridays video about inductors, and one about RC, RL, and RLC circuits?
Caps in SERIES is like ONE cap with a GREAT distance between the plates (the first cap's plate, and the last cap's plate) In PARALLEL it's like ONE giant cap with HUGE plates. So, in series, the capacitance will be smaller than the smallest cap. In paralled, they are all additive (their cap values)
Pull your head outta your asterisk Dave,
You're confusing TO (not two or too) with THROUGH (not threw).
TO! not through. 8+ minutes to get to Displacement Current.
Charge flows TO the plates of the capacitor. Not through.
OMG. Couldn't a cut a long story short... much shorter.
Thank you for the effort anyway.
Dave said voltage charges up in a linear fashion. From my understanding voltage doesn't charge up in a linear fashion in a capacitor, it charges exponentially depending upon the time constant (t=RC) This can be calculated in increments of 5t. It is also possible to find the instantaneous voltage at any point with this formula if increasing and beginning at 0v: v=Vf(1-e^-1/RC) with RC being one time constant.
When he said that he was showing a circuit with an 'ideal' current source connected to the capacitor. In this case, the voltage would charge linearly since the current is linear. However, if you were to look at the voltage across the current source it would not be linear, it would be exponential.
(commenting to you Travis) The voltage across the current source is the same as the cap that it is across, as they are in parallel. However, you are correct that a constant current source would ideally (if it had infinite voltage available and the circuit could take it) drive the voltage across the cap LINEARLY, not exponentially, described by the eq. v = i*t/c. This formula also shows that the voltage rises linearly with both current and time, and that the larger the cap, the slower it'll go. Also, even if a series resistor were inserted, the voltage on the cap would be identical. The exponential formula applies when it's a voltage source and there is a resistance for that voltage to divide across as the cap charges.
Neat treatment of the topic. I graduated 4th in my class (book smart *only*) from Stevens Institute 35 years ago as an engineer, and I can't recall us learning about displacement current. I loved physics and math and of course we learned Maxwell. Doesn't mean we didn't learn about displacement current, but I've never paused to think about the fact that the dielectric does not conduct electrons. As you say, what is the practical use of that fact? Nice job!
The magnets swinging is the displacement current, the observed current on the far coil is the electric current.
So where is the second plate getting the electrons to send on down the circuit? Is it the electrons in the plate itself are stimulated by the electric field to flow?
So are you saying that if you had an ammeter connected between the capacitor and the negative side of the battery, you would get a reading when you closed the circuit switch?
I do like the explanation which means for me that the answer is in between "the current does flow through a capacitor and it doesn't at the same time." This way we can make use of the capacitor accordingly.
:)
However, I look at it as the positive charges of the battery goes to one plate, while the negative goes to another plate. And the two plates are in communication because of the electric magnetic.
But, it still an issue to understand whether the current pass through the capacitor or not, and this can be seen if we put three capacitors in series, how is it possible for the middle capacitor to be charged? Therefore, we just force ourselves to believe that the current does pass the capacitor in order to resolve the problem. Perhaps, you got a better answer for this puzzle of three capacitors in series. And I do appreciate it!
I like the way you teach. Thank you so much!
Nice video, Dave! I'd love to hear your thoughts on conventional current vs. electron flow current. Never did understand why they decided to go the opposite route of the obvious!
Because of that Benjamin Franklin who was banging his chicks all day night
So in a series circuit, the circuit won't have any current after the capacitor is fully charged? So a light bulb added to the circuit will stop glowing when the capacitor is fully charged?
So when you build a radio as a kid that works without a power source, but can power a little earpiece, is that displacement current flowing from the radio tower to the oscillating piece of quartz?
Eletrolytic caps forms an oxide coating on one of the plate materials when exposed to the right polarity. This serves as an insulator. When reversed connected, it does the opposite by unforming the oxide which allow current to flow at a high rate. This produces heat and preasure which must be releaved through the rupture of the cap.
What will be the effect if you consider the high resistance of the capacitor?
Does the graph look like that no matter the capacitance of the capacitor, or does that represent a 1 farad unit, or , would like a 15 micro farad cap , charge a whole lot faster with the volts, compared to the current passing through and coming out it., so the volt line would be a whole lot steeper?
The way I understand it, current enters one side of the capacitor increasing the electrical charge on that plate (+Q becomes more positive) while simultaneously, the electrical charge on the opposing plate decreases (-Q becomes more negative). This matched but opposite change causes an apparent current to be measured through the capacitor (I = dQ/dt). Thus, it appears that current flows in one terminal and out the other.
See 6:11 for the diagram which Dave explains the charged plates.
I should add Dave. I would use the term electron volt. It makes it easier to define what us happening depending on application and use.
This presentation is useful for me. However It can be concluded that electric current can flow through a capacitor like a bulb. But it can't do continuously, electric current give up ( I equal to zero) when maximum voltage of a capacitor reached (showed in the graphic). Is my conclusion true ? Thank.
Hasan Yahya You are correct. Capacitors are by definition an open circuit and incapable of passing any voltage. When they are in a DC condition, be that charged or discharged, they do not conduct power of any kind. But when the charge is changing, they will increase the power they hold with the change in charge. So store power when charging up, and dump power when charging down.
The guys who first investigated electricity called this kind of stored power dielectricity. Modern dudes call it displacement current. Once a dielectric material is charged fully, it will conduct that displacement current just like a conductor. Capacitors break down and short out before they get to this state. Displacement current is real, but does not generate an electric field when it moves, like charge current when it moves.
The present theory that is the most simplified is that dielectricity is the source of magnetic field lines. It flows with charge movement, orients with charge orthogonally and repels other dielectric fields orthogonally when capacity is filled. This is the cause of the magnetic effects people see and measure. It is motivated to move by capacity differentials in circuits, and will surround a conductor which typically would attract more dielectricity than the conductor has capacity for, so it hangs out at the surface, in the insulation and air surrounding a conductor (or magnet). A conductor in this space of another charged conductor can pull it away by activating the movement of it's charge field, or with circuits that have more capacitance.
There are ways to configure circuits to operate with dielectric energy controlling the propagation of the electric field, and these propagate signals faster than light speed.
It should be obvious that electricity is not a particle phenomenon and the fields are not fully contained in the physical structures of circuits. So, the systems are open and external conditions can influence the circuits beyond the typical analysis used that focuses just on components.
Electric fields are everywhere and energy can be harvested simply with the proper activation of electric fields.that draw dielectricity to conductors and then transfer it to the proper mix of charge based on the application.
The real mystery is in how smaller fields will combine to form larger fields that affect other fields without using up the smaller fields. No one knows how this happens, but its there for the taking. It happens all the time, no one notices. Permanent magnets don't wear our by usage any more than a light will wear out by what it illuminates.
Electric induction generators do not require more energy based on how much energy you use unless you force the used energy back into the generators.
Or do I digress? Knowing what is really happening leads to more effective designs.
BMan18 ; thank you for this explanation! It turned on the lightbulb in my head! I'm taking a screen shot so I don't un-remember it, lol!
The ohm meter demonstrates the truth
a light bulb?
A capacitor is about electromagnetism and the potential field difference across the plates.
I haven’t watched the video, but in brief:
• Conduction current flows through both leads/wires of the capacitor, but not through the dielectric/gap between the plates of the capacitor.
• The conduction current in both leads is the same.
• You don’t ever charge (add net charge to) or discharge (remove net charge from) a capacitor as a whole (i.e. considering both plates); the net charge of a capacitor is always constant. But you do charge or discharge the individual plates; their net charge does change with time.
• Displacement current “flows” through the dielectric/gap between the plates of the capacitor, but not through the leads/wires of the capacitor.
• The numerical value of both types of currents is the same in a capacitor.
Sir what will happen after the connection.
If I connect a finite current carrying inductor(say 5A) across a inductor having no current initially. (We assume small series resistance of inductor.)
Will the current just after connection be 5A or 0 A. As we say that inductor resist sudden current change.
To stir up more trouble: ignore vacuum capacitors. Instead... in modern ceramic caps about 99.99% is a real electron current in the dielectric. Those electrons in the dielectric can move a bit, but aren't totally free like in metals. Look up ferroelectrics and PZT ceramics. Not "vacuum polarization," but polarized ceramic.
So, Maxwell's displacement current actually plays an insignificant role, at least in ceramic caps. That's why their dielectric const is 2,000, or 5,000, or higher.
how to calculate the cap size? 50V 1000 uf is enough for 12v 10 amps (with load) motor?
You sound like the real thing, my question on the condenser fan motor instructions says the two brown wires goes on either side of capacitor is there a power wire or black that should go on one side of the capacitor to charge it ? I have two brown wires one brown with white stripe but I forgot if one of the black wire is connected too capacitor?
Reminds me of the old conundrum:
Capacitor C charges to voltage V has energy of 0.5*C*(V)^2 and a charge of Q=CV.
Connect a capacitor of the same capacitance in parallel.
Voltage on each capacitor is now the same charge divided by twice the capacitance or V/2
Energy of each capacitor is now E=0.5*C*(V/2)^2 or (0.5*C*(V)^2)/4.
Energy in the two capacitors combined is (0.5*C*(V)^2)/2.
Where has half the energy gone?
cheers
I remember this lesson from university. The point was that current is not only movement of charged particles, but also a change in electric field is also a current. I think that was on forth year. Another argument was that magnetic effect this current has are the same as those of moving charged particles.
Can't remember any math or details nowdays, don't really use it in practice. Amazing that, at some point in time, I had some understanding of that stuff.
A beginner question, Dave, regarding the matter: if the current flowing through the capacitor isn't coming from the main electron source (i,e, the battery) because, as you explained, its electrons are building up on the capacitor plate, where does the exact same amount of electrons come from? I mean, they most come from somewhere, right? And if it's not from the battery where the main current is flowing from, where is it flowing from actually? Thanks :]
You still have net electron movement to build the potential (in the form of a magnetic field) between the two plates. It is also practical to define it as such, because that is what you will physically see in your circuit.
Can anybody answer the following question. If you use an RC filter on a power supply , does this pull down the input voltage and curent ??
Thank you .
The concept of dipoles that develop in the dielectric more easily explain the concept of current flow through the cap. Also, it is important to remember that the cap will only have current flow during times of a CHANGE in voltage. Current will flow foe AC, but not DC (since DC does not change). This also explains Xc, the phase shift, and why caps block DC (Xc is infinite when F=0)
so there is no stopping the current flow after the cap is charged? or does the cap act like a battery and stop current when it reaches max V? until the polarity is reversed?
But when there is no change in charge density across the plates, how will a change in magnetic flux take place to ultimately give rise to displacement current ? Someone clarify please. Thanks.
You could mention the reasons why capacitor voltage is delayed (phase shifted) compared to current.