EEVblog #486 - Does Current Flow Through A Capacitor?

hi welcome to fundamentals Friday this one comes about because of something I said in a previous video I mentioned that current flows through a capacitor and I had a couple of people comment that no that's wrong current doesn't flow through a capacitor you're crazy Dave well am I does current flow through the capacitor turns out it's an interesting question let's take a quick look at it and when I say quick I do mean quick because to actually answer this and get to the bottom of it and understand it all actually takes a couple of semesters at university and lots of physics and math and Maxwell's equations and all sorts of stuff but to answer the question does current flow through a capacitor the answer is I can tell you right now yes exclamation mark but with a little asterisk next to it and that's where this video comes in we're going to talk about the asterisks and of course it all starts out very basic doesn't it if you've got a battery here a switch and a resistor then if the switch is open no current flows but as soon as you close the switch current I flows through there let's not mix up our conventional and electron current flow shall we that's a whole nother can of worms be closed the switch current flows Ohm's law all that still stuff electronics 101 it really is but what happens if we put a capacitor in here like this well of course that's very easy to in fact it's almost insulting you know what happens the capacitor charges up current still I still flows in the circuit and it charges up a voltage on that capacitor in this case our VC the voltage across the capacitor a capacitor when you first if it's got no charge on it when you first apply start having current flow through the circuit it has a voltage is zero so this black graph here so the X is so the y-axis here is the voltage across the capacitor it starts to charge up until it reaches a point where it's fully charged and the voltage just sits at the fixed level which is your battery voltage over here and of course the current starts off incredibly high because the capacitors effectively a short circuit when you close that switch so the current is at maximum and then it slowly tapers off a direct mirror image of that until the current goes to zero so the answer to this question seems bleeding the obvious does current flow through a capacitor yes I see look current is flowing in that loop it's in series how else where else can it go but through that capacitor and of course you've got some standard formulas you learning electronics 101 to go along with this charge Q equals C times V and then you've got I equals D Q or the change in the charge over DT the change in the time some people you know that they don't like to use D in there you could you know use like Delta or something like that it just means change so the current equals the change in the charge on vs. the change in time and that's where that graph comes from so not rocket science current is flowing in the capacitor and because charge equals C times V you can it write that again I equals C times DV DT like that and you know and there's all sorts of equations for currents flowing in various circuits so current does flow through a capacitor or does it and just so nobody nitpicks yes I know it's actually I T in there I didn't add the T in there and if you put a current source in series with the capacitor like this which is essentially what we did in the previous video which caused all this ruckus to begin with then there are the T drops out of that and I just equals C times V on T and the VC the voltage across the capacitor will charge up litter will rise in a linear fashion like that just as we saw in the previous video so is current flowing in here so it's time to ask ourselves what is current well electronics 101 again right current is the flow of electric charge and when you have conductors like you do in electronic circuits they're made up of conductors wires and resistors and stuff like that then it's the flow of electrons through that circuit now let's not get into drift velocity and stuff like that there's a whole nother can of worms where the electrons you know move it like a 1 millimeter per second through the entire circuit Google that one but yeah it's just the flow of electrons in this circuit that's why when you break the switch no electrons can flow in this circuit at all but is it a capacitor just like an open switch look like this symbol itself tells you that there's a break what's the difference between a switch symbol and a capacitor symbol it's they're all they're identical right and that comes down here what you basically know about capacitors right they block DC and they allow AC or changing currents to pass through them hmm so now we'll take a brief look at inside a capacitor here and that requires a whole separate video so it's going to be very simplistic well you know that a capacitor is just two metal plates like this or it can be two bits of wire just sitting in the air like that like a switch remember a switch has capacitance as well tiny amount between the context any two wires in any space is effectively a capacitor anyway it's got two plates in there and it's got a dielectric material in the middle of it and what that does is actually creates when you charge up a capacitor you're actually are building up a charge on both of these plates they're going to be equal and opposite charges between the two plates and you're going to set up an electric field between the two plates but because it's like an open switch no actual electrons actually flow through the material and once again we're talking about an ideal capacitor here in practice yes there's a tiny little bit of resistance in there like that so some electrons do actually sneak through the dielectric material but we're just going to ignore that Vig as well that's just going to confuse issue no electrons actually pass through a capacitor like this it's just a capacitor is just defined in terms and understood in terms of electric charge building up on the plate yes we've got current flowing into and out of the wire for example and current can flow down these plates because they're actually metal right you can get electron flow up and down these plates like this okay but well nothing can go through the middle it's like a switch it's open no electrons can flow so what's going on how can current flow through a capacitor it's sort of you know we know current flows through because it's a series circuit and well you can actually measure it and it got all of basic electronics 101 formulas that tell you current flows through a capacitor but when you look at it in terms of electric fields and how it's two separate plates and an insulator in between that's what a dielectric is it's an insulator be an air or mica or ceramic or whatever material it happens to be no electrons can actually flow what's going on so to grossly simplify this thing which as I said at the start I have to do because this whole concept and understanding all this sort of stuff takes you know semesters of understanding at university and most people come away from it scratching their head anyway trying to figure this sort of stuff out and it depends on how you look at it and there's a big difference between physics fundamental physics and practical electronics as we'll get into right at the end to close out on this thing so to understand this we have to look at electromagnetism for a minute and Maxwell and the famous Maxwell equations now before Maxwell came along electricity and magnetism were thought to be two separate things but Maxwell credibly smart dude combine the two into theory of electro magnetism and to cut the long story short to make it all work he had to come up with the concept of a displacement current that in this case flows flows through a capacitor and that's how what we're using up here in all of our general electronics equations in a practical sense so what it comes down to is that we've actually got two different types of current there's electric current which we explain right back at the start is you know the flow of charge and electrons and but we know that electrons can't actually go through an insulator in a capacitor or anything else so what Maxwell came up with is a concept called displacement current and that gives us a second definition of current and in practical electronics when anyone uses the term current they're including both of these terms under the same umbrella and to try and separate them then you start getting away from practical electronics into theoretical physics and it gets nasty so Maxwell with this idea of the displacement current it's ah there's many different ways to look at it and try and understand the concept but it is a term which he came up with which was required and is required to complete the theory of electromagnetism and electromagnetic waves and rate you know radio waves and the whole concept that we know is electromagnetism today it's an essential part of it and that displacement current is what actually flows through or flows through I use that in quote marks flows through the capacitor but that's why it's still valid to say that there's current flowing through the capacitor in the world of electronics because essentially there is because our day to day equations all assume and know that there is current flowing in this series circuit with the capacitor there even though it's effectively an open circuit and of course sir Maxwell just considered what didn't even consider the dielectric constant in here even if it's a vacuum he said well a vacuum is just a dielectric like everything else it's just got a dielectric constant of one instead of well most other materials have a dielectric constant above one but there's it's just a dielectric constant so that's how electromagnetic waves can travel through a vacuum and it gets really really nasty and I don't have time to go into it so as you charge up a capacitor you're changing the electric field you'll vary an electric field between the two plates and of course the electric current can flow onto the plates like that and so current is flowing in to one plate and leaving the other but what actually happens in the dielectric material is not electric current flow there's not electrons actually flowing through the dielectric material it's the change in electric field between the plates actually also creates a changing magnetic field and it's that magnetic field concept which causes the displacement current it gives the equivalent current incept in terms of electro magnetics flowing through the dielectric material so current is still flowing through you've got I going in here you've got eye popping out there and well what happens with inside the capacitor ah it's not magic it's just fundamental physics and how you understand a displacement current field in Maxwell's equations and therein lies the trick when you're on the conductor I either wire coming in and the plate you can talk in terms of electric currents and everything's just fine and dandy but once you leave that conductor and get into the insulator ie the dielectric material you've got to start talking in terms of changing electric fields which creates changing magnetic fields which then you can have energy storage and transfer in these magnetic fields and that's how current is able to go through and it all comes out in the wash in Maxwell's equations it would take ten boards worth of equations for it to all come out in the wash but so you know you can go and deep into the concept of how you're actually able to get current flowing through the insulator but it does because Maxwell's equations tell you it does and they're not too dissimilar thing pops up when you start talking about inductors and it's like well how dare an inductor not pass all of its current through how can there not be any current flowing through an inductor is just a bit of wire it should conduct well it's the electromagnetic fields in the thing and that's the sort of inverse relationship between capacitors and inductors in this case I mean capacitors as the rate of change of the electric field goes up the greater the current that can flow through this thing and conversely with inductors then the greater the change of the magnetic field the less current flows through it so there you have it that's it in a nutshell and I'm sorry I'm not going to go into any deeper if you want to know about it then google the word displacement current and electric current go into it electro magnetism and Maxwell's equations are the whole thing and you can spend years and years of your life trying to understand how or if current flows your capacitor but as I said impractical electronics yes current flows through a capacitor and when you use the word current it means it implies the combination of electric current and displacement current ie the flow of actual electrons or electric charge through a conductor and then when you don't have a conductor ie you have a capacitor it's so effectively another case is not a special case it's another case of using a different type of current called displacement current and it's a mathematical concept which depends on how you want to interpret it whether you're not a physicist or in practical electronics person and that's the point to say to say somebody's wrong by no current doesn't flow through a capacitor it's just crazy it's of no practical value whatsoever try and design any practical electronics by thinking that current doesn't throw through capacitor she's stupid doesn't work so really there's two different worlds going on here there's the fundamental physics world of electron flow and charge carriers and all that sort of you know jazz and there's you know the high level of macro practical electronics thing where we use all our basic equations and current flow so the capacitor and all the equations work and everything's just fine and that's how we teach electronics that's how we understand it that's how we design things with it the fact that current does flow through a capacitor so as I said yeah you can get into deep theoretical physics of how and if actually and what the definition of the term current is but that's of no practical value because at the end of the day displacement current it's no point thinking about it because you can't buy a displacement current meter and whack it in between the plates and measure the displacement current it's a mathematical concept effectively so just ah don't worry about it electric current flows through a capacitor through so theoretical physicists go for your life me I'm going back to the bench using my current meter to actually measure the current yeah so does current flow through a capacitor well let's find out yep catch you next time you you

Channel: EEVblog

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Keywords: current, displacement current, what is current, voltage, capacitor, charging, maxwell, maxell's equations, theory, practice, how to, explaination, amps, volts, physics, basics, inductor, electric field, electric charge, electron flow, flow, electron, theoretical, basic physics, 101, curve, graph

Id: ppWBwZS4e7A

Length: 17min 17sec (1037 seconds)

Published: Fri Jun 21 2013