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 a 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 Maxwells 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 asteris and of course it all starts out very basic, doesn't it? If you 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. Uh, conventional and electron current flow, shall we? That's a whole. another can of worms. But you close the switch.

Current flows law. All that St 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 too. 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, Uh, 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 of zero. So that's this black graph here. So the X So the uh, 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.

Uh, which is your battery voltage over here. And of course, the current starts off incredibly. High Because the capacitor is 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 got some standard formulas you learn in Electronics 101 to go along with this charge: q = c * V And then you've got I equals DQ or the change in the charge over DT the change in the time. Or 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 versus the change in time. And that's where that graph comes from. It's not rocket science. Current is flowing in the capacitor and because charge equals C * V, you can H write that again.

I = C * 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 it T in there I didn't add the T in there and if you put a current Source in series with a capacitor like this, which is essentially what we did in the previous video which caused all this Ruckus to begin with, then the T drops out of that and I just equals c * V on T and the uh VC the voltage across the capacitor will charge up, will, uh, rise in a linear fashion like that, just as we saw in the previous video. So there's 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 another can of worms where the electrons, you know, move at like a 1 mm/ second through the entire circuit. Uh, 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 isn't a capacitor just like an open switch? Look that. the 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 to you. What you basically know about capacitors, right? They block DC and they allow AC or changing currents to uh, pass through them. H. 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 contacts. Any two wires in any space is effectively a capacitor. Anyway, it's got two plates in there and it's got a dialectric material in the middle of it.

And what that does is actually creates. When you charge up a capacitor, you're actually, uh, 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 capacity here in practice. Yes, there's a tiny little bit of resistance in there like that. So some electrons do actually sneak through the dialectric material.

But we're just going to ignore that, because, well, that's just going to confuse the 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 cuz 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 the 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 dialectric is. It's an insulator, be it air or mic, 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 uh, physics, fundamental physics, and practical Electronics as we'll uh 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, uh, electricity and magnetism were thought to be two separate things. but Maxwell incredibly smart dude combined the two into theory of electromagnetism. And to cut the long story short, to make it all work, he had to come up with the con concept of a displacement current that um, 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 Uh 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 um, 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 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 R you know Radio W 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: Uh 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 Cu.

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, Uh, Maxwell Just um considered. was didn't even consider the dialectric constant in here, even if it's a vacuum. He said, well, a vacuum is just a dialectric.

Like everything else, it's just got a Um dialectric constant of one instead of. well. most other materials have a dialectric constant above one, but there's It's just a dialectric 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're varying the 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 dialectric material is not electric current flow. There's not electrons actually flowing through the dialectric material. It's the change in electric field between the plates Actually also creates a change in magnetic field.

And it's that magnetic field concept which causes the displacement current. It gives the equivalent current incept in terms of electro. Uh, magnetics flowing through the dialectric material. so current is still flowing through.

You've got I going in here. You got I popping out there. and well, what happens with inside the capacitor? Uh, it's not magic, it's just fundamental physics and how you understand a displacement current field in Maxwell's equations. And there in lies the trick when you're on the conductor.

I.E the 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 I.E the dialectric material, you've got to start talking in terms of uh changing electric field fields which creates change in magnetic fields which then uh, you can have energy, um, storage and transfer in these magnetic uh 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 10 boards worth of equations for it to all come out in the wash. but so you know you can go 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 a 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 Wi.

it should conduct well. it's the electromagnetic fields in the thing and that's uh, the sort of inverse relationship between capacitors and inductors. in this case, um, 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 Um inductors, then the uh 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 and go into it Electromagnetism and Maxwell's equations and the whole thing. and you can spend years and years of your life trying to understand how or if current flows through a capacitor.

But as I said in Practical Electronics Yes, current flows through a capacitor. and when you use the word current, it means it. It implies the combination of electric current and displacement current. I.E the flow of actual electrons or electric charge through a conductor.

And then when you don't have a conductor, I.E You have a capacitor. It's a effectively another case. It's 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 an practical Electronics person.

And that's the point to say To say somebody's Wrong by no current doesn't flow through a capacitor is just crazy. It's of no practical uh, value whatsoever. Try and design any practical Electronics By thinking that current doesn't throw flow through a capacitor, it's just stupid doesn't work. So really, there's two different worlds going on here.

There's the uh, 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 macro practical Electronics thing where we use all our basic equations and current flows of 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 uh, 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, act and what the definition of the term current is. but it's of no practical value because at the end of the day, displacement current? it. 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 go. Don't worry about it. Electric current flows through a capacitor through. So theoretical physicist, go for your life.

Me: I'm going back to the bench using my current meter to actually measure the current. So does current flow through a capacitor? Well, let's find out. Yep, catch you next time.