 I have an induction cooker at home on which I am boiling water and I thought the way it worked was like an iron box. The base would just get hot and that heat would get transferred to the vessel and start cooking whatever is inside. But then I read that that's not how it works. The base of the induction cooker does not get hot at all. And then I tested it out. I took the induction cooker, I switched it on and then I put my hand on top of it for a few minutes and nothing. It wasn't hot at all. It was right. It doesn't produce any heat. But then on that same thing I keep this vessel of water and within minutes it starts boiling. So the question is how does an induction cooker cook things and heat things up without itself getting hot? Well, that's what we're going to find out. And it turns out it's due to the magic of, I mean science of eddy currents. So what are these eddy currents? To answer that question, let's look inside the induction cooker. If you could open up, then you might see a lot of things. But the most important one is a coil at the center. And we know that when you pass current through the coil, it produces a magnetic field and that's exactly what happens. When you turn on the switch, there's a current that starts passing through the coil and that current starts generating magnetic fields. And what's important is that the current is fluctuating. It's changing its direction continuously. It's called alternating current, which we'll talk about in separate videos. And it changes its strength. As a result of that, the magnetic field that it generates also keeps fluctuating in direction and in strength. So imagine this is how the magnetic field is generated. It's continuously fluctuating. And that's all that our induction cooker does. It does not produce any heat. All it does, it produces a fluctuating magnetic field. But how does that boil over water? We'll get to that eventually. But for now, instead of thinking about keeping a vessel on top of this, let's keep a rectangular loop of wire. What will happen? Well, because the magnetic field is fluctuating, we have changing magnetic flux through a coil. And from Faraday's law, we know that that produces an EMF. And as a result, there will be an induced current in this coil. Now, the direction of the current will all depend upon whether the magnetic field is increasing, decreasing what the direction is in terms of lens law, something all we have talked about before. But don't worry too much about the direction as of now. What's important is there is an induced current due to the changing magnetic flux. But now comes the question, what if instead of having a coil like this, what if we kept a rectangular plate? Now what would happen? Well, again, the flux is changing. So there is an induced EMF. But earlier, there was only one path for the current to flow. But now the entire thing is a conductor. The whole thing is a playground for the electrons, which means your currents will be formed in loops everywhere on the surface of this conductor. And so you'll have loops of current formed on the surface. And these loops of current, which are formed on the surface of a conductor due to changing magnetic flux is what we call eddy currents. And I thought that these are called eddy currents because maybe some guy named eddy discovered it because that's how most of the stuff works in physics. But they're called eddies because they're swirling. So if you consider a whirlpool of water, something that's swirling, they're usually called eddies or vortices. And since this is very similar to that, we call them eddy currents. But how does that produce any heat? Well, of course, current produces heat. We've learned that before. When current flows through a wire, the wire heats up. Now there is current everywhere on the surface of the conductor, which means this whole metallic plate heats up. So if we go back to our vessel of water, due to the fluctuating magnetic fields, eddy currents are set up on the base of this vessel. And that's how the vessel gets heated up directly. And then that heat is transferred to the water, making it boil. Well, then why doesn't that same thing happen to our hands? Well, that's because our hands are not all that great conductor. And therefore, the eddy currents formed are incredibly microscopic, and they hardly have any effect. So in short, these induction cookers generate fluctuating magnetic fields. And when there is a conductor nearby, it experiences of changing magnetic flux. That induces currents on the surface, which we call the eddy currents, which heat it up, and that's how you can have induction heating. Incredible, isn't it? But wait, wait, before we start parting, doesn't it mean that whenever you have conductors and changing magnetic fields, you will have eddy currents and as a result you will have heating? Even if you don't want them? Yeah, wouldn't that be a problem? What if you don't want that happen? How do you reduce eddy currents when you don't want them? Again, going back to our metallic plate, how do we reduce these eddy currents when we don't need them? Well, we can reduce current by increasing the resistance of our metallic conductor. We know more resistance means less current. How do I increase the resistance of this conductor? One of the cool ways of doing that is by introducing slots like this. So kind of making it like a comb. In doing so, think of what we are doing. We are introducing air in between and air is an excellent insulator, which means the overall resistance of this now has increased tremendously. And as a result of that, the eddy currents now have reduced tremendously, which means less heat loss. And this also means now there is less area for the current. So you'll have tinier eddy currents. And again, because of this, we'll have less heating effect. And a practical application of this is seen in transformers. Now, the details about the working of transformers is something that we'll tackle in a separate video. But the important thing is transformers also have changing magnetic fluxes and they have this, you can see this giant conductor in between, which means there will be eddy currents formed over here and we don't want the heating effect to happen over here. So how do we reduce it? The whole idea over here is instead of having one giant block of metal where eddy currents can keep dancing wherever they want, we instead have thin slices of these metals laminated and then glued together. In doing so, we are introducing air like just like over here and as a result, increasing the resistance, thereby reducing the eddy currents. And because these things are all laminated, there is no electrical contact over here, which means you can't have a lot of eddy currents over here. So in doing so, we dramatically decrease eddy currents and as a result, we dramatically decrease heating losses. So now we not only know how to use eddy currents to heat up things, but we also know how to reduce them when we don't want them. But wait, there's more. Check out this demo. In this case, again, we have an electromagnet which is going to generate fluctuating magnetic field due to the alternating current and it's pressed that so it's switched on and then he's going to put a metallic and aluminum ring on top of this and see what happens. Here we go. And, voila! Notice the ring just stays there. It's levitating in air. Beautiful, isn't it? Why is it doing that? And see what happens when you let's go off it, it falls down. So why is it doing that? Can you pause the video and think a little bit about this as to why the ring was levitating? All right, let's go back to our drawing board. Here is that picture of levitation one more time. Why is the ring staying in the air like that? Again, this electromagnet is generating a magnetic field. There's a current running and it generates a magnetic field. But importantly, it's generating a fluctuating magnetic field just like with the induction cooker. And as a result of that, we have a conductor placed on top of that and just like how we saw, there's going to be eddy currents. So there'll be currents running on the surface of that aluminum coil. But what does it cause levitation? Well, remember, this current loop itself starts behaving like a tiny magnet. So this aluminum ring now behaves like a magnet. And now from Lenz's law, we've seen that the current induced will be in such a way that it tends to oppose the cause for it. So when you apply Lenz's law, it just turns out that the magnetic field that it generates opposes this. And as a result of that, this starts repelling this magnet. And for that reason, once that repulsion is stronger than gravity, it starts staying in the air. Magnetic levitation. And one of its applications can be seen in these amazing trains called as maglev trains. Some of these trains, you use the same principle. Eddy currents are used to levitate the entire train in the air. And as a result of that, they're not touching the tracks and so friction is dramatically reduced. And as a result, these trains can achieve a high, very high speed. Maglev trains. Magnetic levitation trains. Amazing, isn't it? But wait, there's more. Check out this demonstration. We have a pendulum with an aluminum plate attached to it. And we have a couple of electromagnets. Now, once the power is turned on, this starts producing a steady magnetic field. There are no fluctuating magnetic fields. Now see what happens. And an important thing, aluminum is not a magnetic material. It doesn't get attracted to magnets. Okay. So there goes the pendulum. It's swinging just like usual. And now it turns on and see what happens. What did you see? Can you explain what you saw? Again, pause the video and think about what might be going on. So we saw that the moment the magnetic field was turned on, it immediately stopped. Why did the pendulum stop? What made it stop? Is it eddy currents? How does that work? Let's get back to our drawing board. So let's say this is that constant magnetic field generated by those electromagnets. And here is that moving aluminum piece, pendulum. Why would there be, what's going on over here? So even though the magnetic field is not changing, notice as the piece enters into the field, from its perspective, the magnetic flux associated with it starts increasing. And as a result of that, there is an EMF induced. This is called motional EMF. And now this motional EMF starts producing eddy currents. And the same thing is gonna happen when it exits. As it exits, the magnetic flux is decreasing. That'll produce motional EMF and eddy currents. But why does it slow down? Let's look at it carefully and use Lenzer's law. As it enters into the magnetic field, since the flux increases, the current induced will be in such a way as to try and decrease the flux. In other words, the current eddy currents over here will tend to oppose the magnetic field. And as a result, it'll end up repelling the magnetic field. Which means as the plate enters into the magnetic field, it repels these magnets. And so what happens? If you repel as you're entering, it'll slow it down. But what happens when it exits? Well, now as it's exiting, the magnetic flux is reducing. So the opposite happens. Now the induced current tries to increase the magnetic flux. As a result, it tries to produce magnetic field in the same direction. And therefore, which means it gets attracted by the magnet or gets attracted towards the magnet. And this means now when it's exiting, it gets attracted back to the magnet, which again slows it down. And therefore, every time this thing enters into the field and exits the field, it gets slowed down. And as a result, it eventually stops. And now you can answer this question. What do you think would have happened if we had slots in the aluminum plate and re-read the experiment? What change would you predict this time? Okay, hopefully you made your prediction. Let's see if your prediction matches the experiment. So now you can see there are slots cut into it and we're gonna repeat the experiment. There goes the swing. Now it's gonna turn on the magnet. Now, what do you see? The magnet is turned on. Hard to believe it's turned on, right? It's slowing down, but it's taking much longer. I'm not gonna tell you what's happening. I'm pretty sure you can answer that question yourself now. And one final demo, which will make a lot of sense now. Here it is, again, an aluminum disk this time attached to a motor. So when you turn it on, it spins. That's not the demo. But the question is, what happens now when you bring a magnet close to it? Can you predict what's gonna happen? Very similar things. Well, let's see if your prediction matches. It slows down. Does it make sense? Well, again, as the pieces of aluminum go through into the magnetic field and exit the magnetic field, they experience repulsion and attraction that slows them down. And as a result, the whole disk slows down. Which means you can use any currents to break, to slow down things. We call them electromagnetic breaking. What could be an application of this? We can slow down spinning wheels. Which means we can slow down wheels of a train using electromagnetic breaking and eddy currents. Wow. In short, eddy currents are currents formed on the surface of a conductor whenever magnetic flux through them changes. If you want to reduce them, you just increase the resistance by adding some slots or using lamination. And they can be used to heat things up or maybe levitate things or slow things down. In other words, they are pretty, pretty awesome.