Okay. Now, we're also going to explain to you about transformers and inductances. Maybe transformers, you are familiar with the movie. Where you can transform, a car into a rough robot and vice versa. But in this case, we're not transforming, a car to a robot. We're transforming one type of energy to the other. Here magnetic energy into electrical energy, and see how we can transform the voltage from high voltage to low voltage or vice versa. So, one of the most interesting features of Faraday's discoveries is that a change in current in one coil makes an emf in a second coil. So, in that way in fact, you can transmit energy without a wire. So, you can have a wireless charging. So, wireless charging station for your cell phone, mobile phone or even for your shaving gadgets. That's all based on this principle. So here, your transformer is a combination of two coils usually with an arrangement of iron sheet to guide the magnetic fields. It can transform one emf to another. So, the change in current in one coil is influencing the current induced in the other coil. So, that's an induction. But then, you can also think about, "What does the change of current of one coil influence itself." We call that self-inductance. So, there is a changing flux not only through coil B, but also through coil A and a varying current in coil A produces a varying magnetic field inside itself. So, the flux of this field is continually changing, so there is a self induced emf in coil A, and we call that self-inductance. So, Melody, do you have an experience to use a transformer, to let's say, change the voltage from 220 volt to 110 volt? Yeah. I have a universal adapter for my computer. Exactly. Did you realize sometimes the transformer can get very hot. Yeah. Yes. Why do you think you are creating so many heat out of this gadget? Maybe, it's because of the self-inductance? Maybe that's one case or there's a loss of transmitting the energy from one part to the other. So, that's why it's getting hot and sometimes it is being broken by the heat generated, okay? Now, I'm going to cover the Lenz's rule. Where the emf tries to oppose any magnetic flux change. So, that's like human being. If you go to some new place and you are exposed to new culture. The first reaction is to oppose any changes inside yourself, right? The same thing happens in magnetic coil as well. So, the direction of an induced emf is always such that, if a current were to flow in the direction of the emf, it would produce a flux of magnetic field that opposes the change in B that produces the emf. So here, the change is very important. So, if you already have magnetic flux, then you don't want to lose it. But if you didn't have any magnetic flux, then you don't want to have it. So, it's opposition to change, not the absolute state, right? So, in particular, if there is a change in current in single coil, there is a back emf in the circuit. This emf, acts on the charge flowing in coil A to oppose the change in magnetic field, and so in a direction to oppose the change in current. So imagine, if you have a switch here, and you switched on to create a magnetic flux and suddenly you want to turn it off, then all of a sudden you will create a back emf to oppose your change, oppose turning off. So, you will have a huge current flow, which will maybe harm you in a very dangerous way. So, we have just covered the self-inductance, and that can be understood when we use analogy with mechanical inertia. So, a current in a self-inductance has inertia. Because the inductive effects try to keep the flow constant, just as mechanical inertia tries to keep the velocity of an object constant. So, if you think about inertia. If you want to change your habit. What do you need Melody? A very strong will. A very strong will. That might be a good driving force. You need force, right? So internally, if you create your own will, that will be a force. But externally, if you have painful experience, that will be also force. Or if you have a crush on something, or if you love something so much, then that will be also a strong external force to change your habit, which creates a huge inertia otherwise, right? So, I just mentioned about the self-inductance, the inertia and why it could be dangerous. So, I'll give you some specific examples to enhance your understanding. So, suppose that a battery is connected to the coil of a large electromagnet and the strong magnetic field has been built up. As in the case of this picture. Now, suppose that we try to disconnect by this switch and by opening the switch. So, this will create an enormous emf, right? In the circuit and it would be large enough to develop an arc across the opening contacts of the switch. So then, you can have a safety feature like this, where you can deviate the current through the lamp. So, the lamp will have the current not the switch. So, this is the feature you will see in large electromagnets. So here, as written here, when the switch is opened, the current doesn't change rapidly but remains steady, flowing instead in through the lamp. It could then be turned off safely. So, in this slide, we're going to explain to you about forces on induce currents by bringing up the example that you can probably see in any of the science museums. Namely, electromagnetic ring launcher. So, this is a gadget demonstrating Lenz's rule in a dramatic way. As you can see, you have a coil, which is the electromagnet, and you have an aluminum ring placed on the end of the magnet. When the coil is connected to an AC generator by closing the switch, the ring flies into the air. Why? Because once we have a one way turn of current to create north pole, this will induce a current opposing the change, so we have repulsive force. But as soon as it gets used to it, we're changing the pole to south. So again, they will create another current opposite to that, so you have continuous opposition. So, that will make you fly away, and that's also in the human society. If you turn something on and on and on. Obviously, the community will not be integrated. So, the force comes from the induced currents in the ring, right? That's what we learned from this lesson, and the Lenz's rule. Here's the question, what happens if we make a thin radical cut in the ring? Melody. Well, if you make a cut in the ring then there can't be any circulation of current so therefore the ring will remains stationary. That's true. So, if you make a cut, then there will no close loop, so you will not be able to make repulsive force necessary to propel this ring. This slide shows the magnetic levitation especially using superconductors. These type of materials will show perfect conductivity below critical temperature T sub C and this also shows of perfect dam magnetism. So, meaning no matter in what state, if you apply a magnetic field it will create a current inside its bulk to oppose that change therefore, you will always have repulsive force. So, if you have a sheet of perfect conductor in this case superconductor and put on electro magnet next to it, when they turn the current, the magnet eddy currents appear in the sheets so that no magnetic flux enters. Since the eddy currents are opposing the field, magnets are pulled from the conductor and you will see levitation. People are trying to use this to bullet train. So, minimising the friction between the wheel and the rail will enhance the speed of the train. So, Melody, do you have any experience to ride on a bullet train? Yeah, I think there are a few in Korea, right? Yeah we have KTX in Korea, Shinkasen in Japan, or ICE in Germany and many many, so TGV in France. If you imagine even with the friction force we have, we have such a high-speed train. If we can remove that friction force, then imagine how fast the train will be. Alright now, what if the conductor is not perfect? If the superconductor becomes a normal conductor, then the current will tend to die out because you have joule heating, resistance heating and the magnet will slowly settle down. The eddy-current in an imperfect conductor need an EMF to keep them going. To have an EMF, the flux must keep changing. That's why in the case of the launcher, we were changing the magnetic flux with AC current. So, the flux of the magnetic field gradually penetrates the conductor if the conductor is not perfect and if you don't change the flux. So, here's another way to demonstrate that. So eddy currents, how eddy currents produce sidewise forces. This is another gadget you probably can see in Science Museum and I'm going to ask Melody if she has ever seen this kind of gadget, have you? Yeah, I think so. Yeah, we would have Pendulum, right. You have a normal pendulum and once you turn on the switch, it get arrested, right? It gets arrested. So, in a normal conductor there are not only repulsive force, but there can also be side wise forces. A score sheet of copper is suspended on the end of a rod to make a pendulum as you can see here, and the copper swings back and forth between the poles of an electromagnet. When the magnet is turned on, the pendulum motion is suddenly arrested, and we're going to think about this in the next slide, okay. So, let's take a closer look to the place of our copper score sheet and the magnetic flux. Between the two poles. As the metal plate enters the gap of the magnet, there is a current induced in the plate which acts to oppose the change in the flux through the plane. If the sheet were a perfect conductor, the currents would be so great that they would push the plate out again. So, it will be bounced back, right? With a copper plate, there is some resistance in the plate, so the currents at first bring the plate almost to a dead stop as it starts to enter the field. Then, as currents die down, the plate settles to rest in the magnetic field. So, that's how we can make or break one of this principle. In fact, if you have an experience to ride a Tesla car or other type of electric car, you are using this effect to slow down your vehicle and recharge your battery. Now, this slide shows the effect of the shape of the plate as we discussed for the aluminum ring if I cut it, we learned that it will not have a propulsion, right? Now, what if we make this kind of slits in our copper plate. You see, the strengths and geometry of the currents are quite sensitive to the shape of the plate as we just discussed. If the copper plate is placed by one which has several narrow slicing coordinate, the eddy current effects are drastically reduced. So, you will see a change in the behavior of this pendulum just by merely cutting and making some slits in our copper plate. This slide shows how we can set up a rotating magnetic field. We can make a field like that of a retaining magnet by taking a torus of iron and winding six cores on it. So, we don't have to physically rotate permanent magnet, we can just use a three phase power line to this configuration, and make a rotating magnetic field. So, without mechanical motion, you can create a rotating magnetic field. With that, if you put a ring on top of it, then you can make it rotate. So, the torque produced on a conductor by rotating field is shown by extending a metal ring on an insulating table just above the torus. So, the rotating field causes the ring to spin about a vertical axis. The basic elements seen here are quite the same as those at play in a large commercial three-phase induction motor where you use a large torque and large force. So, maybe you can imagine for motors used in say, cars can also use this principle. This is another example showing how we can use three electromagnets to make a rotating field and motors. This slide shows an interesting experiment namely shaded pull effect whereby, partly shading the magnetic field, you can make aluminum disk to rotate. So, how does it work? We will find out. So, electromagnet M, consisting of a bundle of laminated iron sheets wound with a solenoid coil is powered with alternating current from a generator. So, you can see we're alternating north and south pole as a function of time. The magnet produces a varying flux of B through the aluminum disc, but produces no torque on the disk, hence no rotation. So, you can imagine, if you change the flux up and down, you will probably have repulsive force, right? But there will be no torque. However, if you shade this one, then you will have a current here opposing this in one part of the disk but aluminum plate will shield it and will make the opposite type of curve. So, then you have interesting things to make it kick, to make it rotate. Okay. So, the operation depends on two eddy current effects as written here. First, the eddy currents in the aluminum plate oppose the change of flux rate, so the magnetic field above the plate always lack the field above that half of the pole which is not covered. This is so-called shaded pull effect which produces a field which in the shaded region varies much like that in the unshaded region, except that it is delayed by a constant amount in time. The whole effect is as if there were imagined only half as wide which is continually being moved from unshaded region towards shaded one, like this. So, you're making a motion in sideways. Then the varying fields interact with the eddy currents in the disc to produce the torque on it. So, this is another way to think about how to create a motors. So, using the principle we learned, we can make a different kind of geometry, different kind of structure to fulfill the same function. Now, do you recognize this picture, Melody? Is it the Hoover Dam? That's correct. So, this is a gadget people invented to use the Mother Nature's energy and convert it into electrical energy, right. When Faraday first made public his remarkable discovery that a changing magnetic flux produces an EMF, he was asked, what is the use of it? So, that's the question always ask when you make a proposal, right. His answer was, "What is the use of a newborn baby," right? At that time, electricity Induction, it was a new concept, but now we know for any power plants, not only Hoover Dam, but Nuclear Power Plant, fossil fuel power plant, any power plant. At the end of the day, you need to use generators to rotate the turbine and make a current out of it. So, that's a universal thing. Modern electrical technology began with Faraday's discovery. The useless baby developed into prodigy and changed the face of the earth in ways its proud father could never have imagined. So with that, we're going to wrap up our lecture 15, which was the last lecture of part three. I hope you have a wonderful day and see you in next lecture, bye-bye.
