Hey, guys. So for the next couple of videos, we're going to be talking about a phenomenon called electromagnetic induction. And we're just going to start off by talking about what it is and where it comes from. Let's check it out. From the last couple of videos, we saw that when you have a coil or a loop of wire and you attach some voltage source across it, it has a current. We could determine the direction of that current using our right-hand rule. Well, that voltage source doesn't always have to be a battery. This voltage can actually just be created by changing some things. And the word we are going to use for that is called induced. The idea is that there are actually a couple of common ways to induce a voltage, which induces a current in a coil of wire. When scientists discovered this stuff 100 years ago, they actually found three common scenarios that would always induce a current. Let's check those out.
The first one, and probably the most common one that you will see, might be in a lab or something like that, is when you have a bar magnet and you are basically moving this thing into and out of that coil. The idea is that you take a bar magnet and move it in and out of the coil of wire. What scientists found is that when you move the bar magnet, it would actually create a current inside of the coil that you could read on an instrument. In other words, you are creating some kind of induced current. So, basically, you are creating some kind of induced voltage across this wire. The most important thing is that the bar magnet has to be moving. So when the bar magnet was moving, there was an induced current. When it was stationary and velocity equals 0, there was no induced current.
Let's take a look at the second example. The second example involves varying a current in an electromagnet. So when you have an electromagnet, which is basically just a coil or a solenoid, something like this, and you attach it to some voltage source which can vary, so in other words, this is v, what happens is that it creates a current that is in the solenoid. I'll write I_{\text{solenoid}}. As you vary this current, we know a current in a loop of wire creates a magnetic field like this I_{\text{ind}}. They found that when the current was varying, there was an induced current in the coil of wire. But when this current here in the solenoid was kept constant, there was nothing they could read on the coil.
This last situation here involves the same exact electromagnet, except now we attach a switch to it. The idea is that as we basically turn this solenoid on and off, so right, we are affecting the voltage right here. Now this I_{\text{solenoid}} right here that goes in this direction, as we very rapidly turn this thing on and off, it would create an induced current in some direction over here in the coil of wire. They found that when you turn this thing off very quickly, there was some induced currents. So in other words, there was some I_{\text{ind}}. But when you just kept it on or kept it off, whatever it was, there would be no induced currents.
That's basically the three scenarios that scientists most likely observed. There is actually something in common about all three of these situations. If you take a look, in all three of these situations, what is really changing is actually the magnetic fields. In this first example where you had a bar magnet, we know a bar magnet has magnetic field lines that basically form loops like this, forming big loops. You have this magnetic field, and these magnetic field forms loops. What happens is as you are moving this bar magnet into and out of the coil, the magnetic field that is going through the coil is changing. We have some kind of B field that is changing. In the second case here, we know that an electromagnet basically behaves like a bar magnet. If you take your fingers and you coil it and curl your fingers in the direction of those loops, you have a magnetic field that points straight. So you get the same exact current, loops right here. As you are ramping up the current inside of this electromagnet, it is basically making this B field stronger. We know that the relationship between I is that it is proportional to the B field of a solenoid. So the stronger the currents, the stronger the magnetic field. The same principle happens over here. As you are turning the electromagnet on and off, you are basically creating and destroying that magnetic field. As you very rapidly change this thing, as you very rapidly turn the switch on and off, you are creating and then destroying this magnetic field just by turning that switch. So in all three of these situations, the same thing happens. The magnetic field changes. And because this is an interaction between magnetism and electricity, this phenomenon is known as electromagnetic induction. That is what we are going to be talking about for the next couple of videos. The most important thing that you really need to know from this video is that the magnitude of the induced current through the coil actually depends on how fast all of these changes happen. In the case where you had this bar magnet that was moving into and out of the coil, the idea was that the faster you move this coil in and out, the larger the induced currents. So if you went very, very slowly, the induced current would be very low. If you went very fast, the induced current would be very high. And when you had a current changing in the solenoid or the electromagnet, the faster the current would change, whether it was basically going up or down or you were turning the switch on and off, the larger the induced current was. That is what you need to know. There are no problems with this. It is mostly just conceptual. We really need to know this. Let me know if you guys have any questions, and we will move on to the next one.
