12.8 Lenz's Law

Okay, today's lesson we're going to continue with electromagnetic induction, but we're going to look at it slightly differently. So let's quickly review this. Remember, if we have a wire that we're going to move inside a magnetic field, then that's going to induce a current inside the wire. So, remember with our left-hand rule, we put our B finger out of the page, we put our thumb to the right. Remember, you want to think of this as moving that electron to the right, so our thumb goes right, our B goes out of the page. So then, we would get our force, which would be upwards in this case. So that means our current, the electrons in that thing, are going to flow upwards. That's what we did with electromagnetic induction. Another scientist, Len, thought that maybe there's a better way to do this type of experiment instead of moving the wire through the magnetic field. So that's when we have to spin it, if it was like a...Motor Type Generator Thing, or we'd have to physically push the wire. But we're not gonna be able to do this every time, so he thought, could we be able to do the same thing, but instead of moving the wire, well, what'd happen if we move the magnet? If you remember from Faraday's law, he said all we do need to do is have a changing magnetic field. So that also means that if the magnet moves, we should be able to induce a current as well. So Lenz came up with Lenz's law, and what he did was be able to figure out which way the current would go in a coil of wire if we move a magnet in and out of it. So imagine a coil, kind of like a toilet paper tube, wrapped around with wires, and if we push a magnet through the coil of wire, we should be able to create a current in that coil of wire. So what Lenz figured out was, we're gonna push the magnet in or out of the coil, the wire is going to go, the wire, the direction could go one way or the other. So the trick was, how? fear which way the current's going to go? So, the common-sense way or what most people would think is what we consider the wrong way of doing things. If we were to push the North Pole into the coil, as shown in this diagram, I get the North End of the magnet. It's gonna push in where, let's just assume, the coil current is going to go upwards like that. So, if our current is going upwards, we can use our second hand rule. Our thumb would point to the right. That means that would be our North Pole, and that would be our South Pole. If that's the case, that's the way it would work. And what's the problem? Well, if this is gonna work this way, we're actually not going to have to do anything. All we have to do is let go of the magnet, and it's gonna get attracted to the coil, the North-South coil, and it would create electricity. So, if the poles are arranged Like that, that means we could get electricity by doing nothing. But we know that, from work and conservation of energy, we can't create energy out of nothing. We have to do some sort of work to be able to make that happen. So, that means then we actually have the directions wrong. The right way to interpret Lenz's law is that we have to think that the only way we're going to be able to force that magnet in and do work is if this is the north end of the magnet. We have to have the north end of the pole going against it. So, those are naturally going to repel. So, for us to do work and expel energy, we're gonna have to force those things against each other. So, if we force the north end against the north, that's going to create work, which then means that work then would get converted to energy. So, we still use our second hand rail. Once we figure out that the north end has to be the left side, then our fingers would curl around it, pointing towards the left. Our fingers would coil around it, and we'd see that our Current then is going down the wire. So, the key to remember with Lenz's law then is that we just have to remember that we have to do work. We have to be able to kind of do what's opposite. We got a force to eye magnet in the direction that doesn't want to go to create the electricity. So, let's go through this example. We've got a, in this case, we're going to pull the magnet out of the coil. So, you can see our north is going pulled the way. So, in this case, if we pull the magnet the way, that's going to force us to do work. We would have to make that the south into the coil, and that would be the north end. So, if we're gonna pull the north and south, they're naturally attracted, we're gonna force them apart. So, in this case, now if we put our hand, point our thumb to the north, you'd see that our current, our fingers wrap around. But so our cream would go upwards around the coil. Okay, so it doesn't matter how the magnet's oriented. It can be north or south, either. The way the coil can be north or south, by the way, you just have to make sure that you figure it out so that they repel or they do the opposite of what they want. So we're gonna force it in, we want the north and north to match or the south and south to match, okay? So I'll just reschedule and it doesn't really matter which way the magnet is. So if we were gonna force this into the coil, we are gonna go inwards. We want the cells to be against each other. So then we point our thumb to the north end of the coil and we figure out our direction of the current. That way, if we're gonna pull it away, that's a magnet north and south backwards, then this would be the north in because we're going to break that attraction and our direction would be opposite, okay? So the key thing is for a web direction of the north and south side, to either attract or repel, and then from there, you can use your hand rules to figure out the direction of the current. And that's all.