28. Magnetic Fields and Forces
Magnets and Magnetic Fields
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Hey guys, so in this video we're going to talk about compasses and the Earth's magnetic field. Let's check it out. Alright, so remember magnets have ends or sides or poles that are called north and south. So something like this, this end is north, this end is south, But how do you know which one is which? How do you know which one to label north, for example? Well, the end of the magnet that points to the Earth's north is the one that gets labeled to be the north pole of the magnets. Let me show you. So the Earth's North Pole is somewhere around here, the South Pole somewhere around here. It's actually angled because the earth spins around a tilted axis. Okay. So the earth spins around that that little line there. This is the North Pole, which by the way is just a bunch of water. And then this is the South Pole which is a bunch of ice and Antarctica. And you have sort of like the equator over here. So the way this works is, let's say you have a magnet and it has a, you paint one side red and then you paint one side blue and then you bring this over here and you're in the US somewhere, and you notice that when you're here this thing always orients itself like this. With the red side this way, and the blue side this way. And then you move over to Europe over here and then you'll notice that it always orients itself, with the red side pointing to north and the blue side pointing the other way. So what you're gonna do is you're gonna say, well, this one is pointing North, red is pointing North. So the red side must be what we're gonna call the North side. Okay. So North and South. This is completely arbitrary, they could have done it backwards, They could have done the other way around. But they decided to say, hey if it points north we'll call it north. That makes sense, right? And that is, by the way, how compasses work. Okay? So this is what a compass looks like. It has a magnetic needle. So a very thin metal, very tiny metal inside that is magnetized. And the end of that needle, that points to the earth's north is labeled north. Okay? So you can't see here, but this is North right there. Okay? And you may not be able to see this, but this tip is red. Okay? So on old school magnets, one side is red. And the red side is the one that is the north side of the needle. Okay? The north side of the needle. By the way, sometimes if you don't have colors, you may see this as an arrow. You may see something drawn like this. So for example here, I could have drawn, if I had something over here, I could also have just made an arrow this way, which means that that is a direction of North. Okay? By the way, if you have a magnet here that's pointing directly North, it means that you're probably somewhere close to this line so that it's pointing straight up. Okay? And similarly, if you had a compass that the north, that the north arrow or the north side of the magnet was pointing that way, it means that you're probably somewhere over here. So this is why compasses are able to be used, or used to be used as navigating devices. Cool. So if it points north, it's north, that's that. Another important thing to realize is remembering, that magnetic forces can only exist between 2 magnets. Okay? They can only exist between 2 magnets or more generally between 2 things that are magnetized. Okay? So, if this needle here is attracted to the top of the earth, if that needle is attracted to the top of the earth, and you can only have attraction between 2 magnets, it must be that not only the needle is a magnet, which it is, but that the earth is also a magnet, which it is. So the earth, it's not a magnet in the typical sense that there's a a huge metal bar through it, but it behaves like a magnet. A gigantic magnet. Okay? So you can think of the earth as as though it had a huge metal, but magnetized metal bar, this way so that things can be attracted to its north. Okay? Alright. So that's the first thing. Cool. So the earth is a magnet. Weird. The second thing is, if you realize that opposites attract and the compass's north points to the earth's north, so let's do that slowly. Let's say I have a compass here and the north of the compass is pointing towards the north of the Earth. Remember, opposites attract. So if this end is attracting this end, and we call this North, this must be South. Okay? This must be south. Now you might be thinking, no that's north. We just said that that's north. Well, this is the, what we call, the geographic north. Okay? Which one way to think about this is that it's up there on the map. It's on top of the map. Okay? That's the locational, the geographic North. But it must be the magnetic south. Meaning the earth behaves like a magnet that has its north over here and its south over here. So the thing is, if we wanted to call these magnets, this side of the magnet north because it's pointing to the north of the Earth, we must then recognize that this has to be called the south side of this big imaginary magnet that sits inside of the earth. All this stuff is just convention, but that's how it works. Okay? So north and south. So please get that difference down. And because of this, the north pole of a needle, so here's the compass needle, the north one, the arrow, this is the north side or end of the needle, is also sometimes called south seeking. And again, it's because opposites attract. So if you are the north, you want south. So if you're south, you are a north seeking magnet. Or you're the north seeking side of the magnets, in that case. Cool? Another very important but general point is that any magnet's North is always going to point in the direction of the magnetic field around it. So what does that look like? So I want to redraw the earth over here, and we're gonna draw the magnetic field on the earth. So the top of the earth is gonna be the south magnets and the north magnet on the bottom. Remember magnetic fields go from north to positive through the outside. High to low on the outside. So it's gonna look like this. High to low. High to low. Like that. Right? North to South on the outside. So the magnetic field lines will look like this. Any magnets north will point in the direction exactly in this direction here. So if you had a magnet right here or a compass, it will point exactly in this direction here. Okay? Exactly in this direction here. That means that if you have a magnet, if you have, let's do a different color, if you have something over here it's actually going to point like that. Okay. But then you wouldn't really be navigating. Now you run out of space. But just to make the point that, now you got bigger problems, right? But just to make the point that it's always going to follow this line. So it actually doesn't always directly point north, right? If you're here, if you're in the equator, right here, it's actually going to sort of go up like this. It doesn't really point north directly. Depends on where you are. So you have to sort of adjust for your sort of height on the earth. Okay? Similarly, if you look here it actually points if you draw it down here, it actually points away. It doesn't point towards the north of the earth. What it actually does is it points away from the south. Okay? So that's the last point I'll make and then we'll do a quick example. So it says here, if you, by the way, if you notice in these examples, I was careful to always draw stuff on the northern hemisphere, but if you are on the south of the earth, if you are here on the southern hemisphere, then the compass's south pole will point to the earth's south pole. Which by the way, this is our our geographic South, which will be our magnetic North. Oops, not North. North. Okay? So to wrap it up, in the very beginning of the video I said the North side of a magnet points towards the north of the Earth. But actually if you're in the southern hemisphere, then the south side of the magnet points towards the south of the earth. The easiest way to remember this is just remember how what the lines look like and remember this one statement here that the direction of the magnetic field or the direction of the magnets north is always going to follow the blue lines. Okay? Let's do a quick example here. So the green magnet below is fixed in place and you have a ton of small compasses located around it. We want to draw approximate orientation of the needles. And we're gonna use an arrow to indicate the north direction. So what we're gonna do here is we're gonna 1st draw, we're gonna use this principle right here which is super important, that the North will point in the direction of magnetic field. So what is the direction of the magnetic field? Well magnetic field is always North to South through the outside. So I'm gonna do north to south like this. And then I'm gonna be very careful to do this here. Obviously, I laid out these guys there for a reason, so it all just looks real cute. And it looks like that. You can't really see the last one, but that's cool. So north to south, so it looks like this. North to south, it looks like that. Cool? So that means that this needle here is going to be pointed this needle here is going to be pointed in the direction of the field lines. So the needle is going to point like this. This needle is going to point like this. This needle is going to point like this. This needle is going to point like this, and then finally this needle is going to point down right there. The reason we said approximate is because it depends on how precisely you draw this thing. I just want to make the point super important that the fields, the direction of north on a magnet will follow the the direction of the field. And by the way, this works in any hemisphere. Cool? That's it for this one. Let's get going.
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