12.1 History of Magnetism

Wait, today, we're gonna start looking at our new chapter on magnetism. And we'll start today by going through a little bit of history. It's believed that the Greeks, once again, just like electrostatics, were the first to discover magnetism. It was just by fluke that they happened to notice that a certain block of black rocks in the area would attract metals and other kinds of metal pieces. And they basically thought it was magical, some sort of magical rock. But it ended up being that those rocks just have magnetite in them, which is just an iron mineral that acts like a magnet. What they did discover over the years is that if you took a magnet, like a chunk of magnetic rock, and break it apart, you got smaller magnets. And probably by smashing them apart, they found a little tiny sliver of a magnet. And let's suppose as they're breaking it, that little sliver falls into a little mud puddle or something. They found that if it was floating on water, it would move in a certain direction. So basically, what they did is they invented the first compass. So compasses technically weren't invented until the Chinese invented them in the 11th century or so. But there's always been over the years some sort of use of a compass to help us with direction. And the reason for it is a little compass acts like a mini magnet. We have a north and south part of a compass, and that's gonna be angled a certain direction depending on wherever our magnetic field is of the earth, the North Pole, South Pole, and that type of thing. Okay, so with any magnet, we always call one end north and one end south. And all magnets will have that, no matter how big they are. You keep breaking them smaller and smaller, and you're gonna get smaller magnets with both North and South Poles. And the law of magnetism, you guys should know this from before, basically: like poles repel, opposite poles attract. So if you had two north poles next to each other, they can repel, but north and south will attract. Pretty easy stuff there. One quick note before we look at a magnetic field, most people assume that any kind of metal will stick to a magnet, but that's not indeed the case. There are only a few elements iron, nickel, cadmium, and cobalt, that are known as ferromagnetic, which essentially means that they attract magnets or can turn into magnets themselves. So, a magnetic field is very similar to what we talked about with an electrical charge. When we have a positive charge, we said we have an electric field that goes outward from it based on having a positive charge nearby. The same thing sort of happens except with a magnet, we can only get a magnetic field when we have both the north and the south. So it can either be just one magnet that has a magnetic field, or we could have two magnets side-by-side. Let's suppose we have two magnets where the North and South Poles are a certain distance apart. We'll get magnetic field lines between those. So, with electrical charges, you only need one to get your electric field. With magnets, we have to have both the north and south. We actually measure magnetic fields in Teslas, and we use the symbol B. I'm not sure why B is used, probably because it wasn't used for anything else. But we use the symbol B to indicate magnetic field, and it's a derived unit called Tesla, which is actually not a derived unit but rather an invented unit. There are really no ways of directly measuring magnetic field strength, other than knowing if it's stronger or weaker. So it's based on how much it would twist a compass needle. If you had a very weak magnetic field, the compass would barely move. That would be something like the Earth's magnetic field. Alright, it's not overly strong. We don't see things like metallic objects flying through the air because the magnetic field is so strong. It's very weak, and you have to have a pretty delicate compass to be able to detect it. Something like a fridge magnet is still pretty weak, but obviously quite a bit stronger. And then, if you had a super strong magnet that can pick up heavy objects, obviously we've got stronger magnets. We'd say that would be something like two Tesla's. So let's figure out how we can draw magnetic field lines. So, remember, we think of a compass as being like a mini magnet. So a compass will have a north and a south pole. Okay, so whichever direction the compass is placed is going to help us indicate what the direction of the field is. For example, if we had a north pole of a magnet like this, and if we took our little compass, instead of drawing it as a magnet or so, quite often what we'll do is just show an arrow pointing to the north. Or sometimes they'll give you sort of a needle, let's just have one end shaded as being the north end. There's lots of different ways we'll stick, for this stuff will stick to it as an arrow because it's easy and fast to draw. So the idea then is if the arrow shows the north pole, so if we took a magnet and put a compass nearby, so if we had a compass near the north pole, right, the needle of the compass will point away from the north. Right, north repels the south, and the north would attract. So using that, that's how we're going to keep the field line. So we say that our electromagnetic field will go away from the north, something like that. And then if we had our compass over here, the north pole of the compass would get attracted to the south. So then we'd say our south magnetic field lines go inwards. And for one particular magnet, we basically get these curvy magnetic field lines. So our field would look something like that, going around a compass or around a magnet, right? So let me just redo this a little bit clearer, it's a mouthful. So, pulled around a magnet, our field lines go away from the north towards the south, and they go both directions, something like that. Okay? Magnetic field lines from the north inwards to the south. So that's if you just had one magnet. If we ended up getting where we had, just like we did remember with charges, we put a whole bunch of charges in a line and get straight field lines, the same sort of thing applies if we have two magnets with a gap between them. We basically would get a straight magnetic field between the two. The edges would bulge a little bit, but just like we did with electric fields, as long as we're in between, they should be fairly straight. So our magnetic field, indicated by B in this case, is going from the north to the south. William Gilbert, an English scientist, said back in the 1700s or 1800s, somewhere in there, that the earth itself acts like a giant magnet. So what he did was he pictures the earth because it's spinning and so on, he basically says think of the earth as being one huge magnet from the North Pole to the South Pole. And what he said was because our compass, if we have a Compass on the Earth: it's going to point to the north, right? So our little compass will point north like that. So Gilbert came up with the idea that then our "north" part of the north part of the Earth would actually be the magnetic South Pole, and then the "south" part would actually be the north, just to match how the compass points. So, magnetic poles show up as South and the North's magnetic field lines would go from the north towards the south. So, picture it like a ball shape, going around the Earth in a curved manner from the north towards the south. We just do that in every direction. Okay, so what you've got to be careful of is in terms of a magnet. I'm going to draw the Earth as sort of a magnet. It's so cool, North Pole like that. But in terms of geography, our North Pole is off a bit because it's based on how the Earth rotates. We actually have a North Pole and a South Pole for geography. So you have to be a little bit careful. The first thing to remember is that our magnetic north and south are reversed compared to our geographic north and south. And the real reason for that is it's based on how we define a compass and how we define field lines. So it has to be like that to match our previous definition. Also, be careful to notice that the two poles don't line up. Our magnetic South Pole is not in the same location as our magnetic or geographic north, and vice versa. The magnetic poles are based more straight up and down from where the Earth's magnet location is. So you have to realize that they're different. Our geographic North Pole and magnetic South Pole are roughly about 1500 km apart in the northern part of Canada. So, if you have your compass, it's actually pointing towards the top of the Earth, not towards our geographic North Pole. Because of that, you have to take into account, depending on where you are in Canada and so on, you'd have to adjust your angle a little bit. Otherwise, you might be going in the wrong direction. Obviously, if you're a long way away from the North Pole, it wouldn't really matter because that slight angle isn't going to make much of a difference. And with the advent of GPS, all this has pretty much been eliminated because we don't really use compasses all that much anymore. We just use actual satellite positioning. We talked a little bit about field lines. So, if you kind of imagine the Earth as sort of being like a peeled orange, you have your orange sphere shape. Each slice of orange kind of makes those lines that run from the top to the bottom. Think of those as being like your field lines. So, your slices of orange look sort of like that. We're going from the North Pole, which is at the bottom of the Earth, to the south magnetic pole at the top. So, our field lines would go upwards, kind of like the lines on the orange. One last thing, just a point of interest, is that our north and south magnetic poles, because our field lines are sort of going out from them and then around them, if you would actually look at the very top of the Earth, the magnetic north pole, you would see these magnetic field lines going out from it. So, you'd actually end up getting this hole or depression void at the North Pole. The same thing would happen at the south, where the field lines go inwards. So, we get this sort of dim polar void of the magnetic field lines, and it's due to those magnetic field lines where those holes are that we get our Northern Lights. In the North Pole, the Earth gathers electric charge, and other radiation comes from the Sun, and that's what sort of leaks into our atmosphere. So, it goes through our magnetic field, right at the top and the bottom of our two poles. And as they hit our atmosphere, depending on Temperature, charge in the air, and all those kinds of things determine what we call our Northern Lights. Alternatively, if you were in Australia, they would be referred to as Subing Lights. That's all for today.