 Hi, I'm Zor. Welcome to Unisor Education. Today I would like to talk about magnetic field. Now this lecture is part of the course Physics for Teens presented on Unisor.com. I suggest you to watch this lecture from the website Unisor.com rather than from, let's say, YouTube where you might have found it through a search engine because the lecture is part of the course. So there is a hierarchy of different topics presented on the website. You can just go sequentially or in whatever order you prefer. Every lecture has detailed notes right next to it, which basically is like a textbook. You can use it. Plus many topics have exams, so you can basically check how you know the material. And the site is absolutely free. There are no advertisements, no financial strings attached, and you don't even have to log in if you don't want to. Okay, so magnetic fields. Well, first of all, we have the fact that magnetic force exists and it acts on a distance. Now, I'm sure you have problems with the concept of acting on a distance. The force which acts without really touching each other. I mean, I have problems. Physicists have problems and they have come up with a model. Basically, it's a word which describes the whole thing. It's a field. So it's a force field. Well, introduction of the word doesn't really make it much easier to understand, but well, we deal with this. We live with this. So we know that there is such a concept as a field. We consider this field as a reality and we study it as much as we can. So there is such a thing as magnetic field. And we have to study it, right? Now, before the fields we were studying before were gravitational field and electrostatic fields. They are easier. And here is why. In both cases, in case of gravitation and in case of electrostatic, we actually had absolutely no problem to assume the simplest object, which had the property of the source of the field. It's a point object with certain mass for gravity. It's a point object with certain electrical charge, with certain excess of electrons. Well, there are no point objects, obviously, but you can imagine a very small one. And it has certain number of excess electrons or deficiency of electrons, and that what makes it charged. So geometrically speaking, it's a very simple thing. It's a point. We're talking about mathematical abstraction, obviously, in this particular case. So it's a point and it's easy. And if you would like to have some kind of a more complicated object, we can always break it into small pieces, each one small enough to resemble a point, and basically integrate the fields, which are produced by every part of it, every particles of it. Now, in case of magnets, the situation is significantly more difficult. And the primary reason for this difficulty is that magnets have two poles, which act against each other, so to speak. So if this is the magnet, let's say this is the north, this is the south poles, you can't make it a point. You cannot make it small enough. The most reasonable thing to assume that this is a rod, a thin rod, and still it has certain finite, not infinitesimal, finite lengths, and one part is north and another part is south. You cannot avoid that. Now, that's not the only complication. Another complication is that the force which is produced by the north pole, not only it's opposite to the south pole, but the problem is that everything in between, every point in between, also is a source of some kind of a field. And the source is the weakest in the middle, actually it's zero. There is no magnetizing in the middle of that thing. And it's the maximum on this side and on this side, and then it's diminishing down to zero and then increases again, but in a different direction, it's like positive and negative. Complicated. And to study it, I mean, let's say you would like to know what's my field at that particular point. Well, the field in this particular point is a combination of all the fields produced by each one of these guys. And again, what's the length of it? I mean, how can we establish a probe object? A probe object is supposed to be a magnet as well. So it's supposed to be some kind of a rod, if you wish, or a bar magnet or a very small bar magnet, but still bar with a finite length. And now we have also an angle, not only the distance, but also an angle. Complicated. So what can we do about it? Well, we will try. The first thing which we will probably try to do is to establish how this magnetic field around the bar magnet, well, bar magnet is probably the simplest, but it's still complicated. But in any case, if this is the simplest, let's just try to find out how the field around the bar magnet actually looks like. So if you have a bar magnet, let's say horizontal. This is north, this is south. How can we visualize the forces which are actually around this magnet? Here's how. And this is the experiment which we can do at home, if you wish. I'll just explain it. So what you need is, you need iron shavings. You know, when you're just working with iron, you're polishing or whatever, there are some shavings. So you need these shavings. Now, if you will drop these shavings on the table, let's say, where the magnet is located, they will somehow organize into whatever the figures are. And the figures will be something like this. These are little shavings. So what happens is, you know about temporary magnetism, right? So iron shavings, these pieces of iron, are really tiny temporary magnets. As soon as they are in the field, they are magnetized. So one end of each shaving becomes a north pole, another becomes a south pole. Well, temporarily, while they are in the field. Now, as soon as it happens, all the north poles are connected to south poles or south poles to north poles. And they are actually connecting into these lines. You will see these lines. And in the notes accompanying this lecture on the website, I actually have a picture of this. Well, I took it from internet, obviously. I didn't do experiments myself. But yes, experiments are done in different circumstances, in different environments, etc. That's what's going on. So you will see that these are the lines along which these shavings actually are positioned. Now, if you remember in gravitation fields and electrostatic fields, we do have a certain concept which we were talking about, equipotential surface. If you have a point object as a source of electrostatic field, equipotential problems are spheres on different radiuses. These are equipotential surfaces around the magnet. Now, so it looks like there is some kind of a force which is directed from north to south. And the force is, by definition, chosen north to south. Why north to south, not south to north? There is no why, just by definition. It's a tradition. Same thing as in electricity, when you have a direct current, you, by definition, say that this is from positive to negative. Why? Because probably people have decided that this way, somehow in the very beginning of the studying of electricity, and at that time they didn't even know about electrons. So they didn't know that it's actually electrons which are traveling. They said, okay, let's just define that the direction is from plus to minus. Here is the direction from north to south, so that's it. Now, which is north and which is south, by the way? If you would like to determine, well, considering the earth is a big magnet and the north, by definition, is where the north pole is, more or less, then you just position your magnet in such a way that it will orient itself along the meridian, and whatever the end points to the north is called the north. In theory, it means that the earth as a magnet is actually, if you will consider earth to be like a bar magnet along its axis of rotation. The south pole is on the north, right? Otherwise our north compass wouldn't really point to it. South and north are attracting each other. So this is a little deviation. So this is my earth, this is my north pole. And we are talking about north's magnetic pole. Actually, if you imagine a giant magnet, this is the north and this is the south. So this is supposed to be a south pole of the giant magnet. I mean, whatever it is, we just call it a north magnetic pole just because it's on the north, but from the magnetic lingo it's a south pole of this giant magnet. And our compass actually points with the north towards the north's magnetic pole which is actually south, but it doesn't really matter. I don't want to confuse you too much. In any case, it doesn't really matter what is north and what is south. What there is, there are two of them. And that would complicate the whole picture. Now, before I am going any further, let me check. Okay, basically everything before the main thing of this lecture starts, I have covered. So whatever I was just saying, it was a preliminary thing. Now, what's important is we have to understand why magnetic field actually exists and what makes it, basically, whatever it is. Now, we know that about electrostatic fields, for instance, we know that the source of electrostatic field are electrons. So there is some kind of a material object which is the source of electrostatic field. Well, we don't know how it does it, but it does it. At least there is something which we can hook to this electrostatic field, an electron. Now, in case of gravitation, we always talk about the mass. Any piece of mass actually, any object which has certain mass has this qualities of being the source of gravitational field. What is the source of magnetic field? So that's what I am talking about right now. And this is the main part of this lecture. So let me wipe out this and talk about something else. In many cases, physicists are coming up with a model which kind of describes whatever they want to describe to a certain degree of precision. And if it corresponds to whatever observations they make, well, they say, okay, that's a good model. Maybe the universe is really done this way. Well, maybe not, but chances are that it's a good model and we can use it if it corresponds to everything whatever we know about the universe. Then maybe we can use this model to predict certain qualities of the universe for whatever we don't know yet. So let's just develop this model further and further. Well, that's how models are created. That's how they are coming to a dead end if they find something which cannot be explained within the model. And then they try to build another model, more sophisticated, whatever. So I'm talking about a model right now. That's what I suggest you to use to kind of better understand how a magnetic field works. Now, whatever I'm saying right now, or will say right now, is it really how a universe is built? I have no idea. However, it's a reasonable model. It explains whatever we know about magnets. And I think at least at this particular point in your education you might be satisfied with this model. So here is, let me start from a slightly different concept, but it will basically eventually lead us to the theory behind magnetic field. Now, consider you have a particle which is rotating, this is my particle. It's rotating around the axis in certain field, in certain plane. Well, plane is perpendicular to the axis. So it's rotating. It has certain radius. It has certain speed of rotation, angular speed, whatever it is. Okay, fine. Now, let's imagine that on the same axis in a different plane, so this is my plane. Now, in a different plane, there is another particle which is rotating, let's say this is the direction. Maybe a different radius than this one, maybe with a different speed, along the same axis, and the plane is parallel. Okay. While these two planes are relatively far from each other, well, particles don't interact. But let's try to make them closer and closer together. Well, here is a very interesting point which I think makes sense just intuitively. If they are rotating in the same direction, which means that, let's say, we're positioning our observer here. So this observer looks at this particular thing and it's counterclockwise. And look at this one, and it's also counterclockwise. So it's the same direction. Now, what happens when we are making these two planes closer and closer? Well, these two particles don't really disturb each other. If there is something around them, some kind of a force around them, whatever the force, gravitational force, whatever we know about, electrostatic force, they probably don't really bother each other much. If they are spinning in a different direction, so this is this way and this is this way, they disturb each other. So one particle would probably somehow prevent another particle of moving, especially if their radiuses are relatively close to each other. So there is a very reasonable assumption that if they are close to each other, if they are spinning in the same direction, then there is something which might actually even attract them to each other. So they will join these two planes. But if the direction is different, opposite direction, then there is actually some kind of a resistance from making these two planes closer. And we can talk about repelling. So this is my model, this is a starting model. So I assume that these two particles will bring the planes closer and attract each other if they are spinning in the same direction and will repel each other if they are spinning in different directions, in opposite directions. Is it reasonable? Well, I think it is reasonable, especially considering that around each particle you might have an electrostatic field. Fields somehow interact with the field of this guy and when they are going in the same direction, that's okay. I mean, that's probably fine. But if they are going in the opposite direction, they repel. Well, if this seems to be reasonable to you, let's go one step further. Let's consider that this is an electron. An electron is rotating around the nucleus within the atom and maybe we can think about certain plane where it is actually rotating around the nucleus. Now, this is another atom, another electron, rotating around its own nucleus. Now, let's forget about the plane, this is the common axis of them. Even if it's not common axis, but if they are in the same parallel planes, it still kind of makes it easier to align to each other. So, my point is that if all these atoms are aligned in parallel planes and all are rotating in the same direction, then they actually, each particular atom behaves like a bar magnet with north and south pole here, north and south. And in this particular case, this attraction is really equivalent to the attraction of two bar magnets positioned exactly like this. North to south will be attractive. Now, if, however, my direction is opposite, so instead of this, I will have this. Well, if you will look from another side, it will be south-north, right? We just turn the whole plane upside down. So, the whole, assuming inside there is some kind of a bar magnet because behavior is exactly the same. So, we are turning the whole thing upside down so the poles are changing and now there is a repelling south to south or north to north, whatever it is. So, my point is that if you assume that the model of this spinning particle is reasonable, we can actually make the next step and basically take a look at what's inside the object. Inside there are atoms. Atoms have electrons. Electrons are rotating in certain plane within each atom and if they are properly aligned, it makes the whole thing like a big magnet actually because they are all pointing in the same direction all fields are coming together. So, let me go back to north-south model. So, even if you assume that these two are basically nullifying each other there is no field because they are already touching each other. It's like plus and minus in electrostatic. But still the surface has this property of all these are acting as north pole, right? And all these are acting as south pole. So, this is an object itself. So, the whole object becomes a magnet. Now, if, however, all these atoms are randomly directed and all these electrons are rotating in different non-parallel and not aligned properly planes, then all these north poles and south poles they are randomly distributed inside the object and they nullify each other and there is no external magnetism produced by this particular object. So, my point is that according to this model whether it's true or not, but it explains basically all these properties which we are observing with the magnets. So, just keep it in mind. The model where electrons are rotating around certain axis within certain plane in each atom and if these planes of rotation are properly aligned we do have a magnet. Now, we have permanent magnets and we have temporary magnets, right? So, permanent magnets is basically when it's permanent. Now, the temporary magnet is, let's say, a nail. You have a soft metal, like a soft iron, a nail. If you position it closer to the magnet then the magnetic field of existing magnet will align. So, if these are really very easy to align, they are very flexible, so to speak, then all these north things are aligned if this is a real magnet, a permanent magnet. Now, this is a north pole and this is a south pole. Let's say a horse-shoe magnet, a giant horse-shoe magnet, north and south. So, if these are really flexible, they are aligned and as soon as they are aligned they become a magnet. If you take it away, since they are so flexible after a certain time they will rearrange themselves randomly again and it will lose the magnetic properties. But while there is this field outside, this thing actually orients all these axes, parallel to each other, planes are parallel to each other, and that will be a magnet too. That's the temporary magnetism. And finally, if they are randomly arranged but not flexible then this is a diamagnetic thing which means it cannot be magnetized, like plastic, for instance. It can magnetize the plastic. Okay, basically that's all I wanted to talk about today. Again, this is a model. It's our view which, well, seems to make sense and it seems to explain certain things. And, well, if it explains whatever we observe then, well, let's just use this model. Now, what's very important is that before this lecture everything related to magnetism was basically about magnets. It was not related to electricity. Now, this model actually established a very, very tight relationship between electricity and magnetism because magnetism is produced actually by electrons as well as electrostatic field is produced by electrons. Electrostatic field produced by existence of the electrons. Magnetic field is produced by their movement. So, we have a static and dynamic in mechanics, for instance. We were using the statics and dynamics. One thing is just about positioning and other is about movement. Electrostatic is the same for electromagnetism as a unified theory. So, electrostatic for electromagnetism is what's static for electricity. And magnetism for electromagnetism is both dynamics for mechanics. They're kind of parallel to each other. So, again, electrons and their movement. These are the major sources of electrostatic field and magnetic field. And basically, we will talk about one concept which is called electromagnetic field. And that's why the whole subject which we are talking about is called electromagnetism because they are together. This common source, the electrons is extremely important for understanding all the properties of electricity and all the properties of magnetism. Okay, that's it. We have established the connection. And this connection is extremely important. And in the next lecture we will always talk about electricity and magnetism together in some way or another. I do suggest you to read all the text which is accompanying this lecture on Unisor.com. There are a couple of pictures, much better than I draw. And just, again, try to familiarize yourself with the model. Model explains the source of magnetism and, again, without staging that this is exactly how the universe is built, we do know that the model is relatively sufficient to understand whatever is happening, at least at this particular level of your knowledge about electromagnetism. There are much more complex issues and the theory, the real theory, which is right now, let's say, it's a current physicist's view to electromagnetism, is significantly more complex than whatever I was just explaining. However, it's a good first step. With this, thank you very much and good luck.