 Hi, I'm Zor. Welcome to a new Zor education We're talking about magnetic field and today we'll talk about magnetic field blinds. I Will explain basically what it means Though probably you more most of you at least probably heard about this and know what this is, but anyway There is some sense in discussing these because it's related to the force which magnetic field actually exerts So magnetic field lines now this lecture is part of the course called physics for teens It's presented on unisor.com. I Do suggest you to watch the lecture from the website. So you go to unisor.com then it's physics for teens the course name the topic is Electromagnetism and inside you will see The title called magnetic fields and this is one of the lectures about magnetic fields The same site actually contains a prerequisite mass of teens course, which I do suggest you to take All courses are completely free. There are no ads. No financial Strings attached. Okay. So magnetic field well Field is a concept It basically tries to explain what what happens in the space around something which is a source of the field Now in our case, we are talking about space around Permanent magnet and we know that there is certain force which acts on some other objects On the distance from this magnetic from the source of magnetic field So this is the concept of the field now If something is happening in the field, it means there are some forces which are acting So right now we will talk about the forces. So the force which is exerted by the source of magnetic field Through this concept of the field on some other object, which we can usually Qualify as a probe object So we're talking about the forces now the force Has is it's a vector, right? Remember mathematics force is a vector which has two characteristics The magnitude and direction Well today, we will mostly talk about direction of the force Around the Source of magnetic field okay, so The next thing which Kind of complicates the concept of a magnetic field is that In most cases The source of magnetic field has poles North pole and the south pole This is significantly different from electrostatic field Electrostatic field is produced by an electric charge and We can always talk about something which is which we call point charge. So it's an infinitesimally small Electrically charged object, which we can basically consider as mathematical point You cannot have the mathematical point as the source of magnetic field because If you remember how we were explaining the concept of magnetism the result was two poles north and south pole in in any permanent magnet and It's related to orientation of the electrons which are Rotating around certain axis And if all these axes are aligned properly and the planes of rotation are Parallel to each other with all the electrons and the rotation is in the same direction We have this magnetic Properties of the permanent magnet. So we have two poles, which means we cannot really consider it as a point Maybe a model of like two points which are infinitesimally close to each other might actually serve Certain purpose, but it's not easy. It's not easy And in infinitesimal Distance between these two points is also doesn't make much sense mathematically. So the problem should be Distance from each other on the finite some kind of a finite Distance and this is more It's more corresponding to a Gilbert model of the magnetism, which we were talking about in the previous lecture in any case, we are not talking about a Point as a simplest source of The field we are talking about something a little bit more Material more fine finite so What's the shape? Shape is important Maybe two points. Well, it's a model, but let's talk about more practical sense probably You you understand that depending on the on the shape of the Permanent magnet, which is the source of magnetic field the field itself behaves differently I mean one thing if it's a if it's a bar magnet and another thing if it's a horse shoe magnet obviously things around these Different magnets are different and the forces are different So, I mean we have to choose something. Let's choose something which I consider a simplest Kind of a shape which is just a bar magnet. So we're talking about a Bar magnet and one of them One of these poles is Norse another is sells So this is a source of magnetic field Okay, now there is another component whenever we are talking about the force field and direction of the Vectors which represent the forces. I mean we have to talk about the probe object on which the field acts, right? so Around this there is a field magnetic field and we have to choose Some kind of a probe object and well considering my bar magnet is The simplest form let's choose The small light bar magnet as a probe Okay, fine So we have the shapes it's It's a bar magnet. Okay Now now let's think about this way We have two poles that's extremely important for magnetic field now since we have two poles and we have certain Dimension of Of the probe object Well, it it it can actually involve in two kinds of movement One movement is when the whole object probe object is moving. That's a translational movement and another is when it's turning Right if we're talking about one single point There is only movement if we're talking about something which is actually well like a segment Okay, which has a finite length then we have to consider both Translational movement when the center of this object is moving in some direction and translation and Rotational movement so two different kinds of movement. So the field exerted by this permanent magnet, which is the source exerts two kinds of Motions in two kind well it exerts the force which causes two kinds of motions translational movement and rotational movement of the probe object Okay, so we have talked about this So now we can talk about magnetic ones Now you have seen Some photographs maybe you did yourself the experiment if you will drop certain number of Small pieces of metal of iron Filings, you know when you are filing there you have tiny pieces of Iron if it's iron Which if you will throw it on to this Let's say a table where the permanent magnet is located they will organize themselves into these kinds of lines And we were talking about why because Iron by itself is not a natural permanent magnet There is a special mineral called magnetite if I'm not mistaken However, it can be Temporarily magnetized. So if iron filing small You can you can actually consider them as small Segments, it's a very very thin line of a finite length So this is let's say the five phylic one individual one So in the field of the permanent magnet It's temporarily magnetized which means the electrons Inside of this little piece of iron are Aligned properly under the influence of the magnetic field and as soon as they are aligned They become a temporary magnet themselves, which means they have North Pole and South Pole and now when you have a lot of these we know that Similar poles are repelling each other and opposite poles are attracting each other. So they are actually going North-South North-South Each one of them Eventually connecting to these guys. So if you will drop all these phylics They will connect to each other north-south north-south north-south and Eventually the south goes to the north and the north goes to the south and we will have all these lines Now let's think about what these lines represent basically what I'm saying is that The lines these lines represent something which we can call magnetic field lines Which is again, it's as artificial concept as the concept of a field Now the field is something which we have kind of came up with for our convenience to explain certain qualities Magnetic field lines also explain certain things. So what do they explain? Well for one they explain how these iron filings are Shaping when you're dropping them around this magnet. Well, that's good, but from the from the concept of the force field They represent certain about the forces. So what kind of a forces we are talking about? Well, let's think about Here is a mental experiment consider Around this permanent magnet. There is a water now On the water there is something like a piece of wood whatever whatever is floating and on this piece of wood We have placed another permanent magnet a small one a probe magnet Wow Since this is water and this permanent magnet the probe magnet probe object Is lying on the piece of wood which is which is floating on the surface of the water Well, there is a force between these two guys, right between the source of magnetic field and the probe object so the force will Push or pull or whatever this probe object To move in some direction so the center will move and Also the rotation will be Will be observed So what exactly is the trajectory of the movement of the center of this magnet? Which is floating using this piece of wood or whatever So it's floating freely. So what would be the projector of the center and what would be the rotation? Well, I mean, I'm sure you have already guessed That the movement Will be along these lines Why well because there is an attractive force to one of those there is an An attractive force from this one to to to this one and then there is a repelling force from this to this and this to this so opposite attract and similar Repel each other so we have four different forces Repel attract Repel attract or whatever the differences. So all these four forces are acting together and as a result this Little magnet will float in such a way that its center will always move along this line if it's already close to this one it will Flow to this direction if it's close to this end of the permanent magnet It will flow to this direction and as it flows it will turn in such a way That it's a small one. So the difference in distance between these two is very Small so the first thing with it will do it will turn obviously and it will turn in such a way that North Pole here would be South Pole here and Souths here so north is here because the traction between this and South Pole is stronger Than attraction between this and North Pole so it will turn this way and then since this thing is closer to this It will start moving this thing is still repelling the North Pole, right? I Mean the south south this is south and this is also this house will still repel so that's why it doesn't go straight This way but under the both forces it will so it will go this way, but this will still push it Push it out, right? So this is and this is the resulting Direction of the movement more than that the direction of this Orientation of this probe object will be in such a way that it's always Tengential to this magnetic line So to describe the movement of any probe object we use these magnetic lines magnetic force lines and Again, that's the Trajector of the center and As far as orientation of this probe object. It's always tangential to the trajectory Okay, okay, what else? Now what's important about this is the lines will be Lines cannot cross Why well consider two lines are crossing and they will put the magnet magnetic the probe Magnet in at this point well Since Line is a trajectory and if two lines crossing each other which direction will be Will the probe object be moving? Obviously there is some kind of In consistency here, so magnetic lines magnetic field lines never cross now there are also They do not have the beginning can end Although it seems to be that there are starting here and ending here Or or vice versa doesn't really matter. That's not exactly true because we always Connect them inside Now, why do we do this? Whenever we are talking about not a permanent magnet, which is solid thing inside, but something like a loop of the electric current which has Properties of the magnet the inside is empty. So if you have a loop and And there is an electric current then the magnetic field will be completely enclosed against themselves, so Here we also assume that magnetic field lines are continuing inside. So it's always Closed I mean it's some kind of ellipsoid in this case not exactly the ellipsoid And it's more more like a circle in this case But it's also not exactly a circle because the number of the lines inside here is The same as number of lines Outside right and we have more room here. So here. This is more dense Location more more dense Position of all these lines Now what is the density in in in this particular case now here we also have more dense Position of the lines let's say let's let's say here right because the number of lines in the same So every line whatever is here on any distance eventually ends up here now when we are talking about Magnetic lines in in the abstract sense you obviously understand that there are infinite number of these lines I mean I have drawn like three of them and three so it's six of them. No, it's not six out best. It's infinite number So we are just Pushing this this picture as only like six lines because well first of all we cannot really draw Infinite number and secondly understand that every point on the surface of the North Pole can be the point through which the trajectory goes and Ends up in in the South and then continuing inside So the number of these magnetic field lines is infinite obviously However, when we are picturing them We are picturing them in such a way that the density of these lines is greater here than let's say here And it represents the strengths of the magnetic field. So this picture with magnetic lines actually represents trajectory of the movement And also the position of this magnet tangential to it to this trajectory and also it represents the intensity of the field The greater the density of the lines the greater the field the strengths of the field Obviously if you will put this particular object right in the middle here It will not move because the forces will um Balance each other now if you will put this somewhere here Facing if it's if this is let's say north and you will face itself It will move directly through one straight line here Until it just touches it if you will put the same north Against north it will move into this direction Okay, so one of the lines is this one and it seems to be that these lines are not connected, right? Well, you can always consider them to be connected in the infinitely remote point Okay Now the question is we have this magnet. What is the source? What is the north pole and what is the south pole and any magnet? Well, um, I can answer it this way in the very beginning when people were actually dealing only with permanent magnets, which they have found basically in the earth and They noticed that the the permanent magnet is turning itself in some specific way Pointing with one side always let's say north or to the north star if you wish Now when they have noticed that they have called actually this side of the magnet the north because it points to a geographical north and Obviously the opposite was the south so that was the first magnets. Let's say well Then if you have some other magnets you can always compare it with the ones which you have already determined Which one is north and which one is south so if two sides are attracting Each other it means they are different north and one in the south and another so from these first magnets They can determine the polarity of any other magnet. You don't really have to You know hang it on the on this on the thread and then see how it turns Relative to the north pole So that's how I would say people determine which one is which Obviously, there are much better ways to determine it, but that's how it was in the very beginning Also, what's important is now if if you have a permanent magnet which already marked north and south and You will allow it to freely rotate again. Let's put it on the Piece of wood into the water so it's floating and obviously it will turn in such a way that its north pole Points to the geographical north now. What does it mean? well, it means that the earth as a planet is a permanent magnet and Technically speaking its south magnetic pole is Where the north's geographical pole is so that's why there is a little confusion and we're talking about north magnetic pole What does it mean? Is it the magnetic pole which is close to the north's geographical? Pole which means technically speaking magnetically speaking. It's the south pole So magnetically speaking south pole of the earth is on the north However, when we are talking about quote-unquote north magnetic pole, there is ambiguity in this, you know, some people Actually understand that this is not the magnetically north pole It's a magnetic pole which is close to the geographical in north which is magnetically south. Okay. Have I confused you? anyway So that's about which pole is south and which pole is north Now the direction of these forces now what's also important is We are assigning a direction Now the direction of the if you remember when we're talking about electric current direct electric current We were saying that by definition the direction of the electric current is from positive to negative now When people were Investigating this electricity they didn't really know that electrons are causing this electric current They were just basically thinking about two poles as basically in opposite in some way and They noticed that there is something which is which they call the current between these poles and they Called one of them positive another negative and it's much later They found out that the positive is where the deficiency of electrons is and the negative is where the axis of electrons And the electrons are actually going from negative to positive However, by definition the direction of the current was just defined as from positive to negative So it's purely artificial without any kind of physical Substance in it here again purely artificially If this is a north pole and this is a south pole we put the arrow in this direction from north to south So magnetic lines are going from north To south and then inside the magnet going to the south from the south to north so outside of the magnet is from north to south Inside from south to north. Well, that's how we have defined it. This is a direction of the magnetic field lines By definition not talk about this. That's it What else So we were talking about magnetic force, which which has two directions as I would say the Translational movement of the center plus rotational now there is a rotational it means there is a torque Now I didn't mention it before but this construction of Object like this one magnetic object with with two poles It's called magnetic dipole It's very important dipole Now in electricity we have Monopoles because we have only one point charge positive or another point charge negative Monopole means there is only one pole It's either negative or positive in this case We always deal with dipoles because there are always two poles north and south in magnetism So that's as far as terminology is concerned So this magnet is a dipole in some way and this pole and this magnet is also dipole. This is bigger This is a smaller, but doesn't really matter. They're all dipoles and So okay, so basically this lecture was about direction of the Movement which means it's actually a direction of the forces So there is a force which is going along the magnetic line and there is a there is a torque which turns the probe object to Have a position tangential to the line. So these is more about the Direction of the magnetic forces around the permanent magnet Also every vector has magnitude as well. So they have to measure the force and that is something Which we will talk about in the next lecture. So right now. I wanted to concentrate in kind of geometrical Properties of the magnetic field. So we're talking about mostly about direction of the magnetic forces Magnitude will be next So that's about it. I think that's all I wanted to talk about magnetic field lines Yes, one more thing very important If you will take the permanent magnet it has north and south poles now, why do we have this polarity? Why if we will put it, let's say on a Floating piece of wood why it will turn in some way or another well Here is the very important Thing you see the density of the magnetic lines inside is Significantly greater than outside right outside. We have all the space actual infinite space of both ways and all that infinity should be squeezed into this magnet That what makes it Non-symmetrical in some way this non-symmetrical thing he is basically forcing to have these poles now same thing if we have this Circular movement of electricity of electric current in the loop All these magnetic lines which are infinite of them They are spreading thinner and thinner and the density is significantly greater inside again There is a non similarity What happens if you will break this loop if you will put it like this So there is still a loop, right? Now Let's do it instead of this You will break it in such a way that you will have only one single line and put the current in We will see that it's still magnetic field Now what happens with the magnetic field lines? Well each line is around the piece of This loop right so here we will have lines magnetic lines like this and They're also concentric So this thing is purely symmetrical It doesn't have Norse and south poles. So if you have a line a straight line Not looped just straight line and put electric current these magnetic Field lines will have to open up and that's what will be right I mean if you will take it and close you will have this one If you will cut it and open that will be this one and this one doesn't have magnetic poles Because it's all symmetrical every line is symmetrical So you don't necessarily if you have a magnetic field like in this case you do not Necessarily have something which is Norse and south You might have a situation when the field exists and You can always have a direction because there is a direction here So you will have a direction here So everything is here. However there is no Norse and south poles and if you put this construction With infinite Current line in the magnetic field of the earth it will not actually turn any any way because these loops are Kind of symmetrical Okay, that's it. That's it for today I do suggest you to read the All the nodes for this lecture because on the unisor that come every lecture has a video part and the textual part So read the textual part. It's always helpful and That's it. Thank you very much and good luck