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Current time:0:00Total duration:10:56

Video transcript

- In the previous video we discussed how a magnet produces an influence around itself. Making it's presence feel far away from it. We called this the Magnetic Field. And when other magnets come in contact with this magnetic field they automatically experience a force. So it's the magnetic field's that push and pull other magnets. But guess what imagining magnetic field like this green cloud isn't going to be very useful for us. Because it doesn't tell us anything about where the force is strong, and where it is weak. For example we know the force is strong lose to the magnet, but it get weaker when as it get farther away. It also doesn't tell us in what direction other magnets would experience a force. It's for these reasons today we have a better way of imagining this magnetic field. We do that by drawing lines everywhere. We call them the magnetic field lines. And so in this video we'll see exactly how to draw these magnetic field lines, and how we read them. So to start drawing magnetic field lines the first thing we need to do is to find a direction for this magnetic field. We decided the direction of the magnetic field at any point is going to be a direction in which the north pole of a magnet would experience a force. For example, if I want to know what's the direction of the magnetic field here then I have to keep a tiny magnet at this point. And check what direction of the north pole of that magnet would experience a force. And we can do that by introducing a compass. Because a compass has a tiny magnet which is free to rotate to so it'll show me what direction the force is. And the red of the compass is the North, the black of the compass is the South. So I'm gonna keep my compass over here, and notice the north is being attracted by the South and as a result the North is being pulled this way. And therefore by definition the magnetic field at this point is in this direction. Similarly if I want it over here. I'm gonna keep my compass again at that point. And now we see that the North is being pushed away repelled by this North Pole this direction. And therefore the magnetic field over here is this way. These two points are easy to predict because the North Pole gets attracted, and here the North Pole gets repelled. But what if I pick some arbitrary points something over here, or something over here lets say. And it's not so easy to predict, but experimentally I just have to keep my compass over there and see what direction the needle points. Now with this the north pole points in this direction. And so the magnetic field is this way. And you may be wondering why we choose north pole not the south pole. Well people decided we have to choose one of them as a standard. And so we just decided we'd take north pole as a standard. That's all. There's no other reason for this. And now if I wanted to draw the magnetic field direction everywhere I have to keep repeating this experiment. Just keep moving my compass in different locations and keep drawing the arrow marks. That's fun, but that's a little tedious to do here. So guess what there's a simulation on Khan Academy where you can preform the same experiment but all the software. So it's going to become much faster. So let's go to that simulation, and let's preform this experiment. And figure out what the magnetic field direction is everywhere. So here we are. This is the code of this program which we don't have to worry about. And this is the simulation where we start playing. Now before we start I have put the link of this simulation in the description. So if maybe after the video if you want to come over here and start playing with it. Just click on that. All right so here's the setup. The setup is pretty much similar. We have a bar magnet, and we have a compass. Which will help us direct find the direction of the magnetic field. And it says over here press space bar to draw an arrow. That's what I like about this. So for example over here if I we know now the direction of the magnetic field is the north pole. So it's towards the left. If I press space bar, automatically an arrow comes over there. Again I press space bar, an arrow comes over here. Press space bar arrow comes over there. So what we'll do is we'll press space bar everywhere. We'll find out what the magnetic field looks like everywhere. So let's do this. It's gonna look initially all random. But as we draw more and more arrow marks, hopefully a pattern will emerge. And we'll speed it up so that we'll not waste too much time. So let's paste this picture back to our drawing board. Now if you look at this picture carefully. Can you see an interesting pattern it left over here. Look at all the arrow marks. They're all pointing away from the north. And they're slowly turning, and pointing towards the south. And so the way we represent magnetic field today. is we draw continuous lines not arrow marks, but continuous lines. That start from the north, and move towards the south. We draw lines because it's easier to draw them compared to arrow marks. It's very tedious to draw arrow marks everywhere. So we will draw lines like this. We'll put an arrow mark representing the north to south. This is how we represent the magnetic fields today. From north to south. Everywhere from north to south. Now I know this is a little bit shotty. So I've already drawn a better version of this. So here it is. So let's list down some more important properties of these magnetic field lines. The first property is that the lines start from the north and end into the south. But this is outside the bar magnet. If we're to peak inside the magnet, then you see the lines run south to north. Closing the entire loop. And so in short we see that outside the magnetic field lines run north to south. Inside they run from south to north. And as a result we'll always see that the magnetic field lines are always closed loops. Even this loop is a completely closed loop. All of it. Now you may be wondering why is it inside south to north. How does that work? And the way I like to this about this. Is let's say I break open this magnet. To figure out what the direction the field is inside. Well then all I have to do is bring my compass in between, and see what direction the north pole points. Now you might expect that if I bring my compass over here and keep it over here. The north pole will point this way. Towards the south of that, and as a result the magnetic field line must be from north to south like this. But if you keep the compass in between. Notice that the red needle, the north pole is pointing this direction. Meaning the magnetic field is from south to north. Inside the magnet. But why is that? Well that's because when I broke open this magnet, I did not get a south pole, and a north pole. But I got two tiny magnets each having it's own north and south. And similarly north and south. And just like always this needle is repelled by this new north pole, and being attracted by this new south pole. And as a result the magnetic field inside is from the original south to the original north. Another important property of the field lines is that wherever the lines are closer it means we have more field strength. So if you look at the field, notice they're close to the poles. The lines are very close which means the field is very strong over here. And as you go farther away from the poles notice the lines go farther away. Which means the field is pretty weak over here. And this can be seen experimentally as well. If you sprinkle some iron filings on a bar magnet, you see a huge crowd of them6 close to the poles of the magnet. Reveling to us that the field is very strong close to the poles, and of course as we go farther away the crowd decreases because the field becomes weaker. And the reason we get this pattern, is because when you put an iron a piece of iron close to a magnet it automatically gets magnetized, and starts behaving like a tiny compass. And as a result it gives out this pattern. And another important property is that these field lines will never ever intersect. Now what I mean, is imagine we brought another magnet into the picture. Then we might think at first that the field lines of this magnet will intersect with the field lines of this magnet. But that won't happen. This will not be the picture of the magnetic field lines of two magnets. And the reason for that is because at the point of intersection. If we were to keep a magnetic compass a magnetic meter. Than this field line will make a point in this direction. But this field line will make a point in this direction. And a magnetic needle cannot point in two directions at the same time. That doesn't make any physical sense. It's for that reason field lines will never ever intersect. What will happen over here is that these field lines will now change. It'll become complication, but they'll change in such a way that they'll never ever intersect. So in short today we represent the magnetic field by drawing imaginary lines called the magnetic field lines. These field lines tell us in what direction the north pole of the tiny magnet points. So this line tells us the north pole of tiny magnet kept here points this way. This field line tells us that the north pole of a tiny magnet will point this way and so on. And these field lines always form closed loops. Wherever the field lines are closer, the field is stronger, and they will never ever intersect.