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Magnetic field lines (& their properties)

Let's explore what magnetic field lines are, why we draw them, and what their properties are.

Here is the link to the simulation

https://www.khanacademy.org/science/physics/discoveries/magnetic-fields/pi/tracing-a-magnetic-field

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Created by Mahesh Shenoy.

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  • starky seed style avatar for user Dishita
    Will there be a point wherein the magnet is broken up so many times and into such little pieces that it won't create a magnetic field?
    (3 votes)
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    • leaf red style avatar for user djvsmooth
      A couple of different things can happen when you cut a magnet. If you do it gently you can end up with two magnets. You can think of a magnet as a bundle of tiny magnets, called magnetic domains, that are jammed together. Each one reinforces the magnetic fields of the others. Each one has a tiny north and south pole. If you cut one in half, the newly cut faces will become the new north or south poles of the smaller pieces. Remember, I did say though you only get two magnets if you cut them gently. The magnetic domains in a magnetic material can be knocked loose, by bumping or vibrating the magnet (like when sawing it in half). If knocked loose, the domains are no longer arranged neatly, so they do not reinforce each other. If they are in a random orientation, with their fields pointing all over the place, they cancel each other out.
      (3 votes)
  • blobby green style avatar for user bhavanap1967
    when we break 2 magnets, it creates it own north and south pole so why does the south pole of the needle attracts the north pole of the magnet and vice versa
    (3 votes)
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  • starky seed style avatar for user Dishita
    A magnetic compass when kept within the magnetic field always shows the direction,
    but why are the fields considered as lines(closed loops), if the magnetic property can be detected anywhere within the magnetic field?
    Shouldn't the force be like a 'cloud'( I know it's not the best choice of word but it's easier to imagine)?
    (3 votes)
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    • blobby green style avatar for user Nisarg
      I guess there is a mathematical aspect to it. The cloud could provide us the magnitude of force at any point but we won't get its direction, whereas the tangent to the curve at any point would provide us the direction. But it would be nice to have both lines and cloud gradient to get both direction and magnitude of the magnetic field though we can get the magnitude on the basis of the density of field lines near the point of interest.
      (1 vote)
  • blobby green style avatar for user Pranjali Pal
    Why does a compass needle get deflected when brought near a bar magnet?
    (0 votes)
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  • starky seed style avatar for user Dishita
    If we cut the bar magnet into squares, which side would be the north and south pole?

    Considering that, how do we know that the ends of the magnet when kept vertically has 2 poles at the vertical ends and not the horizontal sides?

    Shouldn't the horizontal sides also have north and south poles as a magnet consists of zillions of tiny magnets?

    I know the questions may seem silly but it would be awesome if you could clear my doubt.
    (2 votes)
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    • scuttlebug green style avatar for user Uma
      Given that the magnetic field lines go straight through the magnet, I guess the new north and south poles would also follow them.
      And some magnets in nature probably have poles along the horizontal sides, but the ones made for commercial use have been shaped to fit the normal shape of a bar magnet.
      (1 vote)
  • stelly green style avatar for user Ankita V Hegde
    In the figure which appears at around , there is a field line which appears to be straight. Does that really happen and if yes, then a closed loop won't be formed?
    (1 vote)
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  • male robot hal style avatar for user Xalibur
    But what exactly are magnetic fields, I mean that we know that the stuff around us are made of atoms so we know what exactly are they, we exactly know what sound is, so upto that extent and in that sense, what exactly are fields?
    (1 vote)
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    • aqualine ultimate style avatar for user Satya
      Fields...or a Magnetic Field, particularly, is said to be the Space in which the Effect of The Magnet is Present or can be Felt. We Can Calculate How much a Magnetic Filed can Stretch By Conducting Experiments of How Strong the Magnet is, And How Far from the Magnet does it's Strenght Effect Objects. :)
      (2 votes)
  • blobby green style avatar for user Joseph Mathew Joy
    if 2 magnets are kept near each other and the magnetic field lines supposedly intersect and u keep a compass on that intersection, wouldn't the compass just point towards the stronger magnetic field instead?
    (1 vote)
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    • piceratops ultimate style avatar for user Nolan Ryzen Terrence
      Hey, @ Joseph Mathew Joy that is a really good question. But in the first place: Magnetic field lines never intersect at all.No matter how much you try. In the second place though dear friend, influence keeps the compass pointer deflected and not point in the direction of a stronger field.
      What do I mean by influence? A compass would feel slight deflection even when placed far away from the magnet.(Correct me if I am wrong). This happens with a single magnet. Therefore imagine the deflection variation when two of them are kept.

      So hence it won't point towards one direction rather it would be towards the middle of the two in a way wherein we can't predict/accurately say which direction is it pointing to.

      Let me know if this helps. Sorry for the long explanation.

      Nolan R.T :)
      (2 votes)
  • eggleston blue style avatar for user Harsh
    The magnetitic field line just between the magnet is a straight line, and we know a line are collection of points that extends infinitely in two directions .How can it form a close loop?
    (1 vote)
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  • blobby green style avatar for user diah
    Why is that in a circular loop, the direction goes from south to north?
    (1 vote)
    Default Khan Academy avatar avatar for user

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.