If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Magnetism and magnetic fields

Forces at a distance are explained by fields (gravitational, electric, and magnetic) permeating space that can transfer energy through space. Magnets or electric currents cause magnetic fields; electric charges or changing magnetic fields cause electric fields. Created by Sal Khan.

Want to join the conversation?

  • scuttlebug green style avatar for user Tzviofen ✡
    At , why does this happen?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • spunky sam orange style avatar for user Elder Fauth
      There's not a reason yet, but here is my hypothesis: the universal law is that things must be as close to equilibrium as possible. water will fill empty space to be completely smooth, every force exerts an equal and opposite force, cold and warm air will balance each other out, air pressure will "even" itself out to be consistent with the rest of the air around them, etc.

      This, I hypothesize, also must be congruent with magnets.

      Let's say I broke the magnet and took one of the sides; the left one, or in the video, the "north." This "north" side becomes "out of place" in nature and needs to be "brought to order." This causes the magnet to "create" a new south pole via shifting the molecular construction of the magnet to compensate for the "error" that took place. This also happens with water: when you take water out of a bowl, water comes back and fills back up the space; the "hole" taken out of the water doesn't stay there. If it did, it would be more like taking cream cheese from the box, where the space that the cream cheese previously resided in becomes empty, the cheese not filling the space. However, breaking cream cheese is not breaking a universal law.

      Feel free to comment on my "hypothesis"

      Hope this helps!
      (3 votes)
  • eggleston blue style avatar for user dena escot
    @ what is meant by "it turns out it does interact with the types of things that tend to interact it will interact with magnets?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • leaf green style avatar for user Lionous
    You know how the video said that when magnetite is hit with lightning the particles align to form a "more coherent" magnetic field, does that imply that all objects have a weak magnetic field
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

- [Instructor] Let's talk a little bit about magnets and magnetic fields and right over here we have a picture of what today we would call a magnet where we have these metal nails all being attracted to the stone and the stone, the modern name for it is magnetite, and human beings have known about magnetite for thousands of years in fact the name comes from Ancient Greece, there was the Magnetes people who settled actually areas that are often talked about in the origin of the name magnet, which are now referred to as Magnesia, and depending on the historical count you see it was one of those Magnesia's where the Ancient Greeks were able to find a lot of what we would call magnetite, and actually the element magnesium which is not related to magnetite was also found in that area, which is where it got its name. But it wasn't just some interesting thing that would attract metal, and then they also obviously observed other properties that if you had two pieces of magnetized magnetite it actually turns out that not all magnetite is magnetized, something interesting has to happen to it we believe it's actually lightning strikes that magnetizes it naturally, that the orientation matters, if you're in one orientation they might attract each other. And then if you're in another orientation if you were to spin this one around they might actually repel each other. And this notion of the orientation that there might be some polarity made it more than just an interesting thing to observe, an Ancient Han China roughly 2000 years ago they invented the first compass, where they realized that if you took some magnetized magnetite which the historical name for it a lodestone, and you allow it to freely move and you could do that by either hanging it from a string or have it float on some still water say in bucket, that it will consistently orient itself, so that it points in the same direction. And so you can use that for things like navigation which the Chinese did roughly a 1000 years ago, and that helped us realize that the Earth itself is acting as a magnet, and just like a small magnet has different sides to it, the Earth does too and that's where the convention for a North and South Pole of a magnet came from. But there's probably a question in your mind from the first time that you noticed a magnet. If you have some piece of metal out here, that's not touching the magnet and in a future video we can even talk what touching even means at a microphone scopic level, but if you have a nail out here that's not touching it, but there's some force that's acting at a distance on that nail, how does that nail know to be pulled towards that magnet? It doesn't have eyes, it doesn't have ears, it doesn't say that there's a magnet there I better somehow move myself towards it. There's something about that region of space that is interacting with that nail. Or if you think about magnets, how does it know the orientation of the other magnet to either be attracted to it or repel it? How does it even know that other magnet is there? And that's where the concept of a magnetic field is useful. And this was introduced by Michael Faraday in the 19th century, as a way of at least thinking about giving us a framework for this force at a distance. It doesn't exactly tell us what it is, but it does gives us a way of predicting and thinking about what is happening. And one way to visualize a magnetic field, is to take a bar magnet or I could even say a bar lodestone, and put it underneath a piece of paper, and then putting metal filings on top of that piece of paper, and you will see something like this, in fact I encourage you to do this to observe this yourself, and you see what look like lines that are essentially connecting the North and South Poles like that. And this notion of field lines we can draw it a little bit clearer in something like this, was introduced by Michael Faraday and he says okay there's this thing called a field, that tells us for every point in space around the magnet, what it would do to something that is interactive with the magnet another magnet or maybe a piece of metal, so for example, if you were to put a small bar magnet right over here, the North side is going to be repelled from the North side of this bigger bar magnet, and the South side is going to be attracted to it, and so what you could do is you could put a freely moving, magnet or you could put a compass, to actually see what the orientation is which direction will accomplish point, and it will point in the direction of these field lines, and if you were to put it over here, it would orient this way if you allow it to freely move, where this is the Northern end, and this is the Southern end, and if you were to put it here and you were allow it to freely move let's say as part of a compass, and if this is the Northern end and this is the Southern end, it would orient like that. And so that helps you draw the field lines and also know the direction. And just by convention the direction is where coming from the North Pole into the South Pole. And what Faraday said is not only does this tell you the direction of the magnetic field, it tells you the magnitude based on the density of these field lines. So for example, the magnetic field is stronger here, where in that unit area that I just made you have very dense field lines, while it would be weaker right over here, I have fewer field lines. Now another amazing thing about magnets and their polarity that people have noticed since ancient times is, you would think at least initially, that if this has a North Pole and a South Pole, then maybe you could separate these two things, maybe if you were to break this in half, then you could have a pure North Pole magnet and you could have a pure South Pole magnet, but that's not what happens when you break this in half, instead you have two magnets each with their own North and South Poles, and then you can keep doing this, and early scientists just kept doing it and said there might not be any limit to how much you can cut this obviously, when you keep cutting and cannot cut any more while retaining the properties of the magnetite you're getting down to the molecular level, and in future videos we'll think about how even at a molecular level you can still have a little tiny magnet that has a polarity to it. Now another really interesting thing about magnets is this connection between magnetism and electricity. People have also observed that if you took a current carrying wire and the current is going from the positive to the negative end so it is going in this direction, and if you make it go through a piece of paper that has metal filings, it looks like field lines are forming here, magnetic field lines, it turns out it does interact with the types of things that tend to interact it will interact with magnets and so this is a magnetic field that is being formed by an electrical current, and so that was the major clue that the phenomena of electricity and magnetism are in fact related, and this relationship is what allows us to do things like have electric motors, or generate electricity from wind or water turbines, and it's out of the scope of this video but I'll give you a little bit of a clue of how these things are connected. We know that things they're made up of atoms which are made up of particles like electrons and protons, which are charged and we know that a current, an electric current is based on the movement of charged particles, that when something is not magnetic, say a non-magnetic piece of magnetite, all of those charges are moving more chaotically, but if something happens to it, maybe a lightning strike, it can align how they move so that they act in concert to have a more coherent magnetic field. So leave I'll there I'm already getting a little bit, ahead of our skis so to speak, but I do think it's really interesting to appreciate how all of these puzzle pieces that we see in nature, fit together.