12.2 Domain Theory

Okay, before we start looking at magnetic fields, directions, calculations, and all that, we have a little bit more theory to discuss. What we're going to talk about is a theory called the main theory, which most modern physicists believe in. I've also seen it on Big Bang Theory, where they mention trying to find a monopole, separating the north from the south pole and having only one of them. However, that is yet to be proved. Until then, we're going to stick with the domain theory, which has been around for quite some time. It explains a lot about how magnets work and how things change with magnets, if they ever do. If they find the monopole, and it turns out to be true, then this whole domain theory may be wrong. But for the time being, we're going to stick with it. So, we've already talked about how if we take a magnet and break it apart, we end up with two magnets, both having a north and south pole. And so on, we could keep going smaller and smaller. What we find is that we just keep getting smaller and smaller magnets. But if we take a chunk of magnet and look under a microscope, scientists say what they actually see are what they call domains. So, what that means is, imagine having a chunk of metal with a magnet on it. If you looked inside, you'd see these little strands of metallic fibers, almost like elements or little hairs. They're about one millimeter long, roughly.If we get a magnet, all these little domains, which act as little mini magnets inside, tend to work together. They all point the same way. For example, the North pole part of one domain points in one way. So, we'd get... You know, we'd get a magnet in that case. In this case, I guess if those were acting like magnets, this amoeba - so cool - this would be the North Pole of the magnet and so on. But the direction or what type of pole it is isn't really that important. We're just sort of worrying about what happens when they're lined up. So if you have a magnetic metal, basically what that means is all those domains are lined up and they're sort of working together. If it's a non-magnetic metal, what we would get is this diagram that's at the bottom of the page. All the domains are sort of scattered randomly and pointing against, against, and against each other, and canceling out, and so on. So the theory itself suggests that the reason these domains occur is because the electrons are spinning in orbit around the nucleus. So, you have a whole bunch of these electrons spinning - they're basically inducing a mini magnetic field. So a whole bunch of electrons working together, we get a magnetic field to create. And if you have a whole bunch of those, you get a little domain. And if you I have a whole bunch of the main ideas about alignment. We end up having a metallic object that turns into a magnet in ferromagnetic materials such as iron, cobalt, and so on. We can get those domains to align. Basically, all we have to do is either put the metal on top of an existing magnet. So, if you take a chunk of iron that's not magnetic and place it on top of a strong magnet, what will happen is those domains that are scrambled (if it's not magnetic) will be forced to line up. This will take these mixed-up domains and get them pointing in the same direction. Then, that will change the metal into a magnet. And we can actually see that this works! If you take a magnet and try to pick up paper clips, what we're basically doing is changing the domains in the paper clips to line up, turning them into little mini magnets temporarily. Then, you can sort of connect a few of them. Together, we need to understand how to determine whether a magnet is permanent or temporary. The key is to examine the behavior of the domains within the magnet. In the case of a permanent magnet, the domains are fixed in place, effectively creating a chain of magnetic moments. This state is known as "hard iron." To achieve this, the iron is initially in a liquid form and placed within a strong magnetic field. As the liquid iron solidifies, the domains become locked in place, hence the term "hard iron." On the other hand, if the domains are easily movable, such that they become randomized when introduced to a nearby magnet and then manipulated, we refer to it as a temporary magnet. Temporary magnet, or a magnet created using regular metal (referred to as soft iron), has its domains all scrambled up. This is commonly used in electromagnets, where software plays a significant role. For instance, the large magnets used to pick up scrap metal or cars rely on this concept. On its own, the magnet is just regular iron. However, when we flick a switch, it becomes a magnet strong enough to pick up a car. We can then move it to a desired location and turn it off, causing the car to drop. The ability to turn the magnets on and off is a valuable feature, which is why soft iron is frequently used in electromagnets. Lastly, it is worth noting that magnets tend to weaken over time, usually due to being dropped or banged around. So, if you drop a magnet on the ground, you may unintentionally observe this phenomenon. When it hits the ground, we're basically shaking it up, or you know, bumping the electrons and the atoms. So, if those electrons move around enough and speed up enough, what we can do is basically mess up the domains. So, if they're not pointing in the same direction anymore, it's no longer a magnet. Or, we could just take a magnet and bump them a little bit, so that some of the domains move. It'll just become a weaker magnet. One other thing that we can do to destroy a magnet is heat it up. If we get a magnet superhot, the heat, usually, most particles speed up when they get hot. So, the electrons start moving faster, they bump around, they change directions, the domains change, and it's no longer going to be a magnet. Once that metal gets really hot, and just two experiments, we know that when we hit a certain temperature, it is impossible to make it a magnet. For example, iron. If we heat it to about 770 degrees Celsius, you'll never be able to make it a magnet. So, they call that the Curie temperature, which was... Named after Madame Curie, who is a fairly famous French physicist, that's all we're gonna get into. Some calculations and figuring out directions will be covered in our next lesson.