Video transcription

Hi, I'm Steve Jones and I'm going to explain the domain theory of magnetism. Now this is a very important thing because ferromagnetics are very different in character from other materials even though all materials exhibit some form of magnetism. Within a ferromagnetic, within for example iron, steel, so on, it is seen that they're in fact are what they call domains. These are large areas in which the magnetism of the individual atoms and molecules are all aligned in the same direction. So overall, the material is not magnetic because there are as many pointing in one direction as the opposite direction. So on average, there is no overall magnetic field. But as soon as a magnetic field is applied to the ferromagnetic, what happens is at first when it's a small field, well the domains in that direction start pulling atoms from the other domains in to that direction and then eventually when the field is large enough, these domains start snapping into the one direction of the field. So for example this one will at some point or other suddenly orientated totally the opposite way in line with the magnetic field that is being applied to it. And this effect when you take away the field, this does not necessarily disappear. In some materials like steel it actually remains like that. In other materials like soft iron, it grudgingly reverts or it suddenly reverts, it depends on the material itself. But the huge advantage of this is that magnetism is affected by whole bunches of atoms which we call domains, and the whole domain actually switched into that direction. So the effect is very large compared to other materials where just atom by atom they'll snap into a certain direction and because of the temperature causing the atoms to move about, they tend to demagnetize as quickly as they magnetize, in other words they don't magnetize at all. So that in a very simple form is the theory of domains and how a ferromagnetic material becomes magnetic.