Hi. It's Mr. Andersen and this is AP Physics essentials video 34. It is on the magnetic
domains. In these pictures right here my daughter and I are both playing with ferro fluids,
which are tiny little liquid magnets that can respond to a magnetic field. And so in
this video what I am doing is I am taking a magnet, holding it underneath the ferro
fluid and you can start to kind of visualize these magnetic fields. I can then turn it
sideways and you can even see the north and the south pole. And so these tiny little ferro
fluids are made up of these 100 angstrom little magnetic domains which are tiny magnets within
the magnet itself. So if you have ferro magnetic material in the form of a magnet, what you
are going to get is a north pole and a south pole. Again you will never have just a north
pole by itself or a south pole by itself. But if we were to look inside that ferro magnetic
material we would find that there are these magnetic domains. What you can think of as
tiny little magnets and each of those tiny magnets have their own tiny little fields.
And so if you break a magnet in half, have you broken it? No. You have just created two
magnets. And you could keep breaking it in half, over and over and over until you get
to the level of these tiny magnetic domains. And if these line up, if they align, then
we have an overall magnetic, ferro magnetic material. And they can do this by putting
them inside an external field. Or sometimes it will just spontaneously or naturally occur.
And so if we have a bar magnet, it has a clear north and a south, we can make sure that it
is magnetized by holding a compass up on either side. And you will find that the south end
of the needle points to the north end of the magnet and vice versa down here. Now if we
break it in half, that bar magnet in half, what have we done? Well let's use our compasses
again. We put them on either side, what you will find is that we have now created a south
pole at this side, and then we have created a north pole up here. So we could do this
again and again and again and we just keep making smaller and smaller magnets. So there
must be magnets within the magnet thats are causing that magnetism. And so you can think
of it like this. In a block of iron we have all of these domains which are going to be
tiny magnets inside it. But if the whole thing is not magnetized they are going to be pointing
in every which direction. And they are going to be separated by these domain walls. And
so if you take something that is not magnetized and you magnetize it, what you are really
doing is you are lining up all of those magnetic domains. If they are all pointed in the same
direction then you have magnetized that overall object. And so a question that might jump
to mind is why do we even have magnetic domains? What is the purpose of having these tiny magnets
within magnets? Well it is an energy kind of question. So if we look at a magnet right
here, it is creating these giant magnetic fields around the outside of it. And so what
it is doing is using magneto static energy, is what scientists refer to. And so if we
can start breaking that down into smaller magnets what we are doing is we are reducing
the amount of that magnetic field and we are reducing the amount of energy. And so we get
a bunch of these domains we have actually eliminated that overall increase in energy.
It is at lower energy state. So let's say I want to go back again. Let's say I want
to go from an object that does not have a charge to one that does. Well this occurs
in nature. Lodestone, scientists and humans have known this for hundreds of years, are
naturally magnetized pieces of magnetite. And so how does that work? Well there is speculation
that is could be that they are being formed, since we mostly find them on the surface of
the earth near a lightning strike, but what it is doing is it is lining up these domains.
If you want to do this in the laboratory what you do is you take a chunk of some ferro magnetic
material, like iron for example. And we put it inside an electric field. So we are going
to put it inside a magnet. And you can see at this point that we have all of these domains,
again they are much smaller than this, but we have all of these domains and they are
pointing in every which direction. But if we can apply a magnetic field to it, watch
carefully, right here what happens if we let it sit there for awhile, is that we are going
to start to eliminate a lot of those domain walls. We are going to start to eliminate
a lot of those domains. And so overall this now is going to have a magnetic field. It
is going to be a magnet. It may last for a certain amount of time before eventually those
domains go back to where they were before. And so do you understand how these magnetic
domains lead qualitatively to the overall behavior of the magnet? I hope so and I hope
that was helpful.