Magnetic Domains
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Magnetic Domains


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.

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