Hi. It’s Mr. Andersen and this is AP Physics essentials video 70. It is on magnetic properties
All material in the universe has magnetic properties because all material in the universe
has electrons which are tiny little magnets. But it depends on what that material is as
to how it responds to magnetic fields. And so these are giant magnets down here on the
bottom. But on top of it we have carbon. And carbon is a diamagnetic. And so it is actually
opposed to these magnetic fields and it will hover over the surface of these magnets. Now
you are diamagnetic as well. And that means that if we were to put you in a magnetic field,
like they did with these frogs, you will actually be pushed away by the magnetic field line.
And you will actually levitate there. Now they have not done humans yet, but they have
done frogs and they have done mice. And it is a way that we can kind of study how we
respond to weightlessness. And so if you take objects and place them inside a magnetic field
they are going to respond in one of three ways. If you are ferromagnetic material and
we put you inside a magnetic field, the little electrons and the atoms and the domains will
start to align to the electromagnetic fields and it will make a magnet of its own with
a north and a south pole. And it will make a permanent magnet. That means if we pull
it out it is going to stay magnetized. And that is because we have aligned these domains
inside the object itself. Now if you take material that is paramagnetic and you put
it inside a magnetic field with really strong magnets on either side, it will actually line
up parallel to those magnetic field lines. And what we are doing is inducing these small
fields within the object itself. Now if you pull it out, even though it is parallel, if
you pull it out thermal motion is quickly going to scramble all those induced fields
inside. If you put paramagnetic material inside really strong magnets it will actually be
attracted to both the north and the south pole. Now if you take diamagnetic material
and put it inside a magnetic field it will actually work in opposition. It still has
these induced fields, but it will be in opposition to the magnetic fields. Again you take it
out, thermal motion is going to scramble all of those induced fields and it goes back to
being nonmagnetic. So why is it that permanent magnets do not do this? Well they do. If you
heat up a permanent magnet above what is called the curie temperature, all those magnetic
fields get scrambled and it is not a magnet anymore. And so this is a quantum property.
And it is based on electrons. Electrons have a spin. And if you have a number of unpaired
electrons they are not counteracted by the other spinning electron. And so it is a tiny
little magnet. And since atoms are made of lots of electrons, atoms are tiny magnets.
And they will align themselves into domains which are bigger magnets, up to about the
size of a millimeter. Then you can have the whole magnet be magnetized. And so even though
it is a large property it is actually a quantum property that is responsible for this magnetism.
And how do we measure that? Well we use something called magnetic permeability. And I have made
a video on that already. But essentially what you do is you take an electromagnet. So we
could take an electromagnet. This would attached to some kind of a power source on the other
side. And then you can put the object that you want to measure in the middle. And this
is what is called a toroid. And we can measure the magnetic field using a magnetic field
sensor. And so if we were to just place this toroid inside a vacuum we could measure the
magnetic permeability of free space. That is going to be the standard that we will compare
other objects to. And then we can simply put the material inside there and measure its
magnetic permeability as well. And so the relative permeability or mu sub r is going
to be a way to determine if this is ferromagnetic, paramagnetic or diamagnetic. And so this chart
should make sense to you. What we are doing is increasing the magnetic fields, so we would
be turning up the strength of that magnetic field. And then we are seeing how the dipoles
or the magnetic permeability is affected by the increases in that magnetic field strength.
And this would be the line for ferromagnetic material. As we increase the magnetic field
it quickly, all of these dipoles align, the domains align and we have a permanent magnet.
Now this right here, the red line, is going to be the magnetic permeability of free space.
So that is our standard. And so paramagnetic material are going to increase their dipole
influence as we increase the magnetic field. You can see it is a linear relationship, but
they are going to be slightly greater than free space. And diamagnetic material slightly
less than free space in their ability to be magnetized. And so ferromagnetic material
would be like iron, cobalt, nickel. If we take any of these materials and put them inside
a magnetic field, so we are going to put them inside a magnet like this, there are a lot
of domains or areas inside that ferromagnetic material, but they are all pointed in that
same direction. In other words all of these tiny magnets are lined up. But if we put them
inside a magnetic field like this, watch what happens to those domains. They grow in size.
And they all start to point in the same direction. So now what you have created is a permanent
magnet. If it is a paramagnetic material, so examples, magnesium, lithium, molybdenum,
if we put those inside a magnetic field it is also going to have these tiny little fields.
And they are going to start to align parallel to that material. And so if we have a really,
really strong magnet on either side, and right here they are dropping, this is liquid oxygen
through the middle, liquid oxygen in paramagnetic, and so it is actually being attracted to one
of these poles. If it were a little closer to this side it would be attracted to this
pole over here. Now once it falls through, if it does not get attracted to one of those
poles, now it goes back to being just regular material again. It does not keep that magnetic
field. If we are looking at a diamagnetic, so what is that going to be? That is you,
but it is also like, wood, it is water. It is pretty much everything you can think of
that does not have all these unpaired electrons, that is diamagnetic. So what happens if we
put that inside a magnetic field? It will actually work in opposition to those magnetic
fields. And so since this frog is diamagnetic, it is kind of being pushed away by these magnetic
fields. And that is a greater push than we have just from gravitational field strength.
So we get this levitation. And so did you learn to use representations to figure out
how different materials respond to magnetic fields? Again they could be ferromagnetic,
quickly responding and becoming permanent. Or they could be paramagnetic, so they are
going to align with it. Or diamagnetic, so they are going align without it. So hopefully
you learned that and I hope that was helpful.