Hi. It's Mr. Andersen and this is AP Physics essentials video 20. It is on magnetic permeability
which is the ability of matter to create internal magnetic fields. And we will get to that in
a second. But first let's address what a magnetic field is. So imagine I take a bar magnet that
has a north and a south pole and I hold another one right next to it and let it go. You know
what is going to happen . The north will quickly be attracted to the south of the other pole.
And that is why magnet are held together. But why is that? They are not touching but
there is something between them that is affecting their behavior. And so to study that we will
use a compass. And so I have the same bar magnet again. But I have a compass right down
below it. What is a compass? It is essentially a small magnet that is floating on a needle.
And you can see, same thing, that the north side of this compass is attracted to the south
side of the bar magnet. Let's move the compass a little bit. It is still pointing at the
south. Now let's move it over here. It is parallel to the magnet. And then we can see
now as we go to the other side the south is now facing the north. And so there is something
out there that is invisible that is affecting the magnet inside the compass. And so to visualize
that, lots of times we will use iron filings So you put a little bit of paper on top of
the magnet. Put some iron filings on there and then those fields are going to show up.
And now the property of the compass starts to make sense. And so it is lining up along
this magnetic field. And so as we move the compass over here it is again parallel to
those fields. Same thing over here. And same thing over here. So there are these magnetic
fields that emanate out from this magnet. And so if we take a compass and put it inside
an magnetic field it is just going to line up parallel. And so what is magnetic permeability?
Well if we take material and put it in that magnetic field its ability to create internal
fields is its magnetic permeability. And so if we put free space there, what is free space?
It is simply a vacuum. We can measure its magnetic permeability. And that is going to
be its internal ability to form magnetic fields. And it is going to remain constant over time.
So we can compare other material to the free space magnetic permeability and it tells us
something about their ability to be magnetized. And so now let's put some material in this
magnetic field. You can see that it is not changing the magnetic field at all. And so
we would say this has low magnetic permeability. If we were to compare that to free space it
looks essentially the same. And so if we were to put a ratio between those two it would
be a ratio of 1. Now let's put some material in the middle, like iron, that has high magnetic
permeability. And watch what happens to the magnetic field. It is warping those magnetic
fields. And so it is really creating these internal magnetic fields. And so if you have
high magnetic permeability, you are more likely to be a magnetic kind of material. So how
could you even measure such a thing? Well luckily we have a electromagnetism. In other
words magnetism is like electricity and vice versa. And so you can use an object like this.
It is essentially a donut wrapped in a wire. We call it a toroid. And then we can start
to measure the magnetic fields. And so as we wrap that wire around the toroid we can
add a current to it. And then we can just hold the magnetic sensor a little bit back
and we can measure the magnetic field that is being created. And so if you wanted to
measure the magnetic permeability of free space, how would you do that? You simply would
not have any thing inside that donut. It would just be wire wrapped around free space. And
so we can measure its free space magnetic permeability. And that iss mu-naught. And
so that is going to be the ability of a vacuum to create these internal magnetic fields.
Now if I wanted to measure the magnetic permeability of a material I could just put something where
the donut is. And so let's make a donut out of iron for example or a donut out of aluminum
or wood or whatever we want to put in the middle. Now we could measure its mu, which
is the magnetic permeability of that object or of that material. And so it is really important
since free space magnetic permeability is a constant that we can compare it to that.
So lots of times what you will see is the relative magnetic permeability. And all that
is is simply a ratio between the material's magnetic permeability and that of free space.
That is how lots of times it will be listed. And so if we were to look at some relative
magnetic permeabilities, what we are looking at here is the ratio of mu to mu-naught. If
we look at wood its value is going to be 1. Well, not exactly 1, but pretty darn close
to 1. And so that means it has a really low ability to form these internal magnetic fields.
If we look at aluminum it is also 1. So what do you know about aluminum? A magnet is not
going to stick to aluminum and that is because it can not induce these magnetic fields. But
if we were to look at elements on the periodic table that are magnetic, they are going to
have a much greater value when we are looking at the relative permeability. And so 250 for
cobalt. 600 for nickel and 200,000 for iron. So why is iron such a good magnet? It is because
its structure, its molecular structure on the inside is easily able to induce these
magnetic fields. And so did you learn the magnetic permeability measures the ability
of material to support the formation of these internal magnetic fields? And it also shows
us how magnetic they are going to be? I hope so. And I hope that was helpful.