We've learned a little bit about gravity.
We've learned a little bit about electrostatic.
So, time to learn about a new fundamental
force of the universe.
And this one is probably second most familiar to us,
next to gravity.
And that's magnetism.
Where does the word come from?
Well, I think several civilizations-- I'm no
historian-- found these lodestones, these objects that
would attract other objects like it, other magnets.
Or would even attract metallic objects like iron.
Ferrous objects.
And they're called lodestones.
That's, I guess, the Western term for it.
And the reason why they're called magnets is because
they're named after lodestones that were found near the Greek
province of Magnesia.
And I actually think the people who lived there were
called Magnetes.
But anyway, you could Wikipedia that and learn more
about it than I know.
But anyway let's focus on what magnetism is.
And I think most of us have at least a working knowledge of
what it is; we've all played with magnets and we've dealt
with compasses.
But I'll tell you this right now, what it really is, is
pretty deep.
And I think it's fairly-- I don't think anyone has-- we
can mathematically understand it and manipulate it and see
how it relates to electricity.
We actually will show you the electrostatic force and the
magnetic force are actually the same thing, just viewed
from different frames of reference.
I know that all of that sounds very
complicated and all of that.
But in our classical Newtonian world we treat them as two
different forces.
But what I'm saying is although we're kind of used to
a magnet just like we're used to gravity, just like gravity
is also fairly mysterious when you really think about what it
is, so is magnetism.
So with that said, let's at least try to get some working
knowledge of how we can deal with magnetism.
So we're all familiar with a magnet.
I didn't want it to be yellow.
I could make the boundary yellow.
No, I didn't want it to be like that either.
So if this is a magnet, we know that a magnet
always has two poles.
It has a north pole and a south pole.
And these were just labeled by convention.
Because when people first discovered these lodestones,
or they took a lodestone and they magnetized a needle with
that lodestone, and then that needle they put on a cork in a
bucket of water, and that needle would point to the
Earth's north pole.
They said, oh, well the side of the needle that is pointing
to the Earth's north, let's call that the north pole.
And the point of the needle that's pointing to the south
pole-- sorry, the point of the needle that's pointing to the
Earth's geographic south, we'll call
that the south pole.
Or another way to put it, if we have a magnet, the
direction of the magnet or the side of the magnet that
orients itself-- if it's allowed to orient freely
without friction-- towards our geographic north, we call that
the north pole.
And the other side is the south pole.
And this is actually a little bit-- obviously we call the
top of the Earth the north pole.
You know, this is the north pole.
And we call this the south pole.
And there's another notion of magnetic north.
And that's where-- I guess, you could kind of say-- that
is where a compass, the north point of a
compass, will point to.
And actually, magnetic north moves around because we have
all of this moving fluid inside of the earth.
And a bunch of other interactions.
It's a very complex interaction.
But magnetic north is actually roughly in northern Canada.
So magnetic north might be here.
So that might be magnetic north.
And magnetic south, I don't know exactly where that is.
But it can kind of move around a little bit.
It's not in the same place.
So it's a little bit off the axis of the geographic north
pole and the south pole.
And this is another slightly confusing thing.
Magnetic north is the geographic location, where the
north pole of a magnet will point to.
But that would actually be the south pole, if you viewed the
Earth as a magnet.
So if the Earth was a big magnet, you would actually
view that as a south pole of the magnet.
And the geographic south pole is the
north pole of the magnet.
You could read more about that on Wikipedia, I know it's a
little bit confusing.
But in general, when most people refer to magnetic
north, or the north pole, they're talking about the
geographic north area.
And the south pole is the geographic south area.
But the reason why I make this distinction is because we know
when we deal with magnets, just like electricity, or
electrostatics-- but I'll show a key difference very
shortly-- is that opposite poles attract.
So if this side of my magnet is attracted to Earth's north
pole then Earth's north pole-- or Earth's magnetic north--
actually must be the south pole of that magnet.
And vice versa.
The south pole of my magnet here is going to be attracted
to Earth's magnetic south.
Which is actually the north pole of the
magnet we call Earth.
Anyway, I'll take Earth out of the equation because it gets a
little bit confusing.
And we'll just stick to bars because that tends to be a
little bit more consistent.
Let me erase this.
There you go.
I'll erase my Magnesia.
I wonder if the element magnesium was first discovered
in Magnesia, as well.
Probably.
And I actually looked up Milk of
Magnesia, which is a laxative.
And it was not discovered in Magnesia, but it has
magnesium in it.
So I guess its roots could be in Magnesia if magnesium was
discovered in Magnesia.
Anyway, enough about Magnesia.
Back to the magnets.
So if this is a magnet, and let me draw another magnet.
Actually, let me erase all of this.
All right.
So let me draw two more magnets.
We know from experimentation when we were all kids, this is
the north pole, this is the south pole.
That the north pole is going to be attracted to the south
pole of another magnet.
And that if I were to flip this magnet around, it would
actually repel north-- two north facing magnets would
repel each other.
And so we have this notion, just like we had in
electrostatics, that a magnet generates a field.
It generates these vectors around it, that if you put
something in that field that can be affected by it, it'll
be some net force acting on it.
So actually, before I go into magnetic field, I actually
want to make one huge distinction between magnetism
and electrostatics.
Magnetism always comes in the form of a dipole.
What does a dipole mean?
It means that we have two poles.
A north and a south.
In electrostatics, you do have two charges.
You have a positive charge and a negative charge.
So you do have two charges.
But they could be by themselves.
You could just have a proton.
You don't have to have an electron there
right next to it.
You could just have a proton and it would create a positive
electrostatic field.
And our field lines are what a positive point
charge would do.
And it would be repelled.
So you don't always have to have a negative charge there.
Similarly you could just have an electron.
And you don't have to have a proton there.
So you could have monopoles.
These are called monopoles, when you just have one charge
when you're talking about electrostatics.
But with magnetism you always have a dipole.
If I were to take this magnet, this one right here, and if I
were to cut it in half, somehow miraculously each of
those halves of that magnet will turn
into two more magnets.
Where this will be the south, this'll be the north, this'll
be the south, this will be the north.
And actually, theoretically, I've read-- my own abilities
don't go this far-- there could be such a thing as a
magnetic monopole, although it has not been
observed yet in nature.
So everything we've seen in nature has been a dipole.
So you could just keep cutting this up, all the way down to
if it's just one electron left.
And it actually turns out that even one electron is still a
magnetic dipole.
It still is generating, it still has a north pole and a
south pole.
And actually it turns out, all magnets, the magnetic field is
actually generated by the electrons within it.
By the spin of electrons and that-- you know, when we talk
about electron spin we imagine some little
ball of charge spinning.
But electrons are-- you know, it's hard to--
they do have mass.
But it starts to get fuzzy whether they
are energy or mass.
And then how does a ball of energy spin?
Et cetera, et cetera.
So it gets very almost metaphysical.
So I don't want to go too far into it.
And frankly, I don't think you really can get an intuition.
It is almost-- it is a realm that we don't
normally operate in.
But even these large magnets you deal with, the magnetic
field is generated by the electron spins inside of it
and by the actual magnetic fields generated by the
electron motion around the protons.
Well, I hope I'm not overwhelming you.
And you might say, well, how come sometimes a metal bar can
be magnetized and sometimes it won't be?
Well, when all of the electrons are doing random
different things in a metal bar, then it's not magnetized.
Because the magnetic spins, or the magnetism created by the
electrons are all canceling each other out,
because it's random.
But if you align the spins of the electrons, and if you
align their rotations, then you will have a magnetically
charged bar.
But anyway, I'm past the ten-minute mark, but hopefully
that gives you a little bit of a working knowledge of
what a magnet is.
And in the next video, I will show what the effect is.
Well, one, I'll explain how we think about a magnetic field.
And then what the effect of a magnetic
field is on an electron.
Or not an electron, on a moving charge.
See you in the next video.