Introduction to Magnetism

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.