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This is the Aurora Borealis.
Or you might know them as the northern lights.
The same mysterious force that creates these awesome displays
also makes possible the work that many of our machines do.
Any guesses what they are?
In this segment of "Physics in Motion",
we'll find out the story behind that force.
Magnetism.
The spectacle of the northern lights is caused
by the Earth's magnetic field
interacting with the solar wind,
or what we call the magnetosphere.
The wind is actually high energy particles
emitted by the sun.
And when the sun emits a huge number of these particles
and they hit the magnetosphere,
you get this.
Can you imagine what people thought
when they first saw these natural special effects?
They couldn't have known it was magnetism
that made these spectacles possible.
People first started to see magnetism as a useful force
more than 2,000 years ago.
The Chinese figured out how to use magnetic material
to make compasses
because the metal always pointed north or south.
That discovery spread to Europe,
paving the way for global exploration,
which led to Columbus reaching America.
But no one knew why the magnetized pointers
behaved the way they did.
In 1600, a little more than a century after Columbus,
an Englishman named William Gilbert
figured that out.
It's because the Earth itself is a magnet.
That's because there is a lot of iron in the Earth,
and iron is naturally very magnetic.
So the Earth generates its own magnetic field,
which looks like this.
See how the fields are concentrated
at the north and south poles?
Now, here's the tricky thing to remember.
The magnetic south pole is actually near
the geographic north pole.
And the magnetic north pole is near
the geographic south pole.
Scientists realized that opposite magnetic poles attract
and like poles repel.
Just like electric charges.
They decided this was a more accurate way
to talk about magnetic poles.
So when a compass needle spins,
the north end is drawn
to align with Earth's magnetic field.
By the 17th century,
people had observed that electricity
could create magnetism, and vice versa.
Magnets can generate electricity.
But then, about 200 years ago,
a guy named James Clerk Maxwell nailed it,
when he realized that electricity and magnetism
are two aspects of the same force.
Electromagnetism.
The interaction of electric currents or fields
with magnetic fields.
It's an alternating kind of effect.
Electricity creates a magnetic field,
which can generate electricity, and so on.
You can see how closely they are related
in this animation of a ray of light.
Electricity and magnetism both arise from
the interaction of electrically charged particles.
We've talked a lot about what electricity is
in earlier segments in this unit.
But what exactly is magnetism?
We can define it as an effect
produced by the motion of electric charges,
resulting in attractive and repulsive forces
between objects.
It's different from electricity in a couple of ways.
One is that a magnetic field occurs
only when charges are moving,
but electric fields are created
whether the particles are moving or not.
Second, magnetic poles are always produced in pairs,
whereas electric charges are not.
Now, one cool thing about a magnetic force
is that it can act at a distance.
Let me do a little magic trick.
You see how it looks like it's levitating?
Well, in fact, it is levitating.
Courtesy of the magnetic field.
How does it work?
This end of the magnet is north.
Usually called the magnet's north pole.
And this one is north too.
So, what do you think happens?
The like poles repel each other,
keeping the magnets apart.
They don't have to touch each other to affect each other.
The force is between them.
Kinda like magic, isn't it?
But it's science.
All magnets are miniature versions of the Earth.
Like our planet, they have two poles,
they create a magnetic field,
and they are affected by other magnets.
At the subatomic level, the electrons spin like tops,
as well as spinning around the nucleus,
creating magnetic fields.
Within any piece of magnetic material are domains.
Areas where the individual magnetic orientations of atoms
are aligned with one another.
So they point in the same direction.
Each of the domains also has a north and south pole.
When the domains line up, that creates a magnet.
So, why isn't everything magnetic?
Most electrons in an atom exist in pairs
that spin in opposite directions.
So the magnetic effect of one electron in a pair
cancels out the effect of its partner.
But if an atom has some unpaired electrons,
iron atoms have four,
these produce magnetic fields that line up with one another
and turn the whole atom into a mini magnet.
In other words, when you put a material like iron for instance
in a magnetic field,
the electrons change their spin
and align with the magnetic field.
Now, magnets can affect other materials
in another way.
They can make a temporary magnet
out of certain kinds of unmagnetized materials.
Let me show you.
One magnet, check. See?
It attracts these iron filings.
One nail.
But it's not a magnet. No attraction.
See?
Now I slide the nail with the magnet
in the same direction,
moving it away from the nail at the end of each pass.
Each time I do this,
more electrons in the nail
align with the magnetic field.
Now, I've been aligning our domains
for about 15 seconds.
Let's see if we've got a magnet.
And we do.
And let me show you something else.
When I sprinkle iron filings on top of a bar magnet,
Look, you can see magnetic field lines.
Like any force, magnetic force has a direction
such as up or down.
If we wanna know the direction of the magnetic force,
we can figure that out using what we call
the "right hand rule".
We know that the magnetic force
acts only on charged particles,
so let's see how the right hand rule works
for a positive charge first.
With my right hand, I make a thumbs up sign.
My thumb represents the direction
of the magnetic force acting on a positive charge.
Now, I point my forefinger at a 90 degree angle
with my thumb.
My pointer finger represents the velocity vector
of the positive charge,
including the direction it is moving in.
Then, I take my middle finger
and I point it at a 90 degree angle
with my pointer finger, like this.
My middle finger represents the direction of the magnetic field
that the charge is moving through.
If I have a negative charge,
do the right hand rule the same way,
but know that the negative charge
is in the opposite direction of the positive charge.
If the positive charge is moving to the right,
the negative charge would be moving to the left.
There are three things that can cause a magnet
to lose its magnetism.
One is heating the magnet.
The heat makes the electrons move faster and faster.
And as they get jumbled up, the magnetic force dissipates.
Another is putting the magnet in a de-magnetizing field.
One that cancels out the magnet's strength.
If you've ever had a magnetic strip on a card
in your wallet stop working,
this may well be the cause.
Finally, dropping a magnet will disrupt the alignment
of its individual magnetic orientations
and the magnetic force.
So, we've learned what magnetism is
and how it behaves.
How charge and motion creates a magnetic field.
And remember, that a charged particle
has to be moving through a magnetic field
in order to experience a magnetic force.
No movement, no force.
Now, let's look at how to calculate magnetic force.
Let's say we have a magnetic field
and there's an electron moving around in the field.
The electron is moving fast
at a velocity of 2 x 10 to the 5th meters per second upwards,
in a magnetic field strength 4 teslas.
Pointing from right to left.
A tesla measures the strength of a magnetic field.
What will happen to that electron?
Will it feel a force?
And if it does, how much?
The magnitude of the magnetic force
that a moving charge feels
depends on three things.
The charge, q, the charge's velocity, v,
and the magnetic field's strength, b.
Once we know these three quantities,
we can solve for magnetic force
by multiplying them all together.
In this case, we've been told
how fast the charge is moving,
and the strength of the field it is in.
And remember that we know
what the charge of an electron is.
Negative 1.6 x 10 to the negative 19 coulombs.
Bringing it all together
to solve for magnetic force magnitude
on the electron,
we have magnetic force on a charge equals
the charge magnitude times its velocity,
times the magnetic field in which the charge is moving.
Plugging in all of our numbers,
we find that this magnetic field
exerts a force of 1.3 x 10 to the negative 13 newtons.
Now, which way does the magnetic force point?
We know that our charge is moving upward
and that it's negative.
Let's use the right hand rule to figure out
which way the magnetic force is pointing.
Our charge velocity vector points up,
and b, the magnetic field, points to the left.
So the magnetic force pushes the force towards me.
But remember, we have a negative charge.
So the magnetic force will point
in the opposite direction.
That is away from me.
At the beginning of this segment,
I said that magnetism is part of what makes
many of our machines work.
But it's not just giant magnets.
It's in the motors and generators that run
almost anything you can think of.
That's it for this segment of "Physics in Motion".
We'll see you next time.
(announcer) For more practice problems,
lab activities, and note-taking guides,
check out the "Physics in Motion" toolkit.