Magnetic force | Middle school physics | Khan Academy

- [Narrator] Gravity
is a fundamental force.

But most of the other forces

that we deal in our everyday
life, like tension, friction,

normal forces, et cetera,

they're part of a fundamental interaction.

A fundamental force called
the electromagnetic forces.

And from the name itself,

we can see there are two parts to it.

We have the electric part
and the magnetic part.

We combine them together
as a single force today,

but we're gonna study them separately.

In this video, let's talk
about the magnetic force.

Now, you played with magnets.

So you probably know that magnets

can put magnetic forces, right?

And you might know that
magnets have two poles.

You have the north pole
and the south pole.

This is very similar to how
you can have a positive charge

and a negative charge, but
there is a big difference,

you know, the big difference is positive

and negative charges can be separated.

But can you separate a
north and a south pole?

Well, at first it seems like,
yeah, just cut a magnet.

You get a north and south pole, right?

No, you don't.

If you cut a magnet, you
know what you'll get?

You'll actually get two tiny
magnets each having its own

north and south poles.

And if you try to create
again, well again,

you'll get tiny magnets each
having north and south poles.

You can never, ever separate
the south and the north pole

of a magnet, they always come together.

Okay, that's pretty interesting, right?

But let's talk about some other
features of magnetic forces.

While playing with magnets,
you may have noticed

that you don't need them to be in contact

to exert a magnetic force,

which means magnetic forces
are non-contact forces.

And again, that brings up the question,

how do these forces work?

Well, just like in the case
of gravity and electricity,

they work via fields.

So in our model, the
bar magnet, for example

will produce a magnetic field, okay?

You can actually see
these magnetic fields.

If you pour tiny iron filings,

they will orient themselves
along the field lines.

And you can actually visualize the field.

You can see the field lines curve.

That's beautiful, isn't it?

So we can now draw these field lines,

and this is kind of what it'll look like.

The field lines usually, I mean,

our convention is they
start from the north

and they end into the south.

But they don't just end
there inside the magnet,

they actually loop back.

I mean, this drawing is
not all that great, okay?

But the whole idea is
these lines will loop back.

Again, this is very
different than what we find

in electric fields or gravitational
fields for that matter.

They don't loop over there.

Here, they loop.

Magnetic field lines will always loop.

Even these ones will go and loop back.

They will always loop.

And if you have another magnet close by,

then that magnetic field can
put a force on that magnet

and they can turn it.

And that's how if you
put a lot of compasses,

which are tiny magnets,
they can turn according

to the direction of the magnetic field.

Okay, so from this,

can we think about the
direction of the magnetic force?

Yes.

Again, you probably know that unlike poles

attract each other,

if you bring a north pole close

to the south pole, it'll attract it.

On the other hand, if you bring
like poles close together,

they'll repel.

Unlike poles attract like
poles repel each other.

So that's about the direction
of the magnetic force.

But what about the strength
of the magnetic force?

Well, if you have strong magnets,

they'll have stronger magnetic fields,

and they will have stronger
magnetic forces, right?

So stronger magnets, well
will exert more force.

But what else decides this, you know,

how strong the force is going to be?

The magnetic force is going to be?

Well, distance.

If you have two magnets
very close to each other

like we have over here,
north and south pole almost,

you know, is touching each other,

the magnetic force will be very strong.

In fact, sometimes it'll be
hard to even pick it apart.

You may have noticed this.

But what happens if you
bring them farther apart?

The magnetic force will
get weaker with distance.

So if you, you know, separate them

and bring them farther apart,

now you can hardly feel the attraction

because the magnetic force
gets weaker with distance.

And you can see that again with like poles

as well like poles repel each other.

You can see that a lot when they're close.

But when you bring them farther apart,

well you can hardly
feel that force at all.

So, magnetic forces get
weaker with distance.

This is very similar to
what we find in electricity

or for that matter in gravity.

Now here's the last question.

These are all permanent magnets.

They produce magnetic fields
and they exert magnetic forces.

What about materials that
are not permanent magnets?

Can they produce magnetic fields
and exert magnetic forces?

Looks like snow, right?

But they can by passing an electricity.

For example, copper wires,
well, they're not magnetic,

they're not permanent magnets at all.

However, if you pass
a current through them

and you coil them up like this,
they will act like a magnet.

They will generate a magnetic field.

We call such magnets electromagnets,

because they're magnetic
due to electricity,

which is pretty cool.

This is very different from,
you know, permanent magnets,

they're not permanent magnets,

they're in fact temporary magnets.

They're magnetic as long as
electricity passes through them.

And you can look at their magnetic field.

And it's very similar

to what you get in a bar magnet, right?

Very similar to that.

One side acts like a north pole,

another side acts like a south pole.

So these are temporary magnets.

And the beautiful thing about
them is you can, you know,

alter their magnetic properties.

You can turn off the electricity boom

and then, you know, the
magnetic fields turn off,

they don't behave like magnets anymore.

You turn on the electricity,
they behave like magnets.

And guess what?

Their magnetic fields depend
on how strong the current is.

So if you put a stronger current,

you get a stronger magnetic field,

you get a stronger electromagnet.

That's what makes
electromagnets so awesome.

You can change their properties,
unlike permanent magnets,

whose properties are
pretty much fixed, right?

So this also means that
if you get a giant coil

and you can put a lot
of current through it,

oh, you can create some
very strong magnets.

That's basically how
your junkyard magnets,

for example, work.

They're so strong that, you know,

there's a current, they're electromagnets.

We use currents, strong
currents generate very strong

magnetic fields, so strong
that they can lift stuff up.

You should never be anywhere close

to such magnets can be very dangerous

if you're wearing something
that is magnetic, you know?

And another example of
that is in your MRI scans,

MRI machines require very
strong magnetic fields

and we generate them via
not permanent magnets.

Nope.

We use electromagnets giant coils

with electricity running through them.

So, long story short, magnetic force

is a part of the fundamental
electromagnetic interaction.

It's a non-contact force.

And what about its direction
will unlike poles attract

and like poles repel?

And what about its strength?

Well, stronger permanent
magnets will exert more force.

But on the other hand, when
it comes to electromagnets,

more currents, if you have more currents,

then they can exert more force.

And on the other hand,
when it comes to distances,

well the magnetic force
gets weaker with distance.