like all materials in the universe can be broken down

into a category of insulator, electrical insulator,

or electrical conductor.

That's not completely true.

There are semi-conductors and super conductors

and other exotic forms of electrical materials

but for most introductory physics classes

and problems and tests, you can get pretty far

assuming that it's either an insulating material

electrically or a conducting material electrically.

Before I talk about the differences between these,

here I have two solid cylinders

of either an insulating material or a conducting material.

Before I talk about the differences, one similarity

is that both insulators and conductors are composed of

a huge number of atoms and molecules

and these atoms and molecules,

whether it be insulator or conductor,

are composed of a positively charged nucleus

and a negatively charged swarm of electrons

that surround that nucleus.

Another similarity is that for both conductors

and insulators, the positively charge nucleus cannot move.

I mean it can wiggle around and jiggle

just from thermal vibrations, maybe a little bit in place,

but it can't travel freely throughout the material

for either an insulator or a conductor

as long as it's a solid.

If it was a fluid, I suppose these things can move

and migrate around, but for a solid

the positively charge nucleus is fixed.

They're stuck.

The thing that might be able to move

are the negatively charged electrons,

and here's the difference.

There are electrons in a conductor

that can move about relatively freely.

These can move around with almost no resistance,

whereas for insulators a key difference

is that these electrons cannot move around freely.

These don't have the right energy levels and bands

in order to make these electrons move around freely.

They are also stuck.

For insulators, everything is basically stuck,

These electrons might be able

to jump around in their own atoms

or get shared in a neighboring atom,

but it can't jump around freely from atom to atom

and travel throughout the insulator.

For the conductors, the electrons can do this.

that's the key difference.

Now the electrons aren't just going to do this on their own,

they have to be compelled to start moving

by hooking this up to a battery

or setting up some sort of electric field or force.

If that did happen, the electrons in a conductor

start migrating down the line

but in an insulator, the electrons are stuck

which might make you think that

"Well, okay, shoot, for electrical materials

"all we really care about are the conductors.

"The insulators we will just use if we don't want

"electrical interaction."

While that is somewhat true, it is not completely true

because if I set this insulator up to a battery

or set up some sort of electric field or force in here

even though the electrons in an insulator

can't jump from atom to atom,

what it can do is it can shift.

This nucleus and the cloud of electrons

can kind of shift a little bit.

Positive may be this way,

and the the negatives over on the other end

so what you get is overall this side of the atom

would be more negative,

and this side of the atom would be more positive.

Even though the electron doesn't move,

and the electrons don't move,

now because this is set up where the positive

is shifted from the negative,

this material, if you get all of them to do this

or a lot of them, this can create

an overall electrical effect where this insulator

can interact with other charges nearby

and exert forces on them.

Even though the charges can't flow through an insulator,

they can still interact electrically.

Now, let's see what happens if we add extra charge

to these insulators or conductors.

I mean, the way they started off right here

we had just as many positives in the nucleus

as there are negatives surrounding them

and that's true for the conductors and insulators.

What happens if we add extra charge?

Maybe we add extra negatives into here.

Then what happens?

Well, it'll get really messy if we try to draw it

with all the atoms, so since these all cancel out

their overall charge, I am not going to draw

every atom and nucleus.

I'm just going to pretend like those are there

and they are all canceling out.

I'm just going to draw the actual extra charge.

Let's say we added extra negative charges

to this insulator.

What would happen?

Let's say I just add a negative charge here

and a negative charge there,

and here and there, I have added a bunch

of negative charges to this insulator.

What would happen?

Well, we know these negatives

can't move throughout and insulator.

Charges can't flow through an insulator so they're stuck

which means for an insulator, I could charge

the whole thing uniformly if I wanted to

where the charge is spread out throughout the whole thing

or I could make them bunch up on one side if I wanted to

and they'd be stuck there.

The point is that they're stuck.

For a conductor, what would happen if I tried

to put a negative here and a negative there,

some extra negative charge on a conductor?

They don't have to stay here if they don't want to.

If you put extra negatives in here,

they are not going to want to

because negatives repel each other

just like opposites attract, like charges repel.

So what are they going to do?

Well, this negative is going to try to get as far away

from this other negative as it can so go over here.

This negative is going to try to get as far away as it can.

It repels it.

Now, it can't jump off the conductor.

That takes a lot more energy,

but it can go to the very edge.

That's what charges do for conductors.

You've got a solid conducting material,

you put extra charge on it, it's all...

All that charge is going to reside on the outside edge

whether you've added extra negative or positive,

always on the outside edge.

You can only add charge to the outside edge

for a conductor, because if it wasn't on the outside edge

it will quickly find its way to the outside edge

because all these negatives repel each other.

I said this is true for positives or negative.

You might wonder, "How do we add a positive?"

Well, the way you add a positive

is by taking away a negative.

If you started off with a material that had

just as many positives as negatives

and you took away a negative,

it's essentially like adding a positive charge in here.

But again, the net positive charge, the net negative charge

always resides on the outside edge of the conductor

because charges try to get

as far away from each other as possible.

So what physical materials actually do this?

What physical materials are insulators?

These are things like glass is an insulator.

Wood is an insulator.

Most plastics are insulators.

All of these display this kind of behavior

where you can distribute charge and the charge

can't flow through it.

You can stick charge on it.

In fact, you can stick charge on the outside edge

and it will stay there.

There's conductors.

These are things like metals, like gold

or copper is typically used because it's kind of cheap.

Cheaper than gold, certainly.

Or any other metal.

Silver works very well.

These are materials where charges

can flow freely through them.

Now that we see how conductors and insulators work,

let's look at an example.

Let's say you have two conducting rods.

Say these are made out of metal.

One of them has a net amount of negative charge on it

which is going to reside on the outside edge

because that's what net charge does on a conductor,

but this other rod, this other metal conducting rod,

does not have any net charge on it.

What would happen if I took this first rod

touched it to the second rod?

You probably guessed, charges want to get

as far away from each other as possible

so these negatives realize "Hey, if we spread out,

"some of us go on to this rod and some of us stay here,

"we can spread out even father away from each other."

That's what they would do.

If these rods were the same size,

you'd have equal amounts on each.

If the second rod was bigger,

more of them would go on to this second one

because that would allow them to spread out even more.

Some would stay on the smaller one.

That's charged by just touching something.

That's easy.

You can charge something also, you can get clever.

You can do something called

charge, you can charge something by induction it's called.

What does this mean?

Charge by induction says alright, first

imagine I just take this and I bring it nearby

but don't touch it.

Just bring it near by this other piece of metal

and I don't touch it.

What would happen?

There is negatives in here, I haven't drawn them.

There's positives in here.

The negatives can move if they wanted to.

Do they want to?

Yeah, they want to!

These negatives are coming nearby,

they want to get as far away from them as possible.

Even though there are already some negatives here,

a net amount of negatives

are going to get moved over to this side.

They were located with their atom on this side,

but they want to get away from this big negative charge

so they can move over here, which leaves

a total amount of positive charge over here.

I.E. There is a deficit of electrons over here,

so this side ends up positively charged.

You might think, "Okay, well that's weird.

"They spread out.

"Does anything else happen?"

Yeah because now these positives are closer to the negatives

than the negatives are,

and these positives in this charge rod

are attracting these positives.

These negatives in this conducting rod

are attracting these positive charges

because like charges repel and opposites attract

but they are also repelling.

These negatives in this rod are repelling these negatives.

Do those forces cancel?

They actually don't because the closer you are

to the charge the bigger the force.

This would cause this rod to get attracted

to the other rod.

That's kind of cool.

If you took a charged rod,

brought it to an empty soda can,

let that can sit on the table

in this orientation so it could roll,

if you bring the rod close

the can will start moving towards the rod.