- [Voiceover] It's useful to pretend
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.
It's kind of cool, you should try it if you can.
But, that's not charge by induction.
Charge by induction is something more.
It says alright, take this piece of metal
and conduct it to ground.
What's ground?
Well, it could be the ground.
If you took a big metal pipe and stuck it in the ground
that would count,
or any other huge supply of electron,
a place where you can gain, steal, basically take
infinitely many electrons or deposit
infinitely many electrons and this ground would not care.
So the frame of your car, the actual metal,
is a good ground because it can
supply a ton of electrons or take them.
Or a metal pipe in the earth.
Some place you can deposit electrons or take them
and that thing won't really notice or care.
Now what would happen?
If I bring this negative rod close to this rod
that was originally had no net charge?
Now instead of going to the other side of this,
they say "Hey, I can just leave.
"Let me get the heck out of here."
These negatives can leave.
A whole bunch of negatives can start leaving
and what happens when that happens is that
your rod is no longer uncharged.
It has a net amount of charge now.
They won't all leave.
You're not going to get left with no electrons in here.
There's going to be some electrons in there,
but some of the electrons will leave
which means that this rod, which used to be uncharged
now has a net amount of positive charge in it.
I've charged this rod without even touching it
because I let the negative electrons leave.
If I'm clever, what I can do is I can just cut this wire
before I take away the thing that induced the charge.
If I remove this now and move it far away,
what these negatives would have done
is they would have said "Shoot, okay,
"I am glad that that's over.
"Now I can rejoin.
"I'm attracted to this positive again.
"I'm going to rejoin my positives."
and this thing will become uncharged again
but now they can't get back.
They're stuck.
There's no way for these to get back
because you've cut the cord here
and you've permanently charged this piece of metal
without even touching it.
It's called charge by induction.
It's a quick way we charge something up.
Let me show you one more example.
Everyone's tried this.
You take a balloon.
What happens? How do you charge it up?
You rub it against your hair.
It steals electrons from your hair
and the balloon becomes negatively charged.
What do you do with it?
You know what you do with it.
You take this thing and you put it near a wall or a ceiling
and if you're lucky, it sticks there,
which is cool!
How does it work?
Well, remember, this is an insulating material rubber.
The ceiling is an insulating material.
Electrons aren't getting transferred
but even in an insulating material, the atom can reorient
or polarize by shifting.
The negatives in that atom can shift to one side
and the other side becomes a little more positive
and what that does, it causes a net force
between the ceiling and the balloon
because these positives are a little closer.
These positives are attracting negatives
and the negatives are attracting the positives
with a little bit greater a force
than these negatives are repelling
the other negatives in the ceiling.
Because of that, because the ceiling
is also attracting the balloon
and the balloon is attracting the ceiling
with greater force than the negatives
are repelling the balloon, the balloon can stick
because of the insulating material's ability
to polarize and cause and electric attraction.
This is what I said earlier.
Even if it's an insulator,
sometimes it can interact with something electric
because the atom can shift and polarize.