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- [Voiceover] It's useful to pretend

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like all materials in the
universe can be broken down

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into a category of insulator,
electrical insulator,

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or electrical conductor.

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That's not completely true.

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There are semi-conductors
and super conductors

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and other exotic forms
of electrical materials

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but for most introductory physics classes

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and problems and tests,
you can get pretty far

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assuming that it's either
an insulating material

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electrically or a conducting
material electrically.

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Before I talk about the
differences between these,

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here I have two solid cylinders

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of either an insulating material
or a conducting material.

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Before I talk about the
differences, one similarity

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is that both insulators and
conductors are composed of

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a huge number of atoms and molecules

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and these atoms and molecules,

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whether it be insulator or conductor,

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are composed of a
positively charged nucleus

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and a negatively charged
swarm of electrons

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that surround that nucleus.

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Another similarity is
that for both conductors

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and insulators, the positively
charge nucleus cannot move.

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I mean it can wiggle around and jiggle

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

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but it can't travel freely
throughout the material

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for either an insulator or a conductor

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as long as it's a solid.

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

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and migrate around, but for a solid

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the positively charge nucleus is fixed.

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They're stuck.

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The thing that might be able to move

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are the negatively charged electrons,

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and here's the difference.

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There are electrons in a conductor

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that can move about relatively freely.

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These can move around
with almost no resistance,

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whereas for insulators a key difference

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is that these electrons
cannot move around freely.

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These don't have the right
energy levels and bands

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in order to make these
electrons move around freely.

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They are also stuck.

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For insulators, everything
is basically stuck,

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These electrons might be able

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to jump around in their own atoms

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or get shared in a neighboring atom,

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

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and travel throughout the insulator.

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For the conductors, the
electrons can do this.

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that's the key difference.

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

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they have to be compelled to start moving

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by hooking this up to a battery

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or setting up some sort of
electric field or force.

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If that did happen, the
electrons in a conductor

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start migrating down the line

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but in an insulator,
the electrons are stuck

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which might make you think that

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"Well, okay, shoot, for
electrical materials

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"all we really care
about are the conductors.

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

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"electrical interaction."

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While that is somewhat true,
it is not completely true

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because if I set this
insulator up to a battery

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

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even though the electrons in an insulator

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can't jump from atom to atom,

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what it can do is it can shift.

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This nucleus and the cloud of electrons

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can kind of shift a little bit.

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Positive may be this way,

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and the the negatives
over on the other end

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so what you get is overall
this side of the atom

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would be more negative,

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and this side of the atom
would be more positive.

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Even though the electron doesn't move,

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and the electrons don't move,

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now because this is set
up where the positive

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is shifted from the negative,

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

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or a lot of them, this can create

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an overall electrical
effect where this insulator

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can interact with other charges nearby

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and exert forces on them.

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

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they can still interact electrically.

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

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to these insulators or conductors.

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I mean, the way they
started off right here

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we had just as many
positives in the nucleus

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as there are negatives surrounding them

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and that's true for the
conductors and insulators.

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What happens if we add extra charge?

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Maybe we add extra negatives into here.

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Then what happens?

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

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with all the atoms, so
since these all cancel out

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their overall charge,
I am not going to draw

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every atom and nucleus.

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I'm just going to pretend
like those are there

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and they are all canceling out.

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

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Let's say we added extra negative charges

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to this insulator.

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What would happen?

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Let's say I just add
a negative charge here

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and a negative charge there,

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and here and there, I have added a bunch

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of negative charges to this insulator.

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What would happen?

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Well, we know these negatives

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can't move throughout and insulator.

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

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which means for an
insulator, I could charge

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the whole thing uniformly if I wanted to

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where the charge is spread
out throughout the whole thing

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

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and they'd be stuck there.

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The point is that they're stuck.

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For a conductor, what
would happen if I tried

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to put a negative here
and a negative there,

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some extra negative charge on a conductor?

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

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If you put extra negatives in here,

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they are not going to want to

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because negatives repel each other

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just like opposites
attract, like charges repel.

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So what are they going to do?

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

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

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

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It repels it.

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Now, it can't jump off the conductor.

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That takes a lot more energy,

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but it can go to the very edge.

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That's what charges do for conductors.

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You've got a solid conducting material,

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

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All that charge is going to
reside on the outside edge

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whether you've added extra
negative or positive,

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always on the outside edge.

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You can only add charge
to the outside edge

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

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it will quickly find its
way to the outside edge

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because all these
negatives repel each other.

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I said this is true for
positives or negative.

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

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Well, the way you add a positive

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is by taking away a negative.

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If you started off with
a material that had

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just as many positives as negatives

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and you took away a negative,

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

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

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always resides on the
outside edge of the conductor

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because charges try to get

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as far away from each other as possible.

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So what physical materials
actually do this?

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What physical materials are insulators?

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These are things like
glass is an insulator.

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Wood is an insulator.

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Most plastics are insulators.

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All of these display this kind of behavior

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where you can distribute
charge and the charge

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can't flow through it.

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You can stick charge on it.

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In fact, you can stick
charge on the outside edge

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and it will stay there.

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There's conductors.

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These are things like metals, like gold

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

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Cheaper than gold, certainly.

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Or any other metal.

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Silver works very well.

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These are materials where charges

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can flow freely through them.

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Now that we see how conductors
and insulators work,

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let's look at an example.

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Let's say you have two conducting rods.

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Say these are made out of metal.

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

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which is going to reside
on the outside edge

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

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but this other rod, this
other metal conducting rod,

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does not have any net charge on it.

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What would happen if I took this first rod

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touched it to the second rod?

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You probably guessed, charges want to get

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as far away from each other as possible

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

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

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

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That's what they would do.

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If these rods were the same size,

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you'd have equal amounts on each.

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If the second rod was bigger,

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more of them would go
on to this second one

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because that would allow
them to spread out even more.

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Some would stay on the smaller one.

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That's charged by just touching something.

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That's easy.

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You can charge something
also, you can get clever.

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You can do something called

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

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What does this mean?

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Charge by induction says alright, first

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imagine I just take this
and I bring it nearby

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but don't touch it.

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Just bring it near by
this other piece of metal

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and I don't touch it.

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What would happen?

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

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There's positives in here.

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The negatives can move if they wanted to.

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Do they want to?

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Yeah, they want to!

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These negatives are coming nearby,

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

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Even though there are
already some negatives here,

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a net amount of negatives

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are going to get moved over to this side.

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They were located with
their atom on this side,

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but they want to get away
from this big negative charge

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so they can move over here, which leaves

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a total amount of
positive charge over here.

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

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so this side ends up positively charged.

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

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"They spread out.

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"Does anything else happen?"

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Yeah because now these positives
are closer to the negatives

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than the negatives are,

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and these positives in this charge rod

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are attracting these positives.

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These negatives in this conducting rod

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are attracting these positive charges

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because like charges repel
and opposites attract

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but they are also repelling.

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These negatives in this rod
are repelling these negatives.

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Do those forces cancel?

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They actually don't
because the closer you are

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to the charge the bigger the force.

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This would cause this rod to get attracted

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to the other rod.

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That's kind of cool.

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If you took a charged rod,

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brought it to an empty soda can,

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let that can sit on the table

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in this orientation so it could roll,

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if you bring the rod close

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the can will start moving towards the rod.

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It's kind of cool, you
should try it if you can.

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But, that's not charge by induction.

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Charge by induction is something more.

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It says alright, take this piece of metal

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and conduct it to ground.

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What's ground?

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Well, it could be the ground.

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If you took a big metal pipe
and stuck it in the ground

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that would count,

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or any other huge supply of electron,

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a place where you can
gain, steal, basically take

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infinitely many electrons or deposit

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infinitely many electrons and
this ground would not care.

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So the frame of your
car, the actual metal,

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is a good ground because it can

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supply a ton of electrons or take them.

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Or a metal pipe in the earth.

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Some place you can deposit
electrons or take them

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and that thing won't
really notice or care.

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Now what would happen?

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If I bring this negative
rod close to this rod

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that was originally had no net charge?

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Now instead of going to
the other side of this,

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they say "Hey, I can just leave.

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"Let me get the heck out of here."

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These negatives can leave.

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A whole bunch of negatives
can start leaving

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and what happens when that happens is that

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your rod is no longer uncharged.

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It has a net amount of charge now.

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They won't all leave.

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You're not going to get left
with no electrons in here.

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There's going to be
some electrons in there,

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but some of the electrons will leave

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which means that this rod,
which used to be uncharged

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now has a net amount of
positive charge in it.

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I've charged this rod
without even touching it

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because I let the
negative electrons leave.

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If I'm clever, what I can do
is I can just cut this wire

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before I take away the thing
that induced the charge.

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00:11:30,923 --> 00:11:33,393
If I remove this now and move it far away,

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what these negatives would have done

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is they would have said "Shoot, okay,

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00:11:36,116 --> 00:11:37,315
"I am glad that that's over.

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00:11:37,315 --> 00:11:38,637
"Now I can rejoin.

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"I'm attracted to this positive again.

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"I'm going to rejoin my positives."

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and this thing will become uncharged again

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but now they can't get back.

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00:11:46,952 --> 00:11:48,810
They're stuck.

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00:11:48,810 --> 00:11:50,796
There's no way for these to get back

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00:11:50,796 --> 00:11:52,915
because you've cut the cord here

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and you've permanently
charged this piece of metal

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without even touching it.

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00:11:58,413 --> 00:12:00,022
It's called charge by induction.

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It's a quick way we charge something up.

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Let me show you one more example.

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Everyone's tried this.

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You take a balloon.

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What happens?
How do you charge it up?

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00:12:06,921 --> 00:12:08,267
You rub it against your hair.

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00:12:08,267 --> 00:12:09,918
It steals electrons from your hair

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00:12:09,918 --> 00:12:12,892
and the balloon becomes
negatively charged.

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00:12:12,892 --> 00:12:13,797
What do you do with it?

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00:12:13,797 --> 00:12:14,963
You know what you do with it.

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00:12:14,963 --> 00:12:17,491
You take this thing and you
put it near a wall or a ceiling

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00:12:17,491 --> 00:12:19,603
and if you're lucky, it sticks there,

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00:12:19,603 --> 00:12:20,934
which is cool!

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00:12:20,934 --> 00:12:21,982
How does it work?

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00:12:21,982 --> 00:12:25,297
Well, remember, this is an
insulating material rubber.

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00:12:25,297 --> 00:12:27,066
The ceiling is an insulating material.

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00:12:27,066 --> 00:12:28,839
Electrons aren't getting transferred

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00:12:28,839 --> 00:12:33,839
but even in an insulating
material, the atom can reorient

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or polarize by shifting.

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00:12:36,148 --> 00:12:38,396
The negatives in that
atom can shift to one side

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00:12:38,396 --> 00:12:40,945
and the other side becomes
a little more positive

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00:12:40,945 --> 00:12:44,003
and what that does, it causes a net force

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00:12:44,003 --> 00:12:47,537
between the ceiling and the balloon

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00:12:47,537 --> 00:12:51,105
because these positives
are a little closer.

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00:12:51,352 --> 00:12:54,121
These positives are attracting negatives

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00:12:54,121 --> 00:12:55,720
and the negatives are
attracting the positives

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00:12:55,720 --> 00:12:57,475
with a little bit greater a force

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00:12:57,475 --> 00:12:59,426
than these negatives are repelling

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00:12:59,426 --> 00:13:01,045
the other negatives in the ceiling.

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00:13:01,045 --> 00:13:04,129
Because of that, because the ceiling

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00:13:04,129 --> 00:13:07,404
is also attracting the balloon

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00:13:07,404 --> 00:13:09,273
and the balloon is attracting the ceiling

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00:13:09,273 --> 00:13:12,735
with greater force than the negatives

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00:13:12,735 --> 00:13:16,170
are repelling the balloon,
the balloon can stick

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00:13:16,170 --> 00:13:19,496
because of the insulating
material's ability

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00:13:19,496 --> 00:13:23,943
to polarize and cause
and electric attraction.

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00:13:23,943 --> 00:13:25,049
This is what I said earlier.

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00:13:25,049 --> 00:13:26,251
Even if it's an insulator,

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00:13:26,251 --> 00:13:29,248
sometimes it can interact
with something electric

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because the atom can shift and polarize.