Hi. It’s Mr. Andersen and this AP Physics essentials video 72. It is on electrostatic
induction, which is the ability of charges in one object to affect charges in another
object. And so an example of this is static electricity. And so as this child went down
the slide, friction of their hair on the slide actually pulled electrons off. And so now
all their hairs have positive charges. And so that induction of the hair itself is pushing
on all the other positive hairs and so we get this crazy looking hair. And a good way
to study this in the science lab is to use something called an electroscope. And so what
we have is a metal ball at the top connected to a metal rod that goes through the middle.
And then we have metal leaves on the bottom. And so these are all insulated from the surroundings
because we have a rubber stopper that goes right around here. And so what we are going
to do is we are going to charge up a glass rod. So we are going to charge up a rod like
that, and so what you are doing is actually pulling electrons off of it and now we have
a positive rod. It is just like the hair of the child. And so as you pull it close to
the electroscope, what is going on is induction here. So it is impacting the charges within
the electroscope. Watch what happens as we pull it away. The leaves go back to where
they were. As we put it closer, there is induction going on. As we pull it away they go back.
What happens if we actually touch it. So if I touch that, now conduction takes place.
So we are actually transferring charges. And watch what happens. As I pull it away those
leaves are going to be held apart. So to understand what is going on at the charge level you really
have to understand what electrostatic induction is. And so a charged object can be charged
through friction, like rubbing your hair on a balloon. Or it could also be conduction,
touching another charged object. And so depending on what that charged object is, maybe it is
a conductor like metal, the electrons in there are mobile. They can move around and that
charge is going to start to dissipate or at least move within the object itself. If it
is an insulator, all of those charges or those electrons are fixed. And so we can add charges
to one part of it. And it does not mean the rest of it has to stay charged. But if we
take a charged object and we add it to a system, we are going to change the distribution of
the charges within that system. And so if we do induction, it means we are going to
move it next to another object but we are not physically touching it. And if we are
doing conduction, then we are going to physically touch it. And so what are these charges? Remember
it is going to be the electrons and the protons in the atoms that make up the material itself.
And so if we have a neutral object that means we are going to have an equal number of positive
and negative charges. They are going to balance out. And there is going to be no net charge.
Now what does a charged object look like? Well is this case we have way more negative
charges and so this is going to be a negatively charged object. What is it going to look like
if we pull those electrons away? Now we are going to have a positively charged object.
But remember the protons are going to stay where they are. It is always the electrons
that are moving. Now how is an insulator different from a conductor? Well in an insulator, like
in glass or in plastic, all those electrons are fixed. And so they can move a little bit
but they are always going to be stuck to that proton of the atom of which they are a part.
Now how is that different in a conductor? In a conductor remember those electrons are
mobile. It is a sea of electrons and so those electrons can move around. And so when we
are looking at a charged conductor and a charged insulator they are going to behave in different
ways. And so if we have a neutral insulator like this, so this is a neutral insulator
up here. So we have an equal number of positive and negative charges. But what I am going
to do is I am going to add some negative charges to it. So when I add those negative charges
like that, those electrons are fixed in their position. They can not dance from atom to
atom. And so what are we going to have now? A charged insulator where the left side, in
this diagram, is going to have a negative charge and the right side is going to have
a neutral charge. Now how is that different than a conductor? Well same thing, we are
going to start with a neutral conductor now. And I am going to add a bunch of charges to
it. Same location, but what happens with these electrons is they can move. They hate being
around other electrons and so they are going to spread out. And that charge is going to
start to be on the outside of that conductor. So remember in a conductor if we are ever
dealing with metal those electrons are moving. But if we are dealing with an insulator they
are going to remain fixed in position. And so what is polarization? It is when I can
take a charged object like this, one where we have a lot of electrons. And as I move
it near a neutral object, what is going on? You can see those electron are moving. So
we know right away that this is going to be a conductor, since the electrons are moving.
But since it is not touching, now we have electrostatic induction taking place. What
happens if I actually touch it? Now we have conduction. Those charges can actually flow
from one to another. So now let’s go back to the electroscope and try to discover what
is going on at the charge level. So again I took a glass rod and then I charge it. And
so what I am really doing is pulling the electrons away. And as I move it close to that electroscope
we can see that those leaves at the bottom, those metal leaves are moving apart. And so
let’s kind of draw the charges out here. So this has a positive charge on our rod.
So we have removed electrons from it. So why do we have a bunch of electrons up here? Well
as we move the positive charges close to the conductor the electrons in the whole conductor
itself, remember this whole thing is connected, those electrons are moving up because they
are attracted to the positive charges in the rod. What does that leave behind? It leave
behind all these positive charges. And since we have positive charges on this side, positive
charges on that side, they are pushing against each other and that force is actually causing
those metal leaves to move apart. If I move the rod away, it goes back to where it way.
Those electrons are just going to dissipate and now we have a neutral electroscope. What
happens however if I touch it? If there is conduction taking place? I pull it away and
those leaves stay there. Again, what is going on? Well we still have these positive charges
up here. We have these electrons that have moved up, so we have these negative charges
over here, but since I am physically touching it, there is now a pathway for those electrons
to move, and what they are going to do is they are going to migrate over there. Now
this is going to be a neutral object over here, but what we have left behind are all
of these positive charges. Since we have pulled those electrons away through conduction, now
we are left with just the positive charges. All those positive charges are pushing on
each other. And the leaves are going to separate. Now let’s say we were to try to set up a
positive object, we were to try a negative object. So let’s say we try a plastic at
this point. We are rubbing the plastic. In this case what we are doing is we are actually
adding electrons to it. As I move it next to that electroscope, same thing happens.
So what is going on there? Well now we have the negative charges over here. So in the
conductor of the electroscope, what happened to those electrons? Again, they are repelled
by the electrons in this plastic rod, and so they are moving down to the bottom. And
since we now have a bunch of negative charges on either side, then it is pushing them away.
What happens when we get conduction with a plastic rod? Again if we look at it at the
level of the charges, since these are negative charges right here, as we pull it closer all
of the negatives go down to the bottom but these are not fixed. Once we touch it these
electrons can actually move through the conductor. And as they do that we add a bunch of electrons
to the conductor. It now has a negative charge. And that is a permanent negative charge. It
is going to stay that way until we can transfer those electrons somewhere else. So did you
learn to make predictions about the distribution of charge using friction? Again, when we are
rubbing an object we can actually pull electrons off or add electrons to it. Conduction, we
are touching two objects so electrons can move. But in induction we are just moving
where they are because they are not physically touching. Could you make predictions about
the distribution when we are polarizing an object? Electrons again are moving. Could
you construct a representation of how charges are going to be different in an insulator
versus a conductor? Remember in an insulator the charges are going to stay. They are going
to be fixed wherever you put them. But in a charged conductor they are going to move
away from each other. And then finally could you explain the results of an electroscope?
For example, using charge and these ideas of induction and conduction. I hope so. And
I hope that was helpful.