Hi. It's Mr. Andersen and this is AP Physics essentials video 19. It is on electric permittivity
which is best understood when you are looking at capacitance. And so for this video I created
a capacitor out of three layers. Two layers of Reynolds Wrap, which are good conductors.
And then one layer of plastic wrap in the middle which has a higher permittivity. And
so in order to make this work and make it a little bit more portable I rolled the whole
thing up and so it looks like that. If you were to look at a real capacitor it is going
to be rolled up. But is essentially the same thing. You have conductors on either side
and then something insulative in the middle. And so let's look at this video for just a
second. Right now I am hooking up my capacitor to a multimeter and you can see that there
is no electric charge on the inside. Now I am connecting it to a couple of batteries.
So I am adding charge now to the capacitor. And now I am going to hook it up to the multimeter
and you can see that the charge increases really quickly. And so I stored charge in
just this little amount of Reynolds Wrap and plastic wrap. You can see it is kind of discharging
now. And so what is permittivity? Well if you have an electric field like this, matter
has an ability to resist that electric field. And that resistance to an electric field is
called its permittivity. And so the permittivity of free space would be the resistance offered
by just a vacuum between those two conducting plates. And so that is a constant value that
we can compare other materials to. And so if we have other materials, their permittivity
is based on their structure. So their atomic or molecule structure is going to be variable.
And as we vary the permittivity we can make a capacitor that is more or less efficient.
And so let's look at a real basic capacitor. And that will show you the elements of capacitance.
This is what they look like today. So if we zoom in like this what you have are two conducting
plates on either side. So those are connected with wires. And then in the middle we have
a dielectric. And so that is going to be an insulator. It is something that can resist
that electric field forming in that capacitor. And so if we look at basic free space permittivity,
so what we would have in the middle now is going to be just a vacuum. And so what we
do is as we hook it up to voltage you are going to see there is an electric field across
that gap. And that is the free space permittivity. Or sometimes we refer to it as the dielectric
constant. And so for a vacuum it is going to be 1. Air is going to be slightly higher
than 1. But it is going to be pretty close to 1. And so what happens when we add a dielectric
in the middle, when we add an insulator in the middle? Well that is going to be made
of atoms. And atoms are made up of protons and electrons. And what happens is as that
field moves through the dielectric, what it does is shifts the electrons. So the electrons
are actually being shifted, in this case, to the left. And that creates an opposite
field which cancels out that electric field. And so if you add a dielectric between these
two conducting plates, what you are doing is resisting that electric field. And what
that does is build up a charge on either side. And that it is how we are storing that energy.
And so we now see that we have a dielectric constant that is going to be higher than our
constant which was 1.And so if we look at this PHET simulation we have a capacitor here
in the middle. And so we are going to add a little bit of voltage to it. And you can
see the charge is starting to build up on both sides of that plate. Now if we take a
dielectric and slide it in the middle, well this dielectric is just free space. And so
you can that it did not change the capacitance. But if I start to increase the permittivity,
watch what happens to the charge on the plates. So now we are at 2. And now we are at 3. And
now we are at 4. You can see charge is building up on the plate. And each material, teflon
2.1, paper 3.5, glass 4.7 is going to have a different constant. And if I pull that dielectric
out then our capacitance dropped down to 0. And so did you learn that electric permittivity
increases resistance in the electric field? We can use it to make a capacitor which could
be made simply out of Reynolds Wrap and plastic wrap. I hope so. And I hope that was helpful.