Hi. It’s Mr. Andersen. And this is AP Physics essentials video 91. It is on the conservation
of electric charge. And this is among objects inside a system. And we are going to start
with a simulation. This is from simbucket.com, a group of physics teachers in Illinois. What
I am going to do is to take a balloon. I am going to rub it on a sweater. And what it
is going to do is it is going to pick up the electrons. It is going to pick up a negative
charge. Now if I hold that next to a conductor, like a bucket, it will be drawn to it. And
the reason why is that it is pushing those negative charges away which can move in a
conductor. And so it is drawn towards the positive charge. If we look at an insulator
those electrons cannot move throughout the material but they can move on either side
of the proton. And so then we can get that positive charge and that attraction. I can
even touch it and through conduction we can transfer some of those charges to the object
as well. But the number of charges that we have in this whole system is going to be conserved.
It is going to remain the same over time. And so the conservation of charge in a system
means that if we have a couple of objects, one let’s say is neutral and the other one
is charged, if I bring that charged object next to the neutral object I can induce a
charge in it. But we are not brining in new charges. We are not losing charges. The amount
of charges in the system is going to remain the same. Even if I touch that other object,
then there is conduction so the movement of charges, the total amount of charges in that
system is going to remain the same. And so how can I ever lose those charges? Well a
really easy way to do that is through grounding. If I conduct those charges away to something
like the earth, then we can actually change the amount of charges inside the system. And
so an example of this, if we take some amber and we rub it on some wool, what we are really
doing is we are transferring charge from the wool to the amber. So if I hold it above a
feather like this, the feather is attracted. And so now let’s look at the conservation
of charge in these objects. This is what all of the objects look like before. And I made
it as simple as I can. So they both have a net charge of 0. All of them have a net charge
of 0. In other words the positive and negative charges are going to be exactly the same.
But when we rub that amber on that wool what we are doing is transferring some of those
electrons to it. And so if we are to look at now this wool here, it has a plus 3 charge.
And this is going to be a minus 3 charge. We have transferred those electrons to the
amber. What happened to the total number of charges before and after? They are exactly
the same. We have just moved where they are. And so when you hold that amber above the
feather what it is doing is it is inducing charge inside the feather. And so we get a
positive charge on one side and that is why it is attracted to the amber. So we could
see the same thing through conduction. So let me charge up that amber again. And I am
going to use a pith ball. So the pith ball, same thing as the feather, it is going to
be attracted to it. But it will quickly be repelled. Why is that? There is conduction.
We are actually transferring some of that charge from the amber to the pith ball. Now
they both have a similar charge and we get that repulsion. So let’s look at the conservation
of charge here. So they all had a net charge of 0 to begin with. We transferred some of
those electrons to the amber. But we then transferred some of those electrons to the
pith ball. And so at the end this has a negative charge, this has a negative charge and so
we have repulsion between the two. What happened to the total number of charges that we have?
It is conserved before and after within the system. Now how can we get rid of some of
that charge? You can see that we have a excess amount of negative charge. If we were to connect
a wire to it and connect that to the earth, then we are going to have movement of those
electrons to an area where there are less electrons. And so what we are going to be
left with is a net charge of zero in the object again. If we want to add charge to it, we
are going to have to charge it up again. And so now let’s kind of apply that using another
simulation. What we have here is an aluminum can. And then I can charge up a glass rod
which has a positive charge or rubber rod which is going to have a negative charge.
So if we have the glass rod that has a positive charge note that wherever I move it that aluminum
can is drawn towards it. So these are my two objects in this system. Now let’s add a
rubber rod that has a negative charge. Wherever it is the aluminum can is going to be drawn
towards it. So if I put it on the left side it is going to be drawn to it as well. And
so you cannot see the charges. And so before I click the button and we move forward and
I show you the charges you should be thinking where are those charges in the two objects
and how are they moving so that we get this attraction. And so it takes awhile to think
about it. Remember that can is a conductor. So the electrons can move within it. And so
let’s start by showing the charges in the glass rod. Glass rod is going to have a positive
charge. And so watch what happens. The electrons are drawn towards that positive charge. You
get a build up or induction on one side of that, so we have a negative charge. And so
there is an attraction between the glass and the can itself. The can is neutral remember,
but we are inducing a charge inside it. Now what would it look like if we look at the
rubber rod. Rubber rod remember is going to have a negative charge. And so what is it
going to do to the electrons? It is going to repel them. So it is pushing them away.
Now we are left with the protons, the positive charge and that is where that attraction is
coming from. We are getting that pull towards that rubber rod. It does not matter what the
charged object is. We still have an attrition and we still have a conservation of those
charges, it is just that they are moving in different places within the objects. So did
you learn to predict where the charges go before and after charging in a system? Remember
the net total of charges before and after is the same. They just might be in different
places. Did you learn to design to collect data to figure out qualitatively, not with
a number, but where the charge is found? And then finally could you add the charges to it?
I hope so. And I hope that was helpful.