Hi. It's Mr. Andersen and this is AP physics essentials double O 7. It is on positive and
negative charges. Imagine I take a balloon that does not have a charge and I rub it on
my head. I am now giving it a charge. So I could move it up to something neutral, like
a wall and if I let it go it will just stay there. Now imagine if I take another balloon
and I charge that in a similar matter. It should have the same charge as the balloon.
And so if I hold it up next to it and let it go, what is going to happen? It will actually
be repelled by that object. And so scientists speculated there are two types of charges.
And we call those positive and negative charges. And so an electric charge can be positive
or negative. If we have a neutral object it is going to have an equal amount of each of
those different charges. But if it is a charged object, like a balloon, then it is going to
have differences in the number of electric charges that it has. If it is a negatively
charged object it is going to have more of the negative charges. And if it is a positively
charged object it is going to have more of the positives. Now let's say I bring two objects
next to each other that have different charges. What is going to happen? There will be an
attractive force between the two. But if we take two objects that have the same charge
and bring them next to each other, that could be both positive or both negative, they are
going to repel each other. Now why did the balloon stick to the wall if the wall did
not have a charge? I had not done anything to it? Well if you move a charge up to a neutral
object what it can do is it can polarize that object. It can move some of the charges to
one end. And what it does is create a similar charged object. So an electric charge can
either be positive or negative. And so if we are looking at a neutral object, how many
of those charges are we are going to have in that object? An equal amount of each. Now
let's say we take a charged object, a charged object like that balloon, what are we going
to have? We are going to have more of a positive charge, if it is a positively charged object.
Or if it is a negatively charged object we are going to have more of the negative charges
inside it. Now let's say we take two neutral objects, so two balloons that we have not
charged and we move them right next to each other. What is going to happen if we let them
go? Nothing is going to happen. There is going to be no attraction between the two because
the charges are equally distributed. Let's say we take two objects that two objects that
have opposite charge. So the one on the left has a negative charge, the one on the right
is going to have a positive charge. If I let these go what is going to happen? There will
be an attractive force between the two. Opposites attract. Let's say we take two objects that
have the same charge and move them next to each other. In this case they both have negative
charges, what will happen when I let go? They are going to push away from each other. Same
thing if they are both positive. If I let them go, they are going to move away from
each other. Now let's say we take a charged object, so that is the one on the left and
then we have a neutral object, that is going to be the one on the right, watch carefully
what happens as these two come together. So as I move the charged object towards the neutral
object what it is doing is it is polarizing those charges. It is moving the negative charges
farther away. Why? Because they are being repelled by these negative charges. The positive
charges are moving towards it. And so now there is an attraction between this polarized,
what used to be neutral object. And so PHET makes a wonderful simulation that kind of
walks you through that. So what we have got here is the address down below and I will
put a link in the video description as well. So what I am going to do is take a balloon
and I can move that balloon around. And if I let it go it just stays where it is. But
let's say I want to charge it what I can do is I can rub it on a sweater like this. And
now I am applying a charge to that balloon so when I let it go it is going to go right
over to the sweater. And so if we move it next to the wall and let it go it will stay
there. But if I get it close to the sweater then it is going to be attracted. And so let's
reset this whole thing and look at the charges now. So you can see that all of these objects
are now neutral. They have equal amounts of charge. But as I rub it on the sweater what
I am really doing is I am pulling those negative charges off the sweater. And now they are
on the balloon. So now that balloon is a negatively charged object. And the sweater is a positively
charged object. So if I let it go it moves right next to it. Now watch what happens when
I move it next to the wall you can see it is polarizing this neutral object of the wall.
So I am not transferring charges. But if I move it next to it it is attracted. Let's
get two balloons now and remove the wall. So let me add negative charges on this green
balloon right here. And now let me add more negative charges on this yellow balloon and
what is going to happen? Well as I move that balloon next to the other balloon these are
like charges and so you can see that they are repelling each other. And it eventually
would like to go back to that sweater. Okay. So did you learn the following, to explain
a two-charge model of electric charge? Again if it is a neutral object we are going to
have equal amounts of each of those two different charges. Did you learn that the distribution
of those charges tells us if the object is going to be positively charged or negatively?
You can see this would be a negatively charged object here. And then finally did you learn
to explain how the polarization of the electric charge, or separation of that charge in a
neutral object can lead to an attraction? So we have an attractive force here between
charged balloon and what used to be a neutral wall. I hope so. And hope that was helpful.