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  • 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.

Hi. It's Mr. Andersen and this is AP physics essentials double O 7. It is on positive and

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