 Okay, so now we're going to move into some new material. We've reviewed the stuff we've already done, but we've got to apply it to some new situations. Now the presentations you're going to see here in these video clips, the graphics in particular are a little bit rougher than I normally do for a presentation, but it will work for in class. So you just have to kind of follow where I'm pointing with the mouse on some things. We're going to start by talking about conductors. Now, if you've watched the pre-lecture video, there's some more details in there. And you'll want to go back and watch that if you need to. But part of what defines a conductor, well, what does it conduct? It can conduct electricity. And how does it do that? It's got free electrons. So any kind of material where there are free electrons, and again the video explains this a little bit more as it contrasts this to other ones, that means the electrons can freely move around in the material. If you exert a force on the electron, the electron can move. Remember, however, it's not the protons and neutrons that move. Most of the time they're sort of solidly fixed in place, but the electrons are free to move. There are some conductors where the entire material is allowed to move, but let's kind of focus on maybe metals for right now. Now in contrast to this is the materials we have been dealing with, insulators. When I've got an insulator, if I put a charge on a surface, it stays right where I put it. Or if I'm putting charges in a volume, I can distribute them evenly through the volume and they're going to stay exactly where I put them. So in conductors, the electrons are able to move, and in the insulators they're going to stay put. And your pre-lecture video that you can go back and watch later explains not only conductors and insulators, but also another material called semi-conductors that we'll work with later this semester. So now that we know what a conductor is, let's think about what happens if I put charge on a conductor. So I've got a sphere, imagine this is a full sphere and not just a circle. And I'm going to add some little charges to it. Maybe I'm just going to throw a whole bunch of electrons on there. Well, what's going to happen to those electrons when they're on here? I've got extra electrons and electrons are going to be repelled from other electrons. So all those electrons are going to try and move as far away from each other as they can. Well, if I've got a metal sphere, the furthest away from each other they can move is to end up on the surface. So all of my extra electrons are going to be on the surface of the sphere. And that's a three-dimensional sphere all the way around the surface. And the extra electrons, none of them are going to be in the middle. Now there's still electrons and protons in the middle, but all the area in the middle, those atoms are all balanced. So they have an equal number of protons and electrons. Now the same sort of thing happens if I take some electrons off of a conductor. Remember, I can't really take the protons off or add the protons because that defines where the metal atoms are. But I can take some electrons off. When I do that, that means there's some atoms in here that are missing an electron. But the electrons that are there are going to move so that the places where the unmatched protons are, where the missing electrons are, are going to be all the way around on the surface again, making all of that unbalanced positive charge as far away from the next unbalanced positive charge as it can possibly be. And again, everything in the middle is going to be balanced out.