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- [Voiceover] It's useful to pretend like all materials in the universe can be broken down into a category of insulator, electrical insulator, or electrical conductor. That's not completely true. There are semi-conductors and super conductors and other exotic forms of electrical materials but for most introductory physics classes and problems and tests, you can get pretty far assuming that it's either an insulating material electrically or a conducting material electrically. Before I talk about the differences between these, here I have two solid cylinders of either an insulating material or a conducting material. Before I talk about the differences, one similarity is that both insulators and conductors are composed of a huge number of atoms and molecules and these atoms and molecules, whether it be insulator or conductor, are composed of a positively charged nucleus and a negatively charged swarm of electrons that surround that nucleus. Another similarity is that for both conductors and insulators, the positively charge nucleus cannot move. I mean it can wiggle around and jiggle just from thermal vibrations, maybe a little bit in place, but it can't travel freely throughout the material for either an insulator or a conductor as long as it's a solid. If it was a fluid, I suppose these things can move and migrate around, but for a solid the positively charge nucleus is fixed. They're stuck. The thing that might be able to move are the negatively charged electrons, and here's the difference. There are electrons in a conductor that can move about relatively freely. These can move around with almost no resistance, whereas for insulators a key difference is that these electrons cannot move around freely. These don't have the right energy levels and bands in order to make these electrons move around freely. They are also stuck. For insulators, everything is basically stuck, These electrons might be able to jump around in their own atoms or get shared in a neighboring atom, but it can't jump around freely from atom to atom and travel throughout the insulator. For the conductors, the electrons can do this. that's the key difference. Now the electrons aren't just going to do this on their own, they have to be compelled to start moving by hooking this up to a battery or setting up some sort of electric field or force. If that did happen, the electrons in a conductor start migrating down the line but in an insulator, the electrons are stuck which might make you think that "Well, okay, shoot, for electrical materials "all we really care about are the conductors. "The insulators we will just use if we don't want "electrical interaction." While that is somewhat true, it is not completely true because if I set this insulator up to a battery or set up some sort of electric field or force in here even though the electrons in an insulator can't jump from atom to atom, what it can do is it can shift. This nucleus and the cloud of electrons can kind of shift a little bit. Positive may be this way, and the the negatives over on the other end so what you get is overall this side of the atom would be more negative, and this side of the atom would be more positive. Even though the electron doesn't move, and the electrons don't move, now because this is set up where the positive is shifted from the negative, this material, if you get all of them to do this or a lot of them, this can create an overall electrical effect where this insulator can interact with other charges nearby and exert forces on them. Even though the charges can't flow through an insulator, they can still interact electrically. Now, let's see what happens if we add extra charge to these insulators or conductors. I mean, the way they started off right here we had just as many positives in the nucleus as there are negatives surrounding them and that's true for the conductors and insulators. What happens if we add extra charge? Maybe we add extra negatives into here. Then what happens? Well, it'll get really messy if we try to draw it with all the atoms, so since these all cancel out their overall charge, I am not going to draw every atom and nucleus. I'm just going to pretend like those are there and they are all canceling out. I'm just going to draw the actual extra charge. Let's say we added extra negative charges to this insulator. What would happen? Let's say I just add a negative charge here and a negative charge there, and here and there, I have added a bunch of negative charges to this insulator. What would happen? Well, we know these negatives can't move throughout and insulator. Charges can't flow through an insulator so they're stuck which means for an insulator, I could charge the whole thing uniformly if I wanted to where the charge is spread out throughout the whole thing or I could make them bunch up on one side if I wanted to and they'd be stuck there. The point is that they're stuck. For a conductor, what would happen if I tried to put a negative here and a negative there, some extra negative charge on a conductor? They don't have to stay here if they don't want to. If you put extra negatives in here, they are not going to want to because negatives repel each other just like opposites attract, like charges repel. So what are they going to do? Well, this negative is going to try to get as far away from this other negative as it can so go over here. This negative is going to try to get as far away as it can. It repels it. Now, it can't jump off the conductor. That takes a lot more energy, but it can go to the very edge. That's what charges do for conductors. You've got a solid conducting material, you put extra charge on it, it's all... All that charge is going to reside on the outside edge whether you've added extra negative or positive, always on the outside edge. You can only add charge to the outside edge for a conductor, because if it wasn't on the outside edge it will quickly find its way to the outside edge because all these negatives repel each other. I said this is true for positives or negative. You might wonder, "How do we add a positive?" Well, the way you add a positive is by taking away a negative. If you started off with a material that had just as many positives as negatives and you took away a negative, it's essentially like adding a positive charge in here. But again, the net positive charge, the net negative charge always resides on the outside edge of the conductor because charges try to get as far away from each other as possible. So what physical materials actually do this? What physical materials are insulators? These are things like glass is an insulator. Wood is an insulator. Most plastics are insulators. All of these display this kind of behavior where you can distribute charge and the charge can't flow through it. You can stick charge on it. In fact, you can stick charge on the outside edge and it will stay there. There's conductors. These are things like metals, like gold or copper is typically used because it's kind of cheap. Cheaper than gold, certainly. Or any other metal. Silver works very well. These are materials where charges can flow freely through them. Now that we see how conductors and insulators work, let's look at an example. Let's say you have two conducting rods. Say these are made out of metal. One of them has a net amount of negative charge on it which is going to reside on the outside edge because that's what net charge does on a conductor, but this other rod, this other metal conducting rod, does not have any net charge on it. What would happen if I took this first rod touched it to the second rod? You probably guessed, charges want to get as far away from each other as possible so these negatives realize "Hey, if we spread out, "some of us go on to this rod and some of us stay here, "we can spread out even father away from each other." That's what they would do. If these rods were the same size, you'd have equal amounts on each. If the second rod was bigger, more of them would go on to this second one because that would allow them to spread out even more. Some would stay on the smaller one. That's charged by just touching something. That's easy. You can charge something also, you can get clever. You can do something called charge, you can charge something by induction it's called. What does this mean? Charge by induction says alright, first imagine I just take this and I bring it nearby but don't touch it. Just bring it near by this other piece of metal and I don't touch it. What would happen? There is negatives in here, I haven't drawn them. There's positives in here. The negatives can move if they wanted to. Do they want to? Yeah, they want to! These negatives are coming nearby, they want to get as far away from them as possible. Even though there are already some negatives here, a net amount of negatives are going to get moved over to this side. They were located with their atom on this side, but they want to get away from this big negative charge so they can move over here, which leaves a total amount of positive charge over here. I.E. There is a deficit of electrons over here, so this side ends up positively charged. You might think, "Okay, well that's weird. "They spread out. "Does anything else happen?" Yeah because now these positives are closer to the negatives than the negatives are, and these positives in this charge rod are attracting these positives. These negatives in this conducting rod are attracting these positive charges because like charges repel and opposites attract but they are also repelling. These negatives in this rod are repelling these negatives. Do those forces cancel? They actually don't because the closer you are to the charge the bigger the force. This would cause this rod to get attracted to the other rod. That's kind of cool. If you took a charged rod, brought it to an empty soda can, let that can sit on the table in this orientation so it could roll, if you bring the rod close the can will start moving towards the rod. It's kind of cool, you should try it if you can. But, that's not charge by induction. Charge by induction is something more. It says alright, take this piece of metal and conduct it to ground. What's ground? Well, it could be the ground. If you took a big metal pipe and stuck it in the ground that would count, or any other huge supply of electron, a place where you can gain, steal, basically take infinitely many electrons or deposit infinitely many electrons and this ground would not care. So the frame of your car, the actual metal, is a good ground because it can supply a ton of electrons or take them. Or a metal pipe in the earth. Some place you can deposit electrons or take them and that thing won't really notice or care. Now what would happen? If I bring this negative rod close to this rod that was originally had no net charge? Now instead of going to the other side of this, they say "Hey, I can just leave. "Let me get the heck out of here." These negatives can leave. A whole bunch of negatives can start leaving and what happens when that happens is that your rod is no longer uncharged. It has a net amount of charge now. They won't all leave. You're not going to get left with no electrons in here. There's going to be some electrons in there, but some of the electrons will leave which means that this rod, which used to be uncharged now has a net amount of positive charge in it. I've charged this rod without even touching it because I let the negative electrons leave. If I'm clever, what I can do is I can just cut this wire before I take away the thing that induced the charge. If I remove this now and move it far away, what these negatives would have done is they would have said "Shoot, okay, "I am glad that that's over. "Now I can rejoin. "I'm attracted to this positive again. "I'm going to rejoin my positives." and this thing will become uncharged again but now they can't get back. They're stuck. There's no way for these to get back because you've cut the cord here and you've permanently charged this piece of metal without even touching it. It's called charge by induction. It's a quick way we charge something up. Let me show you one more example. Everyone's tried this. You take a balloon. What happens? How do you charge it up? You rub it against your hair. It steals electrons from your hair and the balloon becomes negatively charged. What do you do with it? You know what you do with it. You take this thing and you put it near a wall or a ceiling and if you're lucky, it sticks there, which is cool! How does it work? Well, remember, this is an insulating material rubber. The ceiling is an insulating material. Electrons aren't getting transferred but even in an insulating material, the atom can reorient or polarize by shifting. The negatives in that atom can shift to one side and the other side becomes a little more positive and what that does, it causes a net force between the ceiling and the balloon because these positives are a little closer. These positives are attracting negatives and the negatives are attracting the positives with a little bit greater a force than these negatives are repelling the other negatives in the ceiling. Because of that, because the ceiling is also attracting the balloon and the balloon is attracting the ceiling with greater force than the negatives are repelling the balloon, the balloon can stick because of the insulating material's ability to polarize and cause and electric attraction. This is what I said earlier. Even if it's an insulator, sometimes it can interact with something electric because the atom can shift and polarize.