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Video transcript

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 and that's not completely true there are semiconductors and superconductors 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 and before I talk about the differences between these here I have two solid cylinders of either and 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 a conductor 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 charged 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 charged 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 the 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're also stuck for for insulators everything's basically stuck these electrons might be able to jump around in their own atom or get shared with 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 going to just do this on their own that have to be compelled to start moving hooking this up to a battery or setting up some sort of electric field or force and if that did happen then the electrons in a conductor could 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 just use if we don't want electrical interaction and while that's somewhat true it's not completely true because if I set this insulator up to a battery or I set up some sort of electric field or force in here even though the electron 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 maybe this way and then the negatives over on the other end so what you get is that overall this side of the atom would be more negative and this side of the atom would be more positive and 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 so 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 get really messy if we try to draw it with all the atoms so since these all cancel out their overall charge I'm not going to draw every atom in nucleus I'm going to just pretend like those are there and they're all cancelling out I'm just going to draw the actual extra charge so 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 and I've added a bunch of negative charges to this insulator what would happen well we know these negatives can't move throughout an 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 can make them bunch up on one side if I wanted to and they'd be stuck there but 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 and if you put extra negatives in here they're 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 negatives going to try to get as far away from the other negative as it can let's go over here this negatives 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 they can go to the very edge and that's what charges do for conductors if you 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 so you can only add charge to the outside edge for a conductor because if it wasn't on the outside edge it'll quickly find its way to the outside edge because all these negatives repel each other and I said this 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 so if you started off with the 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 a 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 as an insulator wood is an insulator most plastics or 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 would just stay there and there's conductors you things like metals like gold or copper is typically used because it's kind of cheap cheaper than gold certainly and or any other metal silver works very well these are materials where charges can flow freely through them so now that we see how conductors and insulators work let's look at an example let's say you had two conducting rods say these are made out of metal and 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 could probably guess charges want to get as far away from each other as possible so these negatives realize hey if we spread out if some of us go onto this rod and some of us stay here we can spread out even farther 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 this second rod was bigger more of them would go on to the second one because that would allow them to spread out even more and some would stay on the smaller one so 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 I just bring it near by this other piece of metal and I don't touch it what would happen well there's 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 so even though there are already some negatives here some a net amount of negatives are going to get moved over to this side so they were located with their atom on this side but there you want to get away from this big negative charge so they move over to here which leaves a total amount of positive charge over here ie there's a deficit of electrons over here so the side ends up positively charged and you might think okay well that's weird they spread out is that anything else happened yeah because now these positives are closer to the negatives then 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're also repelling these negatives and this rod are repelling these negatives do those forces cancel they actually don't because the closer you are to the charge of the bigger the force this would cause this rod to get attracted to the other rod so that's kind of cool this means if you took a charged rod brought it to an empty soda can but that can sit on the table and this orientation so it could roll if you bring the rod close the can will start moving toward the rod it's kind of cool you should try it if you can but that's not charged by induction charge by induction is something more it says all right 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 stuck it in the ground that would count or any other huge supply of electrons a place where you can gain steel 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 someplace you could deposit electrons and 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 that instead of just going to the other side of this they say hey I could just leave let me get the heck out of here these negatives can leave a whole bunch of negatives will 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 this rod which used to be uncharged now has a net amount of positive charge in it so I've charged this rod without even touching it because I let those negative electrons leave and 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 I move it far away but these negatives would have done is they would've said if you okay I'm glad that's over now I can rejoin I'm attracted to this positive again my rejoin my positive and this thing would become uncharged again but now they can't get back they're stuck there's no way for these to get back because you cut the cord here and you've permanently charged this piece of metal without even touching it it's called charged by induction it's a quick way to charge something up but let me show you one more example everyone's try 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 it that you take this thing 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 ceilings and 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 the other side becomes a little more positive and what that does is it causes a net force between the ceiling and the balloon because these positives are a little closer so these positives are attracting the negatives and the negatives are attracting the positives with a little bit greater force than then these negatives are repelling the other negatives in the ceiling and because of that because the ceiling is also attracting the balloon and the balloon is attracting the ceiling with greater force then the negatives are repelling the balloon the balloon can stick because of the insulating materials ability to polarize and cause an 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