[Music] so we're going to start this semester talking about electrostatics or how electric charges interact with each other and what you probably know about electricity to start with is that there's opposite charges and opposite charges attract and like charges repel each other mathematically this is pretty easily described if we refer to one kind as positive and the other negative and and really the only thing we know about this is that that that charges interact with each other in a certain way it's not like they have any other property that would tell us this one's positive and that's negative in fact the choice of positive and negative is kind of arbitrary uh it's it's historic and what we call positive now is because that's what we always called positive and we'll find out later on that this actually kind of got backwards on us because what we wound up doing was choosing electrons to be negative and protons to be positive and that leads to some trouble when we talk about electrical current later but for now we just know that there's two types of charge whatever it might be there's a positive and a negative and these guys will attract and repel based on opposites attracting and like charges repelling the other thing we know is that the force will drop off with distance if I have two charges and they're repelling each other that the farther away I take them the less the force will be and in fact careful measurement will show us this Force actually drops off as 1/ R 2 it's what's know as a 1 / R 2 Force law and there's some geomet reasons why this is actually very very important but for now we're just going to leave it as it is and put these thoughts together into one rule here that the force obviously has to be proportional to the sizes of the charges involved q1 and Q2 is what I'll use to represent the charge on number one and number two whatever it might be divided by r s the distance between them and this is the start of what is known as koloms law first though let me let me talk about charge just a little more charge is measured in a unit called a kulam named after Augustine de kulam who other than that picture there you'll never think of him again but it it was a uh just a a way of measuring charge it's not like I could tell you oh that's uh seven kums and you'd know what the heck I'm talking about unless you've used them some in fact a a kolom is a rather large amount of charges we'll find later on the force is generated by one kolom are pretty pretty big now a kolom is actually not the base unit the base unit is really an ampere so a kolom is an ampere P an ampere times a second but we'll come back to that later on when we talk about current for now we're just going to go with the idea that charge comes as a kolom and charge is kol is is conserved it can't be created or destroyed if I make a positive charge it's because I had to leave a negative charge behind somewhere it adds up to a net of zero in that case just like anything else I might do momentum or energy uh this conservation law is actually pretty important in the field of phys physics the other thing that's that's significant here is that there's a fundamental charge there's there's a sort of quantization if you will to charge that I can't divide any smaller and the fundamental charge in nature is that of an electron or of course a proton as well any positive or negative charge at the atomic level is that measured in these units and one electron or one proton has a charge of 1.6 * 10us 19 kums so you can tell if I have a bunch of electrons move moving around it would take a whole whole whole bunch before I can measure up to having a full Kum worth of charge as I said before a Kum is actually kind of a large amount of charge now let's go back to that idea of the force I mentioned that this is the kulon force law named after the same guy and the force is proportional to the charge times the charge over R 2 now if I look at the units on this Force comes in Newtons charge comes in kums as we second said a second ago and distance is of course in meters well a Kum Square divid by a meter squ does not give me a Newton so what that means is I need to have some constant go in there to make the units match up all this constant's going to do for me is convert Kum squ over meter squ into Newtons so let's call that constant K FAL K QQ over R 2 and K will be equal to 8.99 * 10 9 Newtons * me^ 2 over Kum squ again this is just converting those Kum squ per meter squ into Newtons so it's just a constant of nature uh has a more subtle meeting that we'll get to at a later date as well now I'll also point out that more advanced physics we don't usually use this version of K instead what we use is 1 over 4 Pi Epsilon KN Epsilon KN something called the permitivity of free space it's a little more fundamental unit or uh constant in terms of the way nature is made and Epsilon KN is 8.85 * 10us 12 either one of these is is useful to us right now and you'll find out as time goes by there are some times when the K version of this equation is useful and there are sometimes where the Epsilon KN version is useful but I want to have both those in your head for right now one way or another it's just some constant times q1 Q2 over R 2 now K is related to Epsilon KN of course there's a 1 over 4 Pi Epsilon KN equal K the 4 Pi there in case you're curious actually comes from the fact that we live in a three-dimensional space the surface of a sphere is 4 pi r s so 4 Pi gets thrown in there for that reason again this is stuff that you'll learn later on if you go further in physics now we still need to worry about the direction this Force takes force is after all a vector quantity and there's nothing on the right side of that equation that looks very Vector likee to me at the moment so we're going to need to come back and revisit this and think about the way the directions of these things should point but it should be obvious if there's a repelling and attracting to this that that direction is going to point either toward the other charge or away from the other charge depending on the signs of the q1 and the Q2 and that's what the next lesson will be all about