all right now we're going to talk about the idea of an electric current and the story about current starts with the idea of charge so we've learned that we have two kinds of charges positive and negative charge we'll just make up two little charges like that and we know if they're the opposite opposite sign that there will be a force of attraction between them and if they have two like Signs here's two here's two charges that are both positive and these charges are going to repel each other so this is the basic electrostatics idea and the same thing for two minus charges they also repel so like charges repel and unlike charges attract that's one idea so we have the idea of charge and now we need a place to get some charge and one of the places we'd like to get charge from is is copper copper wires copper a copper atom looks like this copper has a has a nucleus with some protons in it and it also has uh electrons flying around the outside electrons in orbits around the outside so we'll draw the electrons like this so there'll be orbits around this nucleus pretty good circles and there'll be electrons in these little minus little minus signs there's electrons stacked up in this and even farther out there's electrons so there's a a a kind of a interesting looking Copper at copper the symbol for copper is cu and its atomic number is 29 29 that means there's 29 protons inside here and there's 29 electrons outside and it turns out just as a coincidence for copper that the last orbital out here has just one electron in it that guy right there and that's the one that is the easiest to pull away from copper and pull have it Go participate in in in conduction in in electric current so when uh every if I have a chunk of copper every copper atom every copper atom will have the opportunity to contribute one this this one lone lonely electron out here if we look at another element like for instance silver silver has this same same kind of a electron configuration where there's just one out here and that's why silver and copper are such good good conductors all right so now we're going to build let's build a copper wire here's here's sort of a a copper wire it's just made of solid copper it's all full of copper atoms and I'm going to put a voltage across this there's our little battery this is the minus sign this is the plus side and we'll hook up a battery to this and what's going on in here inside this copper inside this copper is a whole bunch of electrons that are associated with atoms it's a neutral it's a neutral piece of metal there's the same number of protons as there is electrons um but these electrons are a little bit loose so if I put a plus over here that's this situation right here where a plus is attracting a minus and so an electron is going to sort of Wander over this way and go like that and that's going to leave sort of a net positive charge in this region and so these electrons are all going to start moving in this direction and down at the end here an electron is going to come out of this battery travel in here and it's going to go in there and make up the difference so if I had a net positive charge here from the electrons leaving and going to the to the left this battery would fill those in and I'm going to get a net movement of charge of negative charge around in this direction like this the question is well how how do I measure that how do I measure or give a number to that amount of stuff that's going on so we want to quantify that we want to assign a number to the amount of current happening here so what we do is in our heads we put a boundary across here so just make that up in your head and it cuts all the way through the copper and what we know we're going to stand right here and we're going to keep our eye right on this boundary down in here and as we watch what we're going to do is we're going to count the number of electrons that move by here and we're going to have a stopwatch and we're going to time that and so we're going to get basically this is charge it's negative charge and it's moving to the side and what we're going to do is at one little spot right here we're just going to count the number that go by in one second and so we're going to get charge per second and it's going to be a negative charge moving by and that's what we call current and it's the same as water flowing by in a river that's the same idea all right now I'm going to set up a different situation that also get produces a current and this time we're going to do it with water water and salt so here's here's just let's build a tube of salt T of salt water like this so we're going to pretend this is some tube that's all full of water I'm also going to put a battery here let's put a another battery and we'll stick the wire into there and we'll stick the wire into there and this is the plus side of the battery and this is the minus side of the battery so water is H2O and this does not conduct there's no free electrons available here but what I'm going to do is I'm going to put some table salt in it this is ordinary salt that you put on your food and it's made of sodium that's the symbol for sodium and chloride CL is chloride sodium chloride is table salt if we sprinkle some table salt into water what happens is these dissolve and we get a net plus charge here and a net minus charge on the chlorine so out here is floating around Nas with plus signs and CLS nearby very close nearby with minuses let's keep it even all right now when I dip my battery wires into this water what's going to happen is this plus charge this plus charge over here from the battery is going to attract the minus CLS so the CL is going to move that way a little bit and over here the same thing is happening there's a minus sign here there's a minus from the battery and that's going to attract this and it's also going to repel CL minuses and so what we get is a net motion of positive charge plus Q going this way and we get minus Q going this way and how do we measure that current how do we measure that current well we do it the same way as we did up there with copper we put a boundary through here in our heads we stand here and we watch the charges moving by and what we're going to get is some sodiums Nas moving this way and chlorines moving this way just like we showed here na moving this way and so there's going to be plus charges moving through the boundary and minus charges moving through the boundary in the opposite direction and if I take the total sum of that for example if I see one na go this way and one cl go this way that's equivalent to two charges moving through the boundary hope that makes sense it's equivalent to two charges one going this way and one going going this way and because they have opposite signs they add together and make two charges in this case current is equal again to charge per second