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Video transcript

- [Voiceover] All right, now we're gonna talk about the idea of an electric current. The story about currents starts with the idea of charge. 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 sign, that there'll be a force of attraction between them. And if they have two like signs, here's two charges that are both positive, and these charges are gonna 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. We have the idea of charge. And now we need a place to get some charge. One of the places we like to get charge from is copper, copper wires. A copper atom looks like this. Copper atom has a nucleus with some protons in it, and it also has electrons flying around the outside, electrons in orbits around the outside. So we'll draw the electrons like this. There'll be orbits around this nucleus. Pretty good circles. And there'll be electrons in these. Little minus signs. There's electrons stacked up in this. And even farther out, there's electrons. So there's kind of a interesting looking copper atom. Copper, the symbol for copper is Cu, and its atomic number is 29. That means there's 29 protons inside here, and there's 29 electrons outside. 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 have it go participate in conduction, in electric current. If I have a chunk of copper, every copper atom will have the opportunity to contribute one, this one lonely electron out here. If we look at another element, like for instance silver, silver has this same kind of electron configuration, where there's just one out here. And that's why silver and copper are such good, good conductors. Now we're gonna build, let's build a copper wire. Here's sort of a copper wire. It's just made of solid copper. It's all full of copper atoms. And I'm gonna 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. What's going on in here? Inside this copper is a whole bunch of electrons that are associated with atoms. It's a neutral piece of metal. There's the same number of protons as there is electrons. But these electrons are a little bit loose. So if I put a plus over there, that's this situation right here, where a plus is attracting a minus. So an electron is gonna sort of wander over this way and go like that. And that's gonna leave a net positive charge in this region. So these electrons are all gonna start moving in this direction. And down at the end, here, an electron is gonna come out of this battery, travel in here, and it's gonna 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 left, this battery would fill those in. And I'm gonna get a net movement of charge, of negative charge, around in this direction, like this. The question is, how do I measure that? How do I measure or give a number to that amount of stuff that's going on? So we wanna quantify that, we wanna assign a number to the amount of current happening here. 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 gonna stand right here. We're gonna keep our eye right on this boundary down in here. As we watch, what we're gonna do is, we're gonna count the number of electrons that move by here, and we're gonna have a stop watch and we're gonna time that. So we're gonna get, basically, this is charge, it's negative charge, and it's moving to the side. What we're gonna do is, at one little spot right here, we're just gonna count the number that go by in one second. So we're gonna get charge per second. It's gonna be a negative charge moving by. That's what we call current. It's the same as water flowing by in a river. That's the same idea. Now I'm gonna set up a different situation that also produces a current. And this time, we're gonna do it with water, water and salt. Let's build a tube of salt, of salt water, like this. We're gonna pretend this is some tube that's all full of water. I'm also gonna put a battery here. Let's put another battery. And we'll stick the wire into there. We'll stick the wire into there. This is the plus side of the battery, and this is the minus side of the battery. Water is H20, and this does not conduct. There's no free electrons available here. But what I'm gonna do is, I'm gonna put some table salt in it. This is ordinary salt that you put on your food. 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 Na's with plus signs and Cl's nearby, really close nearly, with minuses. Let's keep it even. Now, when I dip my battery wires into this water, what's gonna happen is, this plus charge, this plus charge over here from the battery is gonna attract the minus Cl's. So the Cl's gonna 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. That's going to attract this, and it's also gonna repel Cl minuses. So what we get is a net motion of positive charge, plus q going this way, and we get minus q going this way. 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. What we're gonna get is some sodiums, Na's, moving this way, and chlorines moving this way. Just like we showed here. Na moving this way. So there's gonna be plus charges moving through the boundary and minus charges moving through the boundary in the opposite direction. 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 this way. Because they have opposite signs, they add together and make two charges. In this case, current is equal, again, to charge per second.