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Current time:0:00Total duration:5:28

Video transcript

- [Voiceover] In the last video, we talked about the meaning of current, and current is defined to be the movement of charge, amount of charge per second. We looked at a copper wire where electrons are carrying the current, and we looked also at a salt solution where both positive and negative ions are carrying the current across this imaginary boundary where we keep track of how many charges are moving. So now, next we need to talk about how do we define a positive current? What does a positive current mean? So, I'm gonna move this up a little bit. And we're gonna get to something that causes some confusion. But, we'll try to clear it up here. The positive sign, this is a convention that we use. The positive sign for current is the direction that positive charges move. So, that's the direction... this is the direction a positive sodium ion moves. This is the direction a negative chlorine ion moves, When I assign a positive direction to the current, the overall positive direction is gonna be in this direction. That's plus current. Plus current. Alright, and the same thing actually holds for a wire. If I draw, let's go back and draw my wire again. If I draw my copper wire, and inside there we know that all these little electrons, all the electrons are moving in that direction. So q- is moving in that direction. And when I assign a direction for the arrow for a current, the current arrow points this way. Plus current. So for instance, if I had 10 electrons per second that were moving in this direction here, I would say the current is plus 10 charges per second in this direction. And ask yourself: If it's always electrons carrying a current, why on Earth do we point the arrow in the other way? And that actually goes back, way back in history, to out friend Ben Franklin. Ben Franklin is the person who said, who basically made a decision that said the charge on an electron is minus. Now, when he made that decision, nobody knew there was such a thing as an electron. So Ben Franklin. And that was in the year 1747. Ben Franklin's a person who assigned plus and minus as being the two types of charge that existed. And when he did that he had no idea. Nobody had any idea that there was such a thing as an electron. Electron was discovered by a physicist named J. J. Thomson, In the year 1897. That was when the electron was discovered. And believe it or not, if we do the subtraction here, look at this, it's 150 years. For 150 years of electric research, we were able to figure out a lot of really good things, assigning current going in the opposite direction of electrons in metal. And it's all okay. It's all okay. One of the questions that's often asked by new students, is why don't we switch it around so the current points this way? Why don't we assign an electron, why don't we make current go this way? So, why didn't we just switch it around? Well, we had a 150 years of experience, and now we've actually had another 120, since the discovery of the electron. And we've managed to get by with this. We have not chosen to do it. One of the reasons we don't do it is because it would basically, it would be a big mess. Imagine if for instance, say in the United States we decided, Oh, driving on the right side of the street is the wrong side of the street. We want to all drive on the left. Imagine the chaos that would cause. Now, before we made the change, everybody managed to get where we were going, and after the change, everybody would get there going, but the change over would be just so, so costly. And the same thing with electricity. We can do perfectly well talking about current going the direction of a positive charge, just like we did here with the positive sodium going in this direction. This is actually how electricity is conducted in your body. So, it's not uncommon to have positive charge moving around. That's the definition of current in a nutshell. We put a boundary across, and you watch the charges going through, in either direction, which ever direction they do, and you count up and you get q per second. That's current.