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Anderson Video - Electric Current

Professor Anderson
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Okay. Let's think about a wire. What does a wire look like? A wire looks like this. It's a big chunk of metal cylindrical in shape. We've got these charges that are going to move down the wire. If I think about the cross-sectional area A of this wire. Then what can we say about this current I that's flowing in the wire? The current I is equal to the charge crossing that cross-sectional area A in some unit of time. Okay, all that charge is going to have to go through that cross-sectional area A in some amount of time. If you increase the amount of charge doing that you increase the current if you decrease the amount of time it takes to do that you increase the current. Now, when you look at this wire you look at these things that I just drew there's some sort of charge carrier. But what we really know is a wire has atoms in it. That have protons and electrons. And so there is some positive charge here and there is some negative charge right next to it. And when I apply a field to this thing to generate this current it's actually the electrons that move. Okay? The positives stay where they are. The electrons are the things that move. And here's the deal. If the current I is to the right then the current I here is still to the right even though the electrons are moving to the left. Okay, so the electrons move opposite the current direction. And that's because they are negatively charged. Right, Q for an e minus is negative. And so a positive current to the right is really the same as negative electrons moving to the left. So whenever we draw current you're always talking about which way do the positive charges go. Even though in reality we know the positive charges more or less stay where they are the electrons move this way. Now, if electrons are moving to the left there ends up a net positive charge over here on the right. And so it's effectively like those positive charges move to the right.