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Anderson Video - Electric Circuits Intro

Professor Anderson
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Hi class. How's everybody doing? Have a good weekend? Yeah? Anybody watch that World Cup game? It was, uh, was a good game very very exciting game very very neat and tidy. I wished it was a little more "Messi." [Laughter} Yeah I know, it's early. [Laughter] It's Monday. All right, let's talk about electric circuits and current. Some of you guys have already started playing around with electric circuits in your laboratory courses so you'll hopefully be familiar with a lot of this stuff. But the the idea with an electric circuit is this. You're going to put some sort of battery V and typically anytime you write a battery you always make the long side the positive side the short side the negative side okay so this is what a battery looks like in a circuit diagram long bar and short bar You have a metal wire. There is a resistor which looks like that. And that makes a very simple circuit, okay this is the most basic circuit that you could have. This portion of the circuit is a metal wire. Okay, everywhere you have this straight line that's a metal wire. When we get to capacitors we'll have a different circuit element when we get to inductors we'll have a different circuit element but for now we have two. We have a battery and we have a resistor and they are connected by metal wires. Now, one thing that you've been working on in your labs is what are the voltages at different points around the circuit. And when you do that remember about metal wires. What is a metal wire? A metal wire is a piece of metal. Literally it's a long thin strand of metal. But as we learned in class metals are equipotentials they have the same potential everywhere. So the potential anywhere from this side of the battery all the way around to this side of the resistor is exactly the same. That potential is the same along that wire. Likewise on the bottom side here from the negative side of the battery all the way to the bottom side of the resistor that potential is also equipotential it's constant around there. Okay, so when you were doing your lab experiments it doesn't really matter if I take the voltage there or I take the voltage there or I take the voltage there or there. Anywhere along that wire I'm going to measure the same voltage. Likewise along the top wire. Okay. What happens in this circuit? Well, in this circuit the battery is just like a capacitor. Okay you have a bunch of positive charge on one side you have a bunch of negative charge on the other. And if I put a bunch of positive charge on this side and i put negative charge on this side it's going to do something to the charges that are in the wire. It's going to push on them. Okay and so it's going to generate a current. Current flows in this circuit. Now you might think that all these positive charges on the top of the battery would push positive charges around. But that's not what actually moves. What actually moves in a metal is electrons and so we'll talk about the sine of current in a second. Okay. But what is this current? Current I We write it with a capital I is just how much charge you move in how much time. It's Delta Q over Delta T. How much charge moves in a given amount of time. Okay so before we were always talking about statics right, nothing was moving. Now we have movement of charge charges moving around in the circuit. What are the units on current? The units on current are Amperes and we write it with a capital A. Okay, you can also say Amps. All right, we just watched Apollo 13 yesterday with my daughters and my oldest daughter said, you know this is pretty good for an old movie. Like old? That's an old movie? They talk a lot about Amps in there because they were trying to figure out how to power up the spacecraft with a very limited amount of batteries available very limited amount of battery energy available so they always talked about well how many Amps is that going to drop how many Amps is this going to drop and so they had to get really creative in how they discuss that. What is an Amp? Well, it is apparently Delta Q over Delta T. But we know what that is. Delta Q is coulomb. Delta T is seconds. Can you guys still see that over there on the screen? Okay good. Coulomb per second is an Amp. One Amp equals one coulomb per second. All right so when we think about a wire now let's think about what is actually happening in this wire.