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Anderson Video - Ohm's Law

Professor Anderson
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 All right, let's talk about one of the big ones for dealing with circuits which is of course Ohm's law. Georg Simon Ohm was a German, lived from 1789 to 1854 and he came up with the law that we now of course call Ohm's law which is simply this: V equals IR. Okay, V is of course volts. I is current, which is amperes. R is ohms. And the way we write this stuff is capital V for volts, capital A for amps, and the Greek letter for Ohms right there, just capital Omega. All right, let's try a very simple Ohm's law problem and it's the light bulb problem. And we're going to say that we have a light bulb that's connected in a circuit. Okay, a light bulb and incandescent light bulb is like a very thin resistor. Okay, it's a wire essentially that's in a glass housing that is in a vacuum, and so the thing won't oxidize, and when it gets hot it emits light. Okay, so it looks just like a resistor even though there is this glass housing around it. This is our light bulb, and when you get that little filament hot enough, it starts to emit light. Okay. If you puncture a hole in the light bulb, breaks a vacuum, the filament burns out very quickly, but if you can keep a good vacuum in there, then it will last a reasonably long time. All right, let's say that this is a -- not a house light bulb but a flashlight bulb. Okay, so in your flashlight you've got a couple D batteries. A D battery also has a voltage of 1.5 volts but let's say we have two of them in a row. So we'll say V is 3 volts and the current that runs through it is maybe 400 milliamps, which is 0.4 amps, and let's figure out what the resistance of the bulb is. Current I, resistance R. All right, Ohm told us how to do that, piece of cake right? R is just V over I, and we know V, V is 3 volts, I is 0.4 amps, and that means we're gonna get a resistance of 7.5 ohms. Okay, which is pretty small, when you guys were playing with circuits in the lab you were dealing with hundreds of ohms, thousands of ohms, okay? A light bulb has a very low resistance because it's essentially a piece of wire. Okay, it's a little piece of wire that you run current through and that thing gets hot, you're not trying to necessarily impede the current, you're trying to get that thing to get really hot and so it will emit light. Let's try your clothes dryer, you have a electric clothes dryer at home, and let's ask the question how much current is going to go through the clothes dryer. Now electric clothes dryers are typically run off of the 240 volt supply and the resistance is on the order of 11 ohms, and let's ask the question what is the current that's going to your clothes dryer? Alright, no problem. I is V over R and we have those numbers, so this is 240 volts divided by the resistance, 11 ohms, and if you punch in those numbers you should get 21.8 amps. Now that is a lot of current. Right, 21.8 amps. So when you think about the current that's going into your house, right, we had a hundred amps that was going into our house, but now on one device you're using 1/5 of that, 21.8 amps, and when the power comes into your house it goes into this big metal box, right? Here's your house and the power comes in from the line and then there is a big box, which is the first thing that it attaches to, this is the power line. Okay, and then inside your house somewhere you have the clothes dryer. Yeah, question? >> (student speaking) Does it work the same way if the power line is in the ground? >> Yeah, same idea, doesn't really matter if it's above ground or underground. Typically in older neighborhoods it was all above ground, in newer neighborhoods it's underground. It just depends on where they put the power lines and it turns out to be a little cheaper to put them above ground than to dig all these big trenches underground, but it's a little bit nicer for the community and ultimately safer if they're under the ground, right? Because if they're under the ground you don't have these big telephone poles that can get blown over in hurricanes and, you know, injure people or animals or things. So, when the power line comes into your house, right? The first thing that it hits right here, that is called the circuit breaker or the breakout box, and usually it's outside next to your garage, and in there there's a whole bunch of switches. What are those switches do? Every now and then you have to go flip one of those switches, right? You say oh we tripped the circuit breaker, and you go out there and you flip the switch and stuff turns back on again. What is that there for? Anybody know? Your clothes dryer is trying to draw 21.8 amps but let's say the clothes dryer malfunctions and it starts to draw four times that amount 40 amps. If it does that, bad things can happen. Namely the wires that are going to the clothes dryer can get hot and burn up, and if they do that then your house can burn down and so to prevent that they put circuit breakers right here, which is essentially a little safety switch. And what the circuit breaker does is I'm gonna let through up to 25 amps or 30 amps but I'm not gonna let through 40 amps or 50 amps, and if it does that if it tries to draw too much current the circuit breaker flips and it disconnects the supply, and so it immediately cuts off the electricity. And so for this number right here your circuit breaker value will be 25 to 30 amps. So it allowed this much current but if any more comes through, any more than 25 or 30, then it closes it off. And that's what you're doing when you go reset it, you're just resetting the switch and saying okay I fix the problem I'm gonna do it again. If that circuit breaker keeps flipping a lot then you probably need to like look at the problem a little more closely, maybe hire an electrician say no this thing keeps flipping -- keeps flipping back and forth. Okay, what you don't want to do is just tape it close and hold it shut, alright. All right, let's take a five-minute break and then we will be back here in just a few to continue this discussion.