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Anderson Video - Spark Plug

Professor Anderson
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All right, let's talk about something relevant to your everyday life which is the following. Let's say I take a piece of metal. And I take another piece of metal. that are shaped kind of like that and I house them in a big piece of ceramic and now I charge up these pieces of metal. Okay, and I'm gonna put a whole bunch of negative charge on this piece and I'm gonna put a whole bunch of positive charge on this piece. Okay, and this is some sort of ceramic can that is holding them so it doesn't conduct any electricity it's an insulator. Ceramic. And these things are both metal. So I got this funny-looking funky looking piece of metal here I've got another piece coming out and I've got a whole bunch of positive charge on this one a whole bunch of negative charge on that one. Is there an electric field in this region between the two? Sure. Electric fields leave positive charges and go to negative charges. Okay. And if we think about the electric field in there let's say the following. Let's say that the electric field E has a strength of 2.8 times 10 to the 6 volts per meter. And let's say that the gap distance here D is 0.75 millimeters. Let's calculate what Delta V is required to generate those fields. All right, how do we do it? Well, we go back to our relationship for E. E was negative Delta V divided by the distance D. And so Delta V is going to be negative E times D. And I have those numbers, right? E we said is 2.8 times 10 to the 6 volts per meter. D is 0.75 and it's millimeters so we have a times 10 to the minus 3 meters. The meters cancel out and we're going to end up with volts. And I've already run these numbers so I'll just tell you what we get. We get 2.1 times 10 to the 3 volts which is 2.1 kilovolts. That seems like a pretty big number right? 2,100 volts. When you think about volts and you think about voltages that you're used to you can talk about the voltage of a AA battery it's 1.5 volts. A 9-volt battery is obviously 9 volts. Your wall plug coming out of the wall has an RMS voltage of about 115 volts. And now we're talking about 2,100 volts so that seems like a lot and it seems a little dangerous and yet this sort of device you are all extremely familiar with. And you just don't know it yet. What is this thing that I just drew? Does anybody out there know? Or anybody watching know? What is this device that I just drew? Any thoughts? Catherine, what do you think? Okay. I can guarantee Okay, that's a good guess a water heater right? Maybe if we put some water in there we could do something with it. That's a very good guess. I can guarantee you that you have come across this thing certainly in your lifetime likely today, you were very near this device at some point today and you likely will be near this device again on your way home today because this is a device that's where? In your car. This is something that is in your car. Let me draw it slightly differently and we'll see if you can guess what it is. Okay, that ceramic piece is actually shaped a little bit differently. There are some threads up here. And then the ceramic comes down like this. And then there's an electrode coming out the bottom. Not quite to scale but that's a little bit better. Anybody know what this device? Something in your car. What is it? It is a spark plug. Okay it's not the best drawing maybe that's why you couldn't guess it but this is a spark plug. This is a ceramic part of the spark plug. It's sort of a white material. And then it has these two electrodes that are coming out the end. There's a center cylinder and then there's an outer sort of bent piece of metal that comes up like that and there's a very small gap. Right, this is the gap distance D which is 0.75 millimeters. And the whole point of the spark plug is to do what? It's to make a spark. If you can generate enough electric field in between those two electrodes it will in fact generate a little spark. Just like we talked about when you're walking across the floor, the the carpet and you touch the doorknob and you shock yourself. If you do it in the dark you will see a little tiny spark right there. And the reason that happens is the electric field gets so strong that it starts to pull electrons off of other atoms and molecules that are floating around. And so in a spark plug there is a combination of gas and oxygen and when you apply this electric field and the electric field is high enough it creates a little spark. And it ignites that fuel air mixture and creates a little mini explosion right there and that explosion rapidly expands the gas pushes down on the piston attaches to the rest of the crankshaft and so forth to drive your vehicle. And so there's a little piston that goes up and down and every time it comes up the spark plug fires off. Okay? Now, to do that you have to apply a voltage to those electrodes. And it turns out that voltage is pretty big. Right? That's pretty big voltage. What's the voltage of the battery in your car? It's 12 volts right? That big old honking battery in the front of your car it's only 12 volts. So there's something else in the car that takes that 12 volts and generates kilovolts out of it and that's called your distributor. And the distributor ramps up that 12 volt voltage all the way to thousands of volts and that creates a spark. And this is why when you're working on your car you don't want to just grab on to the spark plug cables and turn on the engine. That's not a good thing to do. Because there are thousands of volts in those cables. And you likely won't kill yourself but you could certainly hurt yourself a lot. Okay, so when you're working on your car make sure that the spark plug and the cable is grounded. And you'll see this sometimes, when people test out the spark plug they will pull out the spark plug with the cable attached and then they'll ground it to the car. That means connecting this metal electrode to the car and therefore when you turn on the car it will go back into the ground system of the car. It won't shock you and then you can see if the spark plug is working and literally you can look down there and just see a little spark go, "boop boop boop boop boop." Kind of cool.