12.3 1st and 2nd Hand Rules

All right, in this lesson, we're going to look at what happens when we have electricity and magnetism working together. So, we already know that there are some similarities between electric fields and magnetic fields. They are very similar, and we know that from the domain theory that if we have an electron spinning or any electron moving, that's going to induce a magnetic field. So there's got to be some sort of connection between the two. What happened in 1820, Hans Christian Oersted did a bunch of experiments to show that electric currents and electric flow could actually create a magnetic field and it would affect the magnet. He actually stumbled upon this by accident, in stories that I read. So, basically, if we imagine we just have a wire, some sort of wire, and we have current flowing through the wire, what Oersted noticed was just by fluke, he had this wire set up and happened to have a compass nearby, and he saw that the compass needle moved every time he turned on the current. So when he hooked it up to a battery and we've got electrons flowing through the wire, he noticed that the compass needle moved depending on where it was located. So what he did was, if he put the compass needle on top of the wire, he got that the magnetic field, the compass pointed to the left. And then he found that if he put the compass underneath the wire, on the bottom of the wire, he saw that it pointed to the right. And then when he put the compass to the right of the wire, nothing happened. But if he turned the compass so it was sort of perpendicular, he saw that the compass ended up pointing upwards or out of the page (we're going to get to these symbols in a second), and then the same thing on the other side, he saw that it went downwards. So, as he went around the wire, the compass needle sort of changed accordingly. So what he concluded was that the electric wire created a magnetic field, and vice versa. If he took the electric wire and switched it around so that the current went the other way, what he found out was that the magnetic field switched directions as well. So he was the first to sort of connect the current to a magnetic field. We're going to do a bunch of different hand rules to help us figure this stuff out. So the first rule is going to be how do we know which hand to use? We're going to use our left hand for anything that has to do with negatives - electrons flowing, current, or basically anything in general that's negative. If we're dealing with something that's positive, like protons moving or conventional current, we use our right hand. So we're going to use two hands - one for positive and one for negative. The second thing we've got to do is we just have to be careful with our directions. We're always going to use directions as referred to our paper. So if you're drawing on your paper, we would say the top of the page is north or up. Right (paper being the top of your page), the bottom of the page would be south or down. And we'd say the right of our page is east or to the right, and then the other side would be left or west. So we're always going to do it in reference to drawing it on paper. If you were to draw up, that would be our north. Left would be west, so on. But then we also have to worry about three-dimensional directions because now with magnetic fields being perpendicular, we have to worry about three-dimensional directions as well. So what we're going to say is coming out of the page, so upwards (not up), but coming out of the page towards you, we're going to use the symbol ">" Circle with a dot in the middle - okay, so that's upwards or out of the page, or towards would be another way of thinking about it. And then we're going to use an X to show into the page or away from you. The reason they use a dot and then X is sort of representing an arrow. So if you were to shoot an arrow at somebody, the dot would be coming towards you. Right, you'd see the arrow coming towards you to see the point of the arrow. And if we shoot it away from you, you can see the back of the arrow, the feathers of the arrow, which would be the X. Okay, on this page, I've got these one, two, three, four members listed. This is off a diploma, and this is how sometimes they'll test you on direction. So they might give you a diagram and say, "What is the direction of the magnetic field at a certain point?" And you'd figure out the direction and your answer would be 1, 2, 3, 4, 5, 6, 7, 8, 9, or 0, depending on whichever direction it would be. The only thing we have to be careful of is we don't want to use the word "up" as coming over the page or "down" as going into the page. You want to just keep it simple as being "up" and "down" as being north and south, and then "in" and "out" is being into the page or to the page. Okay, so our first hand rule then we're gonna use to figure out how it works. Ted did his calculations or came up with his law rule. So if we have a straight wire that's going to the right. So that's what our current's flowing to the right. We're gonna use our left hand to figure out which direction the magnetic field is because it's a negatively charged current. So what we want to do is we want to point our thumb in the direction of the current. So we're going to point our thumb to the right. And then they form a new image like grabbing onto this wire, our fingers would curl around it. So our fingers would go sort of upwards and around the backside of it. Okay, so that's gonna indicate our magnetic field. So if it's negative and goes to the right, our fingers curl around it. So our magnetic field goes up and over the wire. It's at the top of that page. Our magnetic field would be inwards, right? So if we're going straight above the wire, our magnetic field will be going into the page. At the bottom, it'd be coming out towards us. And right in front of the wire, it would be going up. I'm in the back of the wire, it'd be going down. So you get the idea of how it's kind of going circular around them. So the example at the bottom of this page works the same way, except our wire now is going interrupt. So our wire is the circle, and it's showing that the current is going into the page. So you point your left thumb down into the page, and our fingers then curl around it in a counterclockwise kind of direction. Okay, so that means then for this question of saying what is the direction of the magnetic field immediately to the right. For at the right, our magnetic field is going up. Above the wire, our magnetic field is going to the left. If we were going to the left of the wire, it'd be going down. And at the bottom of the wire, it would be going to the right. So you see this going around in a circle in a counterclockwise fashion. The second hand rule is basically a modification of the first. And what it is, if we have a straight wire, we get a round magnetic field. So the opposite logic is, what happens if we have a wire that's going around in a circle? Then our magnetic field is likely going to be straight. So if you imagine grabbing onto the wire, sort of like being a steering wheel, so your left hand would be on the left side, generally. Your thumb is pointing up the way I drew the arrow. So you'd see that your fingers then would curl around the steering wheel, and they'd be coming out of the middle. Your fingers would go hope. Through the middle, we've had a magnetic field going upwards. If we changed it and grabbed it onto the top, if you move your left hand to the top of the stream wheel, your thumb pointing to the right, your fingers would still curl around so that they'd come up words. And if we moved our left hand to this right-sided sumo, you thought would be going down, but your fingers would still curl outwards. And at the bottom, it's kind of hard to do because you're on would be upside down, but you've got to get the idea or just sort of put your thumb at the very bottom, pointing to the left. The fingers would still come over. So you can see no matter where we are on the circle, if our thumb is pointing in the direction, our magnetic field is still up through the middle of the circle. So when we get questions where they talk about a coil of wire, and that's usually using it's a solenoid, and it's basically a solenoid is this the fancy name for an electromagnet. So if we run current through the wire, that's gonna turn that chunk of metal into a magnet because if we have all the magnetic field pushing the same direction, it's going to act just like the magnet. So quite often these kind of questions are going to ask which way is north itself for that magnet. So we usually draw the coil like a space though it's just so it's easier to see. So let's suppose for this one, our current is going downwards, so our wire would be going down. So we could use our first hand rule point our thumb downwards, our fingers would wrap around, and our magnetic field would go in this direction. And if we know our magnetic field is in that direction, that means we know that that end would be north, and the other end would be self because that's just the way north-south magnetic fields work. Okay, the second hand rule then, I like I said, is just a modification, and some people liked it better because it's just a little bit easier to manipulate your hand. So we just trade our fingers and our thumb around. So first-hand rule, our thumb was the direction of the wire current, our fingers were showing you the magnetic field. So for the second hand rule, we just do the opposite. Our fingers wrap around in the direction of current, so imagine wrapping your hand around the way the wire is going, and your thumb then points to North, or points in the direction of the magnetic field. Okay, so that's kind of what the second-hand rule does, is just says wrap your fingers around, your thumb points to the north. So either way, it really makes no difference how you want to figure it up. So let's do one more example. So let's suppose I have a cylinder of metal, and let's suppose we're going to wrap it around with wire. So we're going to put that wire around, I'm just going to draw one loop. Okay, so our wire goes around it like that, so it makes sort of like a steering wheel type shape. But in reality, a coil of wires would just wrap around and around and around, it go around hundreds of thousands of times. But we don't want to make it complicated, so we're just going to think of it as one wire coming down. So this picture, one wire wrapping around and going around in a circle. Let's suppose the current comes downwards, so in this case, whichever hand rule you'd use, but your current going downwards, and we'd see that our North Pole or magnetic field would point to the left, right, our thumb points to the left. So that means our magnet then has to be north on that end, too cell phone. Okay, if we have the same situation except our current was going upwards, if you grab your fingers upwards and your thumb at point Goselin so current going through wire is going to tell you which direction the North and South Pole of the magnet is or vice versa if I gave you a question where I tell you that we have a certain magnet direction. So let's suppose we draw our magnet north and South, like that, and put a wrap somewhere around it. So let's suppose our wire goes around the coil, something like that. And I'll draw a few of them this time. So, the question then would be: which way is the current going if that north is up? Which ways to print? So we can still use our same rule, just kind of do the backwards. So our current is going up, therefore our magnets are going up. So, I thought my point up and our fingers would show the direction of the current, which in this case would be around it like that. So, you can use both rules either way to help you figure out either the direction of the north or the direction of the current flow. And that's all for today!