12.6 Motors

Okay, today's lesson we're going to look at how a motor works and what happens when we have more than one wire in a magnetic field. But before we get to that, we're going to start with just the basics here. So, what happens if we have two wires with current? They're both going to create a magnetic field, so there's going to be some sort of interaction between the wires. We know that in any kind of way, alright? So, let's just start with a simple one. If we have a wire that's coming out of the page, we can use our left-hand rule to see that our magnetic field then would go around it in a clockwise fashion. So, if we have one wire doing that, the question then is: what happens if we have another wire doing the exact same thing nearby? Those two magnetic fields are going to interact, and it can be dangerous in some cases, like an airplane. If those things are creating a force that's not good, it could cause some problems. So, we have to be really careful. With the two or more wires carrying current, André Ampère later called it Amps. He did a bunch of research and figured out that there were two key things: the length of the wire and the distance between them. The longer the wire, the stronger the force between them. Similarly, if the wires are closer together, the force between them will be stronger. Additionally, the amount of current in the wires determines the strength of the magnetic field, which in turn creates a stronger attraction or repulsion. Ampère came up with a law to calculate the amount of force, but for Physics 30, we don't need to determine the exact force. We just need to know whether the wires will attract or repel each other. If we have two wires going in the same direction, the magnetic fields around them will be clockwise. Looking at a sketch, we can see that where the wires are near each other, one magnetic field is going down while the other magnetic field is going up. The magnetic field is going up. If we think of the magnetic fields as being like a north-south magnet, we know it goes from north to south. So, that side would be the magnet, something like that, and we'd have North and South kind of like that. So, you can see that the two magnets are in opposite directions. If they're opposites, they attract. So, even though the currents are running in the same direction, the magnetic field is opposite, which then would attract. So, any two wires that are either both in or both out, the force between them is going to attract. Okay, we can use the same logic for the reverse. The one wire going in one way or going out, in this case using our left hand, the rule is that this one would go clockwise, while the outward one would go counterclockwise. And then, using the same logic, we'd have a north-south magnet. Matching up so those would be the same, so they're going to repel. Okay, so there are a few different ways of kind of thinking about this or logically doing it, but that's part of the simplest way. Just think of if our magnetic fields are opposites, they're going to attract. If they're the same, they're going to repel. Okay, so what we're going to look at now is what happens if we put a wire with the current in a magnetic field. We already know that there's going to be a force acting on it using our left-hand rule. So now, what happens if there are two wires in the magnetic field? What we were getting to is the basis of creating a motor, an electric motor. So a galvanometer is just a simple electric motor where they both kind of be the same thing. But a galvanometer only rotates part ways, so it's like a voltmeter or some sort of device. If we want to measure it, it would kind of look like a speedometer. It just goes from one side to the other. However, for a motor, we need it to spin around and around and around as long as we want. So it has to be able to. Spin all the way around, but the concept behind them is both pretty much the same. So, we're gonna draw a simple diagram here and figure out what's going to go on with a wire inside of a magnetic field. In this diagram, we've got the current going up, so it's going up in the order of BCDE. If we use our left-hand rule, let's look at point B. We know our current is going upwards, so we use our thumb going up. Our magnetic field would go from north to south, and our finger at B would go to the right. That means our force acting on wire B should be out of the page, so that length of wire B is gonna come upwards. If we do the same thing at D, you can see the current is gonna go around. Now, our current is going downwards, so we put our thumb down. The finger at B still goes to the right, but now our force finger is gonna be into the page. Based on those two wires, you can see one is going to come upwards, and one is going to go down. At points A, C, and E, those wires are parallel to each other. The magnetic field, and remember, they have to be perpendicular. So there's going to be no force acting on any of those wires, they're just going to be doing nothing. So in our loop or a coil of wire, B is going to come up, D is going to go down. So what's that going to do? It's going to cause that thing to rotate. So this whole loop of wire is going to, in this case, rotate in sort of a clockwise fashion. Okay, so let's before we continue, let's kind of re-examine this picture now that what's going to happen once it rotates. So if B spins upwards and D is spinning downwards, what's it going to look like once those things rotate 180 degrees? Let me kind of re-sketch the diagram here. So our B is now going to be here and our D is now going to be here. So you can see the way the current is, it kind of twists in the middle here, but we're still going in the same directions. So that means it's gonna go up, and it's still gonna be going up through B. Come around and go down through D. Our magnet doesn't change, so now if you do our left-hand rule, it's gonna be the same except at Point D. Now it's going downwards, and at point B, it's gonna be going upwards. So you can see what's gonna happen is it's going to reverse. Once that loop spins 180 degrees and flips around, then the forces are going to be opposite. So then it's gonna spin back and forth. So a galvanometer doesn't stop, but it would just kind of flip back and forth 180 degrees. That's why a lot of galvanometers only go to 180 usually, and that's it. We don't go all the way around. So a motor, then we basically have to figure out how we can get this thing to spin around and around without flipping back and forth. So before we do that, let's do a quick example. This one is, we have a. galvanometer tells us our length of wire is three point eight centimeters, and inside the galvanometer barometer, the spring is going to get pushed a certain distance. So, in this case, what we want to do is to see, well, we have a magnetic force. We know that it's il because that part's pretty easy. We have our current. The question is asking what is the current, so we have il be. And then that magnetic force inside the meter, if you kind of imagine like a little speedometer tip, a meter, that force is going to push that needle over. So, that magnetic force then is going to equal to our force acting on the spring. So that means our force on the spring, if you remember from physics 20, was KX. So, all we have to do is put those two forces equal to each other and we can solve it. So, our K of 87.6, our x distance is point zero zero five five. We're looking for I dal is three point. Eight centimeters, 0.038, and V is 1.25. We just need to rearrange and solve for I. When we do that, we will get 5.1 amps. It's a simple example, similar to what we did before balancing forces and solving them. Our next topic is how to get a motor to work instead of a galvanometer. Let's go back to this diagram. So far, the galvanometer is just spinning back and forth. That's a problem. We need to get the force to go in the same direction to make it spin continuously. Looking at the bottom diagram, we don't want the force directions to change. We want to keep one going downwards and the other going upwards. To achieve that, we need to flip the direction of the current. Our wires were twisted in the current one way, so there may be a way to... Somehow, always keep the current flowing to the left and around the loop. Then we should be in business. So, if every 180 degrees, the current changes direction, that'll flip the direction of the force to keep matching it the same over and over again. So, the trick for a motor then is to be able to flip the current every 180 degrees. What we do is, at the end, when you're just going to be sketched this diagram, we put instead of just having two wires, we use what we call a commutator. And that's when we use a split ring. I'll explain why we split it in a second. So what we do is we have this round ring on the end. We connect our positive and negative wires to that. So that's going to provide the current. But instead of the wires getting twisted and changing, this thing is just going to touch these. Two rings are used to pass the hurdle. So, the way it is now, our current is going around the loop in that direction. As the thing spins, the force of that thing will also spin. You can see the little split ring thing in this diagram starting to turn clockwise. When it reaches the 180 mark, the gaps in the ring line up with your wire, meaning that there will no longer be a current flowing through. The current will stop once it keeps going past that point. Then, you can see that it switches the wires. The round part of the loop that was touching the negative is now touching the positive, and the other one is the reverse. Basically, we will be alternating from positive to negative and back every 180 degrees. The reason we put the split in it is to stop it at 180, so that it doesn't continue. We want to be able to alternate, and the reason for this is to create variations in the flow of electricity. Keeps spinning is to do just the momentum. Once this thing is spinning, momentum will keep it spinning. So even though the current stops for a second, it'll continue in the same direction. It'll touch the ring on the opposite current, and that will cause it to keep on blowing. So that's pretty much the only trick to a motor: we just need to be able to flip the current back and forth every hundred eighty degrees, and that'll keep the motor spinning around. We can use that then to turn things like a lawnmower blade, wheels on our car, or any other kind of motor device. And that's all.