Learn about Magnetism 10: Electric Motors Video

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Learn about Magnetism 10: Electric Motors

Where I left off in the last video, we saw that if we had a magnetic field coming in from the right and we had slip loop of—I guess we call it metal or a circuit and it’s carrying a current. And this is where the current is coming in this direction. You could imagine positive protons although we know the electrons go to the other direction but the current is coming in this direction then going out that direction. We figured out using the right hand rule and just this formula that the net force of the magnetic field coming in this direction on this arm of the wire or the circuit is net downwards, right? And on this arm, it was net upwards. And so it provided a net torque on this circuit or as I said in the last video or paper clipping where this dotted line is the axis of rotation. Then this is how I showed you, it would rotate, right? Where the magnetic field is essentially pushing up on the right hand side and pushing down on the left hand side. It has no effect over here on the top and the bottom. So, it would rotate in this direction. And then this was kind of what it looks like after it rotates a little bit and when I—the whole reason why I did this—I said, “Well, this arm which is the same as this arm; the net force is still upwards, out of our screen but that upwards direction is now no longer going to be completely perpendicular to the moment arm distance; that’s the moment arm distance. Now, the moment arm distance is kind of coming out an angle out of the page, so only some of this net outward force where the magnetic field is going to be perpendicular to the moment arm. And so, the torque on, it will be left but it’s still going to be torque in that same direction; kind of coming out of the page on the right and into the page on the left and the same is true of the left hand side. And then you all the way to the point that the coil is actually vertical where this side right here is on top and this side is on bottom below the plane of your video screen and at that point, the torques actually—there is no net torque and why is that? Because on this top part when it’s pointing straight out at you; when it’s right here, the magnetic field, the force of it. The force that it’s affecting the circuit is pushing straight up. So there’s no longer any net torque, right? Because the force is pushing straight up and that moment arm distance; this distance is now also pointing straight up and torque is also across product, so you actually care about the perpendicular forces. So there, at this vertical point, there is no net torque and the same is true at the bottom of the circuit because at the bottom, the magnetic field force is going to be downwards which is parallel with the moment arm distance. So there is no net torque. And I say, “Well maybe, there’s a little bit of angular momentum that keeps this object rotating. And then it will rotate to—and let me—and this is where it gets interesting. I’ll draw it neatly. Then it will rotate to this point. Once again, I want to have the perspective. Its perspective to rotate here, so let me just make sure I have— So here, it was rotating in this direction and in that direction, right? And then here, maybe some—there’s no longer any torque on it but it still might on the top be moving to the left and on the bottom moving to the right, up to a point that is going to get into this configuration where this side is—so it’s rotated at this point. It has rotated more than 90°. So this edge is now this edge. It had rotated from here all the way—it’s still pointing out of the screen. But if this edge is the same as this edge, now, this—the current direction is going to be like this, right? Because this edge has rotated down all—so it’s rotated from that position all the way to the back to this position. So the current is now coming—let me make sure and let me draw the right. The current is coming like that, like that, like that, going up here into the right up like that, right? So the current now in this left hand side, although, it was the former right hand side is still going in that upwards direction. So when you take the cross product, what is going to be the net magnetic field on that or the force of the magnetic field? Well, you do the same right hand rule; point your index finger up. Put your middle finger in the direction of the magnetic field. This is the palm. This is your other two fingers. Let me draw the fingernails just so—they’re painted finger nails, not that mine are. Then your thumb points upwards. So, on this side of the coil, we still have an upwards force. And if you do the cross products, we drew the right hand rule on the bottom side or the behind side—you could have even imagine it. You’re still going to have a net downward force. So now, all of a sudden, you could imagine the thing had rotated so, it had rotated and the way I drew it here where it pops out on this side and it goes on that side. And I had done it all the way to the point where we had rotated more than 90° but now, all of a sudden, the net force through the magnetic field is going to reverse, right? Because the side that has a current going upward is now the left hand side. So now, the force of the magnetic field is out on this side and you’re going to want to rotate in the opposite direction. Hopefully that makes sense. Just think about what happens. Visualize this coil rotating. So what essentially is going to happen is, you’re going to rotate like I did here on the top. Maybe once you get to this level, you’re going to have a little bit of angular momentum that will keep you rotating or rotational inertia that will keep you rotating until you’re in something like this configuration. Maybe you go all the way back to this configuration where it’s essentially complete 180° turn, right? And then—the since—on this side, the current is going to be going up and on this side of the current is going down because you’ve essentially flipped this thing over. Then the effect of the magnetic field is going to say, well upwards on the left, downwards on the right, right? And so it’s going to turn the other way. So, if you think about it, it’s going to keep oscillating. Let me draw it from a—well I don’t want to draw it from that angle because I don’t want to confuse you. So we have a problem. If we wanted to turn this into some type of electric motor and keep it spinning, we would either have to reverse the current once you get into this configuration or either turn off the magnetic field or maybe you could reverse the magnetic field to get it going in the other direction. And actually you have anther problem which is a slightly lesser problem is if this was a circuit and you just kept turning over and over to the circuit, the wires would get twisted here, so you couldn’t do it indefinitely. So the solution here is something called a commutator. So let me draw a commutator. So what if—let me—I have—let’s see, the same circuit which I have not drawn messier. But it has these two leads. It has these leads that essentially curve. You could imagine them curving out of the page. And then we have a circuit. You could imagine leads here too. And this round thing in this thing are—they’re touching each other the whole time. So current could pass through it, right? Let me draw my battery. This is positive and this is negative. So up here on the circuit, the current is always going to be flowing in this direction. It’s always going to be flowing in this direction. It’s always going to be flowing up and like this, right? Now, when you’re in this configuration, what’s going to happen? Well, the current is going to flow down here. That’s going to be I and that is going to be I. And when you do your right hand rule, we have the same magnetic field. I have changed the magnetic field. It’s coming in from the left. So just like we did before I clear the screen, you use the right hand rule and you’ll figure out. Well, the net force of the magnetic field is going to be upwards here and downwards here and that’s what going to create that net torque and then you’re going to rotate this part. So this part of this contraption is going to rotate. You could imagine maybe there’s like a little pole here. Many it’s a non conducting pole so that none of that—you know and it’s connected to an axle somewhere. So you could rotate along that axis, right? So, the force of the magnetic field is going to create a torque. We’re going to rotate up on this side, out of the page on that site and into the page on that side and then behind the page and then back out of the page, right? That is what the net torque would be. And then we would get it—and we would keep doing that until you get to the vertical configuration. So, at the vertical configuration, this is the circuit on the top stays exactly the same. So at the vertical configuration—and I am trying my best to draw this properly. And the vertical configuration; one of two things can happen and probably the best thing is that we actually lose contact with the two leads. So maybe the actually current stops flowing when we’re in the vertical configuration. I’ll do it in the same color. So when we’re vertical, we just see—you know the top. We see this and then we see—you know it pops out a little bit and then we see this arm right there and then we see that pole that’s maybe holding it or that it’s helping it rotate, all right. But we’re still having some—you know the current has seize, so there’s not going to be any torque, no force through the magnetic field because we’ve lost touched of that point, right. Because these things kind of point out. Hopefully you could visualize how to build such a thing. And we’re still rotating in this direction because of some type of rotational inertia. Then, this is what the interesting part is, what happens when we rotate more than 90°? And I just realized that I am pushing over ten minutes, so you can think about that a little bit while I stop here and continue this in the next video. See you soon.

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Where I left off in the last video, we saw that if we had a magnetic field coming in from the right and we had slip loop of—I guess we call it metal or a circuit and it’s carrying a current. And this is where the current is coming in this direction. You could imagine positive protons although we know the electrons go to the other direction but the current is coming in this direction then going out that direction.

We figured out using the right hand rule and just this formula that the net force of the magnetic field coming in this direction on this arm of the wire or the circuit is net downwards, right? And on this arm, it was net upwards. And so it provided a net torque on this circuit or as I said in the last video or paper clipping where this dotted line is the axis of rotation. Then this is how I showed you, it would rotate, right? Where the magnetic field is essentially pushing up on the right hand side and pushing down on the left hand side. It has no effect over here on the top and the bottom.

So, it would rotate in this direction. And then this was kind of what it looks like after it rotates a little bit and when I—the whole reason why I did this—I said, “Well, this arm which is the same as this arm; the net force is still upwards, out of our screen but that upwards direction is now no longer going to be completely perpendicular to the moment arm distance; that’s the moment arm distance.

Now, the moment arm distance is kind of coming out an angle out of the page, so only some of this net outward force where the magnetic field is going to be perpendicular to the moment arm. And so, the torque on, it will be left but it’s still going to be torque in that same direction; kind of coming out of the page on the right and into the page on the left and the same is true of the left hand side.

And then you all the way to the point that the coil is actually vertical where this side right here is on top and this side is on bottom below the plane of your video screen and at that point, the torques actually—there is no net torque and why is that? Because on this top part when it’s pointing straight out at you; when it’s right here, the magnetic field, the force of it. The force that it’s affecting the circuit is pushing straight up. So there’s no longer any net torque, right? Because the force is pushing straight up and that moment arm distance; this distance is now also pointing straight up and torque is also across product, so you actually care about the perpendicular forces.

So there, at this vertical point, there is no net torque and the same is true at the bottom of the circuit because at the bottom, the magnetic field force is going to be downwards which is parallel with the moment arm distance. So there is no net torque.

And I say, “Well maybe, there’s a little bit of angular momentum that keeps this object rotating. And then it will rotate to—and let me—and this is where it gets interesting. I’ll draw it neatly. Then it will rotate to this point. Once again, I want to have the perspective. Its perspective to rotate here, so let me just make sure I have—

So here, it was rotating in this direction and in that direction, right? And then here, maybe some—there’s no longer any torque on it but it still might on the top be moving to the left and on the bottom moving to the right, up to a point that is going to get into this configuration where this side is—so it’s rotated at this point. It has rotated more than 90°. So this edge is now this edge. It had rotated from here all the way—it’s still pointing out of the screen. But if this edge is the same as this edge, now, this—the current direction is going to be like this, right? Because this edge has rotated down all—so it’s rotated from that position all the way to the back to this position.

So the current is now coming—let me make sure and let me draw the right. The current is coming like that, like that, like that, going up here into the right up like that, right? So the current now in this left hand side, although, it was the former right hand side is still going in that upwards direction. So when you take the cross product, what is going to be the net magnetic field on that or the force of the magnetic field?

Well, you do the same right hand rule; point your index finger up. Put your middle finger in the direction of the magnetic field. This is the palm. This is your other two fingers. Let me draw the fingernails just so—they’re painted finger nails, not that mine are. Then your thumb points upwards.

So, on this side of the coil, we still have an upwards force. And if you do the cross products, we drew the right hand rule on the bottom side or the behind side—you could have even imagine it. You’re still going to have a net downward force.

So now, all of a sudden, you could imagine the thing had rotated so, it had rotated and the way I drew it here where it pops out on this side and it goes on that side. And I had done it all the way to the point where we had rotated more than 90° but now, all of a sudden, the net force through the magnetic field is going to reverse, right? Because the side that has a current going upward is now the left hand side. So now, the force of the magnetic field is out on this side and you’re going to want to rotate in the opposite direction.

Hopefully that makes sense. Just think about what happens. Visualize this coil rotating. So what essentially is going to happen is, you’re going to rotate like I did here on the top. Maybe once you get to this level, you’re going to have a little bit of angular momentum that will keep you rotating or rotational inertia that will keep you rotating until you’re in something like this configuration. Maybe you go all the way back to this configuration where it’s essentially complete 180° turn, right?

And then—the since—on this side, the current is going to be going up and on this side of the current is going down because you’ve essentially flipped this thing over. Then the effect of the magnetic field is going to say, well upwards on the left, downwards on the right, right? And so it’s going to turn the other way.

So, if you think about it, it’s going to keep oscillating. Let me draw it from a—well I don’t want to draw it from that angle because I don’t want to confuse you. So we have a problem. If we wanted to turn this into some type of electric motor and keep it spinning, we would either have to reverse the current once you get into this configuration or either turn off the magnetic field or maybe you could reverse the magnetic field to get it going in the other direction. And actually you have anther problem which is a slightly lesser problem is if this was a circuit and you just kept turning over and over to the circuit, the wires would get twisted here, so you couldn’t do it indefinitely.

So the solution here is something called a commutator. So let me draw a commutator. So what if—let me—I have—let’s see, the same circuit which I have not drawn messier. But it has these two leads. It has these leads that essentially curve. You could imagine them curving out of the page. And then we have a circuit. You could imagine leads here too.

And this round thing in this thing are—they’re touching each other the whole time. So current could pass through it, right? Let me draw my battery. This is positive and this is negative. So up here on the circuit, the current is always going to be flowing in this direction. It’s always going to be flowing in this direction. It’s always going to be flowing up and like this, right?

Now, when you’re in this configuration, what’s going to happen? Well, the current is going to flow down here. That’s going to be I and that is going to be I. And when you do your right hand rule, we have the same magnetic field. I have changed the magnetic field. It’s coming in from the left.

So just like we did before I clear the screen, you use the right hand rule and you’ll figure out. Well, the net force of the magnetic field is going to be upwards here and downwards here and that’s what going to create that net torque and then you’re going to rotate this part. So this part of this contraption is going to rotate. You could imagine maybe there’s like a little pole here. Many it’s a non conducting pole so that none of that—you know and it’s connected to an axle somewhere. So you could rotate along that axis, right?

So, the force of the magnetic field is going to create a torque. We’re going to rotate up on this side, out of the page on that site and into the page on that side and then behind the page and then back out of the page, right? That is what the net torque would be.

And then we would get it—and we would keep doing that until you get to the vertical configuration. So, at the vertical configuration, this is the circuit on the top stays exactly the same. So at the vertical configuration—and I am trying my best to draw this properly. And the vertical configuration; one of two things can happen and probably the best thing is that we actually lose contact with the two leads. So maybe the actually current stops flowing when we’re in the vertical configuration. I’ll do it in the same color.

So when we’re vertical, we just see—you know the top. We see this and then we see—you know it pops out a little bit and then we see this arm right there and then we see that pole that’s maybe holding it or that it’s helping it rotate, all right.

But we’re still having some—you know the current has seize, so there’s not going to be any torque, no force through the magnetic field because we’ve lost touched of that point, right. Because these things kind of point out. Hopefully you could visualize how to build such a thing. And we’re still rotating in this direction because of some type of rotational inertia.

Then, this is what the interesting part is, what happens when we rotate more than 90°? And I just realized that I am pushing over ten minutes, so you can think about that a little bit while I stop here and continue this in the next video. See you soon.

Transcription by: Scribe4you Transcription Services