Anderson Video - Lenz's Law Example

Professor Anderson
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So let's do the following: we're gonna draw a region of magnetic field here, B, where it's everywhere pointing out of the page. Okay? When it's pointing out of the screen and we take a ring and we start to move it through, something's gonna happen. There's going to be current that develops in the ring. Now, before it gets to the magnetic field region, what's the current in the ring? No current, right? Current's zero. Okay? When it is in the center of this region and it's still moving to the right, What is the current in the ring? What do you guys think? What is it, ninety percent of the time when I ask you? Zero. The current's zero. Why? Because, the flux is not changing in this region. It's moving through a constant magnetic field -- a uniform magnetic field. So, the only time something interesting happens is right here where it crosses and right here where it crosses. Okay? And when it crosses there and it crosses there, the current is no longer zero. And now, we have to think about the flux. Okay? So, the flux is out of the page which means that the ring wants to make a current that is going into the page. Right? This helps if you look over there at the computer monitor instead of looking at me. Okay. So, B is coming out of the screen, but my ring wants to make something -- wants to make a field-- that's going into the screen. Okay? So, how does it do that? It does it like this: If you take your right hand and you put your fingers in the direction of the current, right? I'm going to wrap my fingers around in the direction of the current. My thumb is telling me that that will make a B field going into the screen. Okay. So, the currents going to develop in the ring, in that direction, And now, you know exactly what's going to happen on this side. The exact opposite is gonna happen. So over here, the current, in fact, generates is generated in that direction because the B field that it was in is decreasing. The B field coming out of the screen is decreasing but it wants to keep that going. It doesn't want the change. And so by putting a B field in this direction, take your right hand, wrap your fingers around, and now we got our thumb coming out of the screen. And so, it's making a current in that direction. Then finally, when it gets back out here and it's still moving along at V, then current goes back to zero. So the only time current develops, is at these boundaries. Anywhere it's uniform or zero, then there is no current that develops in the loop. Okay. Any questions about that? Did I get the right-hand rule right? Did everybody agree with what I was doing? Okay. I wrapping around this way gives me a B that's pointing into the screen. I wrapping around this way gives me a B that's pointing out of the screen.
So let's do the following: we're gonna draw a region of magnetic field here, B, where it's everywhere pointing out of the page. Okay? When it's pointing out of the screen and we take a ring and we start to move it through, something's gonna happen. There's going to be current that develops in the ring. Now, before it gets to the magnetic field region, what's the current in the ring? No current, right? Current's zero. Okay? When it is in the center of this region and it's still moving to the right, What is the current in the ring? What do you guys think? What is it, ninety percent of the time when I ask you? Zero. The current's zero. Why? Because, the flux is not changing in this region. It's moving through a constant magnetic field -- a uniform magnetic field. So, the only time something interesting happens is right here where it crosses and right here where it crosses. Okay? And when it crosses there and it crosses there, the current is no longer zero. And now, we have to think about the flux. Okay? So, the flux is out of the page which means that the ring wants to make a current that is going into the page. Right? This helps if you look over there at the computer monitor instead of looking at me. Okay. So, B is coming out of the screen, but my ring wants to make something -- wants to make a field-- that's going into the screen. Okay? So, how does it do that? It does it like this: If you take your right hand and you put your fingers in the direction of the current, right? I'm going to wrap my fingers around in the direction of the current. My thumb is telling me that that will make a B field going into the screen. Okay. So, the currents going to develop in the ring, in that direction, And now, you know exactly what's going to happen on this side. The exact opposite is gonna happen. So over here, the current, in fact, generates is generated in that direction because the B field that it was in is decreasing. The B field coming out of the screen is decreasing but it wants to keep that going. It doesn't want the change. And so by putting a B field in this direction, take your right hand, wrap your fingers around, and now we got our thumb coming out of the screen. And so, it's making a current in that direction. Then finally, when it gets back out here and it's still moving along at V, then current goes back to zero. So the only time current develops, is at these boundaries. Anywhere it's uniform or zero, then there is no current that develops in the loop. Okay. Any questions about that? Did I get the right-hand rule right? Did everybody agree with what I was doing? Okay. I wrapping around this way gives me a B that's pointing into the screen. I wrapping around this way gives me a B that's pointing out of the screen.