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Anderson Video - Lenz's Law

Professor Anderson
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Dave how you doing? You have a good weekend? >>Oh I had a great weekend. >>Did you go down to comic-con? >>No, I was gardening in the rain. >>We checked out the-- there was a truck down there from the movie Into the Storm. >>Oh yeah. >> And uh, my kid was looking at this truck and saying "wow what does that do? What does that do? What does that do?" And the lady was giving him the whole spiel and then finally she leans over and she goes "it's not real, it's a fake truck for a fake storm." And he was like awe. I'm so bummed. I said yeah I got a car that goes into the storm but it's in San Diego. There it goes into the light breeze. Okay. So let's talk a little bit more about Lenz's Law for a second. We introduced this last time but let's just review what Lenz's Law says. Lenz's Law is solely for determining the direction of the current. Okay, Faraday's Law tells us about the EMF, the voltage that develops. But Lenz's Law tells us which way that current is going to get pushed. And it's very simple right? We said nature abhors a change in flux. Nature abhors a change in flux. So Let's say I do the following: let's say I have a magnet that is approaching a current loop like so north and south. And we want to figure out what direction the current is going to go. Well the magnetic field is pointing up out of this thing, okay? So the magnetic field from the bar magnet is up, and we know it's spreading out. And some of it is poking through our loop like so. So the flux is increasing in that loop and it doesn't like that. It doesn't like an increase in flux and so it wants to make a B field that is going to point down. Okay? The loop wants to generate a current such that the B field is pointing down. Which means it's going to develop in this direction. If I use my right-hand rule what I said was if you put your fingers in the direction of the current then your thumb will be the direction of the B field. So for this one, we've got a B field that is going down because the current loop is coming around like this. Right? This is the direction of the current And so the B field is in fact pointing down. And this is B due to the loop. Okay? I drew it without the magnet here for simplicity. But the idea is that nature abhors a change in flux and so if flux is increasing from some other source it's gonna do whatever it can to make it decrease. Okay?