Lenz’s Law

Lenz’s Law

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Oh, hello. Welcome to Michael’s Toys the first and only show on YouTube made by, of, and for teenagers who like to cook I’m your host Michael Stevens and today We are going to be talking about magnets, specifically the strongest magnetic field my body has ever been inside The 3 tesla MRI at UC Irvine Which I guess kind of makes this an episode of UC Irvine’s toys because they own the MRI, I don’t. Anywho, I had a lot of MRI scans done of My brain for MindField season two and I learned a lot about them, and they are… just beautiful and wonderful. Here’s a question how strong are different magnets? Well, one way of measuring a magnet’s magnetic field strength, really its magnetic flux density, is the tesla. Alright now to put a tesla into perspective, our planet is a big magnet, really. It’s not a really strong magnet though– its magnetic field strength is just about maybe 31 microteslas. In comparison and everyday ordinary refrigerator magnet has a magnetic flux density of about maybe like five milliteslas. A typical sunspot can have a magnetic flux density of around a third of a tesla, but the surface of a neodymium rare-earth magnet can have a magnetic flux density around 1.25 tesla. The MRI machine I got in many many times for MindField season 2 was a 3 tesla machine Which means you can do some pretty weird things with it. You have to be very very careful: No ferromagnetic metals are allowed into the room it is in, or even near it in the room outside because it attracts ferromagnetic materials so quickly that they could be pulled from your hand race through the air at an ever-accelerating rate until they strike the machine at, like, super-lethal and at least very dangerous speeds. There are videos on YouTube where people show off just how dangerous the magnetic power of an MRI can be: it can lift chairs off the ground, but what about metals that aren’t attracted to magnets? Well, they exhibit some v ery strange behaviors and Craig Stark at UC Irvine was kind enough to allow me to bring in a giant block of aluminum (chuckles wickedly) Now I’m gonna have to just talk over this clip because we obviously couldn’t bring cameras or microphones into the room to pick up our voices we had to set the camera far away and then zoom into myself pause Let’s just take a brief moment to appreciate the tan line on my face caused by my glasses But here’s an aluminum block and I’m gonna set it up on its side, and then let it fall okay, pretty easy That’s a great demonstration of gravity But watch what happens when I set the aluminum block near the inside of the MRI and let it go Anti-gravity But not really. It’s actually a demonstration of Lenz’s law Veritasium has a fantastic video on the topic and I see it coming up on Reddit Quite frequently so I had to do it myself, it’s really fun these are very powerful magnets And I’m gonna drop them so that they fall Only due to gravity. Here we go: three, two, one there it is 9.8 m/s^2, for those of you counting at home, but now I’m gonna drop them through a copper pipe. Copper has no interest in magnets, or maybe magnets don’t care about them Or do they? Well we’re gonna find out. Notice that if I try to stick the magnets magnets to the copper, it not Doesn’t work, but if I drop them through watch what happens. Three two one They fall really slowly. Here we go one more time three two one Something’s slowing them down. You can hear that they are hitting the sides of the copper But they don’t have to do that if my magnet was a better shape it wouldn’t happen, and they would just glide right on down They really are more slowly. Let’s look at it from above. Here are the magnets being dropped by themselves. That’s how long it takes for them to fall to the table three two one That’s it But now let’s drop them through this copper pipe. Okay ready three two one Look at that! Let’s try it one more time. Three, two one, and they hit. Much much slower. To see what’s going on, let’s play around with magnets. One of my favorite magnetic fun recipes is a one that I learned from Science Bob Fantastic guy. It involves breakfast cereals and a bowl of water. So here’s my bowl of water now what I need to do is grab myself one little flake of this corn flake-like cereal Now many breakfast cereals are fortified with iron And it’s not iron in any kind of compound. It’s literally just elemental iron. If I float a little piece of cereal there in the water looks pretty boring But now let’s bring a magnet near it. Ohhhh! Look at that! It follows the magnet! I’ll come in from the other side so you can see that it wasn’t just some sort of property of the water C’mon, little cereal flakey. Yeah. Look at that! I’m able to move the cereal across the water by attracting the iron inside it with a magnet. When you eat many foods you’re eating just straight-up elemental iron like the kind We make nails out of just like a lot less than would be in a nail. Oh I love that I hope it’s clear for you guys to see this demo wasn’t really related to MRIs I just thought it would be really fun to do to really start talking about why that block of aluminium fell So slowly around that strong magnetic field we need a more advanced recipe but I think you guys are ready for it what we’re gonna need for this recipe is a really nice big nail You’re also going to need… let’s see this one looks good Oh, perfect, yeah. You’re gonna need some copper wire that’s thinly insulated. You might need more than this, but what you’re gonna want to do is take your wire and coil it around your nail. Make the coil nice and tight Keep them all bunched together, so you get as many wrap arounds of the wire around the nail as you can get and then When you’re done You’ll want to connect the two ends of the wire to a battery now That might take a while so because of the magic of a TV cooking show I prepared an electromagnet earlier Oh yeah it’s ready, this is good this is really good. Okay…now… Perfect. Ohh yeah. Hey…! Just like mom used to make. Um, we’ve got here a big battery, and we’ve got a nail that is just coiled with lots and lots of wire. I think that We let the paperclip simmer long enough. I’m gonna go ahead and put them There on our aluminum foil and I’ll demonstrate that at this moment Because the wires are not completing a circuit the nail Is not magnetic, has no effect on the paper clips But now let’s connect this wire to the negative terminal and see what happens Ooh, OK, we’re live, and… Yeah! Now let’s turn off the current. Oh! Hoohoohoo! All right, so what we’ve learned is that current flowing creates a magnetic field. What’s even more fascinating is that a moving magnetic field can also generate electric current there are fantastic videos online showing you how to do this. By simply spinning a magnet around a coil of wire. You can light a light up This is how electric generators work. All you need is something to keep the magnet spinning, like wind or falling water, and Your coil will supply electric current There’s a very interesting loop going on here: electric current produces a magnetic field but a moving magnetic field can produce electric current Now, this is key to why the block of aluminum falls so slowly near a strong magnet. It’s also why even things like living frogs can be levitated if your magnet is strong enough Not all materials are attracted to magnets this nail is made of iron and Well the magnet loves it a whole lot but this copper tube… Nothing. The magnet just doesn’t care at all. But what copper *can* do is conduct electricity and since a moving magnetic field can induce electric current in a conductor If we put these together and move one of them well then we should be able to produce some current But we also know that electric current creates a magnetic field Which means we could make the copper act like a magnet by inducing current in it. Let’s try that using this conveniently positioned ribbon I’m gonna take these Neodymium disk magnets and very carefully slide them apart It’s very important to be safe with magnets They are attracted to each other so much more strongly the closer they get that they can easily pinch you, so please be careful with strong magnets. Okay, here we go… Perfect. If I bring this nail near the hanging magnets, ooh they love it. They love it. But the copper they don’t really care much for. Not much interest. Hello? Wake up! No, they don’t care. Watch what happens if I move this copper pipe quickly near the magnets, oooh Look at that! Now they’re not touching at all, but I’m able to get the magnets moving because As I move the pipe across the magnets- look at that! well This is really fun- as I move the pipe across the magnets the magnetic flux density at each point along this piece of copper changes that creates an electric current and that electric current, by Lenz’s law, will produce a magnetic field That is opposed to the magnetic field of these magnets causing the hanging magnets to move. Now the faster you move the materials, the more dramatic the effects. Oh, yeah! Beautiful! This is why the magnets fall through this copper tube so slowly By Lenz’s law the magnetic field they induce in the copper pipe is counter to their own magnetic fields you can think about this phenomenon in terms of conservation of energy Where does the energy come from that produces the electric current in the pipe and the magnetic field? Well it comes from the falling magnets. They fall more slowly because some of that energy is being converted into electric current. The currents created by a moving magnetic field are called eddy currents, and if you want to become more eddy-cated about them, there are links down below where you can learn more. Now let me address a quick question You might be having why is a video like this on the DONG channel and not on Vsauce1? Well I answered this question on Reddit yesterday and The long and the short of it is that in my opinion both channels have sort of different goals. My goal on Vsauce1 is to upload videos where I get to share the things and the new framings that caused a concept to finally click in my head Concepts that I never thought I’d be able to wrap my head around before Now doing that can take a long time I’ve been working for a while on an episode about rotational phenomenon specifically some counterintuitive ones And I’m not happy on Vsauce 1 just using vocab words like torque and moment of inertia to describe what’s going on I want to define what those words mean and ask why over and over again so far back that we’re left with nothing but Geometric principles and symmetries of the universe that can take a while, so thank you for your patience I hope that you you share my passion for the topic when the episode comes out and think that it’s worth it But on the DONG channel, I’m able to share things really quickly without disappointing people who expect something deeper in fact in this year So far in just 2018. I’ve already made now 10 episodes on DONG That’s more than I made on Vsauce 1 all of last year so I love DONG But I remain steadfastly committed to Both Channels Now DONG is made possible, especially today, by our sponsor Audible. Audible contains, I mean just an unmatched collection of audiobooks as you guys know I love reading and now that I live in LA I’m in a car a lot, so audio books come in extremely handy. Audible is offering You guys out there watching Michael’s Toys today a free audiobook download with a 30 day trial membership What’s great about Audible is that, when you download an audiobook, it’s yours to keep, like, forever. Okay? If you end your membership, you still own the book. It’s fantastic. Um, I would highly recommend the audiobook of Carl Sagan’s Cosmos The voices narrating it are just unbeatable. One of the narrators is Ann Druyan. She was the creative director of NASA’s golden records project that put the golden records on Voyager 1 & 2 a Recording of her brain waves are on that record that someday other life-forms out there might find Uh, she, while she had her brain waves recorded. She was thinking about uh, the history of Earth civilizations, she was thinking about, uh, the problems that we face here on earth And she even thought about what it feels like to fall in love Wonderful stuff she co-wrote Cosmos the documentary and a year later Married Carl Sagan, but it’s not just her voice on there. It’s also got Neil deGrasse Tyson, LeVar Burton, Seth Macfarlane. I mean holy cow Totally hugely recommend that. By going to audible.com/michaelstoys You can take advantage of the offer we have today. You can also text michaelstoys to 500 500 Awesome stuff, thank you Audible. Remember, audible.com/michaelstoys And, as always, thanks for watching. That MRI at UC Irvine by the way is a giant electromagnet, but it doesn’t take a huge amount of power to run because It’s really cool Cold, like, really cold. It’s cooled by liquid helium near absolute zero so it becomes a superconductor And there’s almost no resistance You literally throw some charge in there, some current starts going through, and it gets so cold that it becomes a persistent charge. And it just remains an electromagnet unlike my nail which is not a superconductor and well when the currents turned off it stops being a magnet In fact, if you want to turn off the magnet in their MRI, you have to quench the MRI There’s this big emergency button you can push if some emergency happens And the liquid helium is shot out of a big vent, probably on the roof of the hospital building; it’s this big plume of Helium gas and condensed liquid water from the air because it’s just so cold and it’s well It’s very dramatic There’re videos on YouTube where you can see MRIs being quenched That will cause resistance to come back into the coil and it will cease to be an electromagnet. But I love the fact that they just kinda plug it in once, Power up that electromagnet, end then after that is the energy to keep the helium cool.

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