In the last few weeks, I’ve been wrapping up E&M, which has included talking about Faraday’s Law and induced currents. I did the traditional demonstration using a PASCO ring launcher to demonstrate Lenz’s Law, showing that the induced current flows in a direction that creates a field opposing the change in magnetic flux. The ring launcher uses an alternating current in a solenoid to shoot a metal ring a meter or so up in the air, which always gets a good reaction.
The extreme version of the same basic physics is the Meissner Effect, in which currents in a piece of superconductor completely cancel any magnetic field attempting to enter the superconductor, allowing you to levitate small magnets, or trains. We have a Meissner Effect demo kit, with a piece of high-T superconductor and a rare-earth magnet, so I brought some liquid nitrogen up from the basement to demonstrate this.
Of course, having lugged four liters of LN2 upstairs, I needed something more than just a floating magnet to show for it, but I didn’t have anything breakable lying around. I did, however, have the aforementioned ring launcher…
If you cool aluminum to liquid nitrogen temperature, the resistance drops by about a factor of five. That means that the same changing magnetic field induces a current that’s five times larger, and thus a launching force that’s five times bigger.
At room temperature, the launcher will pop an aluminum ring up into the air to about my head height. After cooling the ring in liquid nitrogen, it hit the ceiling with a satisfying thunk. Watching the students in the front row scramble away from the cold aluminum ring like it was fresh from the forges of Mordor was a hoot, too.
I doubt they’ll really take away anything useful from the demo itself (we talked about the temperature dependence of resistance a few weeks back, but I didn’t spend much time on it today), but it was a fun way to spend part of the last class of new material…
If you want another impressive demo of this, I’ve got one from my thesis advisor. He had a big NdFeB rare-earth magnet about 5 cm in diameter and maybe 3 cm thick, and he had a plate of OFHC copper about the size of a salad plate and 2 cm thick. Moving the magnet over the room temperature copper took no real effort; clearly there were no significant forces between the magnet and the copper, just as you’d expect since copper isn’t magnetic. Then he would cool the copper plate to LN2 temperatures by immersing it in a wide-mouthed dewar full of the stuff for a few minutes. This takes a little while, since the plate has a pretty big heat capacity. Now take out the plate and put it on the bench (he did this using a handy piece of twine threaded through a hole in the plate). Waving the magnet over the cold Cu plate at a height of a couple of cm feels like you’re pulling the magnet through molasses. If you drop the magnet from a few cm above the plate, it falls as if in slow motion, drifting down to settle on the plate top. Eddy currents are pretty amazing, and the conductivity of OFHC Cu goes up quite a bit (maybe a factor of 20 or so) between 300 K and 77 K.
Ok, I’m going to be pedantic: That’s not thermal resistance! Thermal resistance is the resistance to heat flow (i.e., the inverse of thermal conductance). Of course, “Fun with the temperature-dependence of electrical resistivity” doesn’t have the same ring (hah!) for a post title…
Mmmm, thermal resistance…
As an electrical engineer I immediately think in terms of power transistors and heatsinks. The thermal resistance of a power transistor would be measured in centigrade degrees per Watt.
I’ve got a simpler, cheaper demonstration using a couple feet of hardware store copper tubing, a penny, and a powerful rare-earth disk magnet with a diameter smaller than the penny.
It’s best to let the observer conduct the experiment. Have him hold the tubing vertically over a soft surface (a magazine is fine), then drop the penny down the tube. (It lands in about 1/3 second.) Then have him test the magnet against the copper for attraction, finding none. Lastly, have him drop the magnet down the tube. When it lands a few seconds later, have him repeat it while looking down the tube.
The only explanation is that the moving magnet induces a field in the conductor, opposing the direction of motion.
@6EQUJ5 — we used to do the same experiment at the physics table during Cal Day, Berkeley’s open house. We had a section of copper tubing that had a slot cut into it so you could watch either a nonmagnetic slug or a cylindrical magnet of the same dimensions fall through. The slot stopped short of either end, so there was still a path for current to flow around the tube. The kids loved it.
http://www.unitednuclear.com/magnets.htm
Supermagnet #33, 800 lbs of pull in your palm
http://www.gaussboys.com/
http://www.gaussboys.com/ndfeb-magnets/75mm-circular-dipole-halbach-array.html
1.1 tesla circular dipole halbach array
Lethal *at several feet distance* to CRTs and magnetic storage including credit cards.
As an electrical engineer I immediately think in terms of power transistors and heatsinks.
Copper slugs in kovar cases all the way, brother. I was highly disappointed, lured into this article on pretenses that we’d be talking about the actual thermal resistance of materials….
And also, having been a high school and college aged kid sitting through physics class demonstrations, Chad, they weren’t shying away from the equipment. They were shying away from you. I’m trying, and failing, to remember a physics demonstration class that didn’t have at least one unplanned event. (Best was the time the instructor used the soup-can cannon to put a tennis ball right through the ceiling tiles. Then, two minutes later, did it again.)
I remember Prof. Fielding Brown doing the PASCO ring launcher demo in sophomore E&M back at Williams, a long time ago. He first did the regular demo, and then cooled the ring in LN2 and launched it much higher, the same way you did. But, to my total amazement, he actually *caught* the cooled ring with his free hand on its way back down, and then put it down on the desk. I don’t recommend this, but he seemed fine (no hand damage) and I certainly remember that demo a lot better than the other ones!
I remember there being some subtlety to the ring launcher demo. In particular, I’m pretty sure that naively you wouldn’t expect the thickness of the ring to matter. The current carried (and thus the force) scales linearly with the thickness, but so does the mass, and so you’d think the acceleration and thus the final height should be the same regardless. Yet it is not.
I remember a similar demo from my freshman E&M class, and the professor pointed out that since resistance decreases with temperature, the effect should be more spectacular if he used the ring that had been sitting in liquid nitrogen during the lecture. But there was a twist: the second ring had a gap in it (i.e., the current could not flow completely around the ring), so nothing happened. He then dipped the first ring in the liquid nitrogen, and as predicted the effect was more spectacular.
Based on some of the comments, maybe a post on the best failures of lecture demos is in order.
I concur with Asad. It’s not thermal resistance. It’s thermally controlled (?) electrical resistance.
I love making students jump. Every year I nearly electrocute myself in class (not on purpose, but that’s what you get when you have a theorist handling equipment) and the students always freak out (and I doubt it is because they are concerned for my safety).
For #11, my favorite experiment that NEVER works was mentioned in a blog last year (item 2 on this page)
http://doctorpion.blogspot.com/2007/04/ultimate-demo-day.html
You cool a const-vol “thermometer” (sphere with absolute pressure gauge attached, normally used for absolute zero measurement using water bath observations) to LN temperatures, predicting the pressure at the final T before starting the experiment.
It always gives the wrong answer because air is not an ideal gas at that temperature. The oxygen condenses, effectively changing n. With that correction, experiment and theory agree.
Thanks for sharing this one… Tying together different ideas in a dramatic way is great. Even better if you can get the students to guess what will happen (they should have known after all).
Too bad evolutionary biology and pop gen don’t really lend themselves to cool demos.