Regarding magnetic “silverware” in restaurants from NPR’s Car talk (week of 9/22)

PUZZLER: A Magnetizing Dinner Chez Magliozzi
Over dinner, Ray’s wife notices that her knife and fork are stuck to each other, her knife was magnetized. Ray’s son’s knife was magnetized too, but the polarity was the reverse of her’s. Ray’s niece offers an explanation. What was her theory?

RAY: It wasn’t a family dinner at home, we were at a restaurant. And restaurants have such large losses of silverware, that they throw into their trash receptacle an enormous magnet so that when silverware is mistakenly thrown into the trash can, the forks, spoons and knives all get stuck to this magnet, and if it’s on there long enough it gets pretty magnetized.

TOM: Wow.

RAY: So that’s how it happened. So the next time you’re at a restaurant you can do a little parlor trick. Find a utensil that’s magnetized, and you can pick up somebody’s keys and drop their keys in their soup.

There was a brief discussion of this on a teachers’ listserv I’m on, where one teacher asked, but isn’t restaurant flatware stainless steel, and isn’t stainless steel nonmagnetic?

Another informed us:

Some stainless steel is not magnet, but not all. The kind of steel in cutlery is usually magnetic. Check out this link.

Several companies make magnetic catchers. For example, check out this link.


This post is primarily for college physics teachers.

Hey, if you’re ever presiding over a conference session, here’s a tip for you. If one of your presenters has technical issues, don’t give her a hard time after the talk is over, even in jest, about having gone over time. Trust me. She’s already been beating herself up for it over the last half hour.

So, technical issues aside (and I tried four times to check my presentation on the conference computer prior to the session and was thwarted in some way each time, I swear!), my talk tonight went well. Here’s what I’ve been working on.

In our department, we’ve made a lot of changes to the freshman level courses, adding things like Tutorials and clicker questions and peer instruction that have been shown to improve student learning, because they get students really thinking about the material and engaged (and thus learning) instead of sitting passively and waiting for knowledge to be imparted upon them by the instructor. At the upper-division, however, there’s this sense that we need to stop “coddling” our students with these kinds of techniques, that we learned by lecture, so why shouldn’t they? If they’re not learning, maybe they should consider changing major at this point. But, I argue, that how you learn doesn’t suddenly change between the sophomore and the junior year, and we might do better by our students to try using some techniques that have proven effective at the lower division.

So, in our course we developed

  • Lists of what we concepts and skills we wanted students to learn
  • Homework questions that targeted those concepts and skills
  • A new assessment exam to see if we taught them those concepts and skills
  • New tutorials for teaching those concepts and skills
  • Interactive lecture techniques
  • Clicker questions

We found that when we compared students in a Traditional course to those who took a Transformed course, even though they were similar to one another coming in to the course, at the end of the course those in the Transformed course scored significantly better than those in the Traditional course on common exam questions and a conceptual exam. So, it worked! We can teach our majors better.

We’ve got all those materials on our website for other instructors to use.

And here is the Powerpoint of my talk (PDF) I’ve also got two posters — see the website above for those.

ferrofluidI just realized I haven’t yet written about ferrofluid… I was mentioning it to a few folks last night over dinner (no, dinner with me isn’t as geeky as I’d generally like) and they hadn’t heard of it.

Ferrofluid is a thick oil with tiny suspended particles of iron (magnetite) in it. That means that it behaves like a magnetic fluid — it’s attracted to magnetic fields. You can make this fluid jump with a bar magnet. The picture on the left shows a ferrofluid that is on a surface above a bar magnet. The “hedgehog” shape shows the lines of the magnetic field.

Remember when you were in middle school and you sprinkled iron filings on a piece of paper above a bar magnet? You could see the “magnetic field lines” as the filings lined up on either side of the bar magnet, and made spiky things on the ends. Here’s an image of that, to the right.magnet0873.png Well, this is the same sort of thing. The hedgehog spikes just show the same thing as the iron filings, but in three dimensions.
It’s hard to make ferrofluid, though. I know, I’ve tried. You can take corn oil and put iron filings in it, and that’s kind of fun, but the iron filings just sit in the bottom of the jar. Stick a magnet to it and the filings still look spiky, they don’t flow smoothly. I’ve tried taking magnetic toner powder (used for printing magnetic ink on checks) and suspending that in motor oil, but it’s not magnetic enough. The first problem is the oil — it has to be something very thin (30W motor oil is supposed to be thin enough) so that it can flow, but not so thin (like mineral oil) that the magnetite settles out. Also, big particles, like filings, tend to settle out because they’re heavy. So they make tiny (often nano-sized) particles so that their weight can be held up by the oil. But, when you make particles that tiny, they have a tendency to clump together. So you need some sort of “surfactant” — a coating on the particles that keeps them apart. (Soap is a “surfactant” because it keeps grease molecules apart). Plus, you need the surface tension on the fluid to counterbalance the magnetic attraction in just the right amount so the fluid holds together but it responds nicely to the magnetic fields. You can imagine, for instance, if this had a low surface tension (like water) the stuff would just stream onto the magnet and not hold together enough to make these shapes.
So, it’s hard to make good ferrofluid. But once you do, it’s a lot of fun. You can make it defy gravity by bringing a magnet down towards it from above, until it jumps up to the magnet. (Word to the wise, put the magnet inside a test tube or something, or else you’ll *never* get the ferrofluid off). You can make beautiful patterns, as in “Ebb Protrude Flow”, in this YouTube video.

Or this one also beautiful

In practice, ferrofluid is used to dampen high-end speaker systems (I don’t know much about that), and also in space, where you can’t make hydraulics work like they do on earth because there’s no gravity. Instead, the hydraulics can be driven and precisely controlled using magnetic fields.