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Friday, October 09, 2009

Superconductivity

This is a little post about superconductivity, lecturing and liquid nitrogen.

The lecture I remember most clearly was when I first demonstrated the Meissner effect in a superconductor. You can buy a little kit to help with this. It contains a little powerful magnet, a disk of a high temperature superconductor and a polystyrene dish. Put superconductor in dish, add liquid nitrogen to dish, wait for bubbling to subside then drop small magnet onto superconductor and this happens:




(A video is better, see here)
The little magnet just sits there, suspended above the superconductor, if you give it a prod it'll spin around on it's axis. It's magic! Now the first time I did this was live in a lecture theatre in front of fifty students. I'd not had a chance to try it out in advance, and I must admit I was a bit underwhelmed by the equipment provided. So I did the tippy-out-the-liquid-nitrogen and wotnot, and my first words thereafter were "Bloody hell - it works!" - the students seemed impressed too. Much poking of the little magnet with the plastic tweezers was done, and we also splashed around the liquid nitrogen for more fun. I did the demonstration the following year, but it wasn't the same without my genuine surprise and excitement.

Lecturing is a bit of performance (quite literally), I struggled with the format because I found it hard to get meaningful feedback from a large group of students. If you do it passionately and enthusiastically it comes across to the students, but that's difficult to sustain for lecture after lecture. If you get it spot on, it's brilliant but usually its just a chore (for both student and lecturer).

Just to explain a little more about superconductors: a superconductor is a material which conducts electricity perfectly - it's resistance is zero (not just small, zero). A light bulb, an electric fire or kettle would be utterly useless with a superconducting element, the electric current would flow through it without emitting any light or heat. Heike Kamerlingh Onnes discovered superconductivity in 1911 (having first worked out how to liquify helium to cool his samples). More recently a bunch of so-called high temperature superconductors have been discovered, the weird thing is these materials are ceramics - they don't conduct at all at room temperature and yet cool them down to liquid nitrogen temperature (-196degrees centigrade) and they conduct really well. As I've mentioned in earlier blog posts, superconductors are used for the making of big magnets and there are also some applications in very sensitive detectors. In principle they would be great for electrical power transmission, but the requirement to cool everything down to at least liquid nitrogen temperatures has meant they've not been economically viable.

Laboratory scientists take liquid nitrogen for granted but it's an utterly alien material, like furiously boiling water but at the same time deep-bitingly cold. It hisses as it's poured into a new vessel, wreathed in clouds of condensing water vapour. Liquid nitrogen splashed on a laboratory floor will chase dust bunnies around with distinct droplets of fiercely boiling liquid, like tiny hovercraft. The droplets vanish without a trace.

7 comments:

Luoluo said...

I think I saw the same experiment when I was doing my undergraduate, though I could not remember very well about the whole story. Thanks for refreshing!

You must really enjoyed in demonstrating this to your students.

SomeBeans said...

I must admit I don't remember doing superconductivity as a student - so it was all new to me too!

Nora Lumiere said...

I'm an artist and this is all madly exotic and fascinating to me. Especially the "furiously boiling droplets" of liquid nitrogen chasing dust bunnies and disappearing without a trace.
I wonder what would happen if you dropped them on powdered paint pigments on a sheet of paper.

SomeBeans said...

Good question! I have no idea...

I'm guessing you'd end up with something like a splash. You need to find a friendly local scientist with a polished floor.

If you do give it a go - please be careful.

Anonymous said...

If I have my facts right a superconducting power cable could also be used for mag
lev. The cooling of it would turn water to ice, and the nearer we could get to a vacuum the less heat losses due to convection.

ice+maglev+vacuum all elements of super fast transportation system
@mjgeek

SomeBeans said...

@mjgeek technically you're right - see here: http://www.maglev2000.com/works/how-02.html
Supercon magnets are on the train. Some figures on maglev in action:
http://en.wikipedia.org/wiki/Shanghai_Maglev_Train

Practically I don't think it'll happen for quite some time because the infrastructure cost would be enormous - look how long the Channel Tunnel took.

Luoluo said...

If I didn't see your post, I nearly forgot the demo I saw when I was in university. There is a national laboratory for magnetic suspension train. We had a tour in the lab and saw some demos. I remember one of them is the superconductor, though I was not very sure about the theory, just feel excited to see the little thing was lifted into air…