GOOGLE PHYSICS VIDEOS

Hi everyone
I have added a new "element" to my blog - videos from Google with the keyword "physics". I don't yet know how useful/suitable they will be, but the first lot are pretty good.
I hope you enjoy these videos, and that they make you want to know more about the physics behind them.
There is one here which is silent and therefore requires some explanation - it involves pouring liquid air (or nitrogen) onto a material which then becomes supercooled, and shows the phenomenon called "superconductivity" - essentially it has no resistance.
The silvery disc is a magnet - when it is placed above the superconductor it will fall towards it, inducing a current in the superconductor which will oppose the change inducing it and produce a magnetic field that opposes the field of the magnet, making it "levitate" . Year 13 students who have studied electromagnetism (Physics 3.6) will recognise this as Lenz's Law in action.
You can achieve the same effect by replacing the superconductor with a permanent magnet, but in this case you have to spin the "levitating" magnet so that gyroscopic precession takes care of any derangement of the floating magnet's spin.
Towards the end of the clip, the magnet is pulled upwards away from the superconductor. In response to this, the currents induced in the superconductor set up a magnetic field that tries to hold the magnet closer - again opposing the change!
Then as the superconductor heats up again on the side of the vessel, the currents in it will reduce because its resistance is increasing, and the magnet gently falls.