damockturtle

[damockturtle]

Here we have a type II superconductor. Unlike a type I, a type II superconductor can be penetrated by magnetic fields and still be superconducting. It isn't penetrated uniformly, but in narrow columns called vortices. These vortices are pinned to defects in the superconductor. This results in the Superconductor being locked to the magnet. To tear away the SC from the magnet, you have to break vortex pinning.

El efecto Meissner en los imanes.

[damockturtle]

This is a good video that discusses how pinned vortices can lock a levitating type II superconductor in place. In 5 minutes it discusses what vortices are and how they are pinned.

[damockturtle]

Don't do this. People might die.

Organizers thought it would be a great idea to create fog using liquid nitrogen at a Jagermeister pool party in Mexico. Not long after, people started passing out in the pool, with several being hospitalized and one in a coma.

Almost immediately, the media started claiming that this was due to some exotic reaction between the liquid nitrogen and the chlorine in the water. These claims were insane because N2 is very stable, available in air, and the low temperatures should have slowed, rather than accelerated, any reaction.

Instead, the liquid nitrogen went to a gas form very quickly, the air near the pool. Since this layer of nitrogen was colder than the surrounding air, it stayed near the pool. This meant that people swimming were breathing almost pure N2 and asphyxiating.

When working with liquid nitrogen, keep it out of closed containers and keep your head a few feet above the ground. The risk of a high pressure explosion or asphyxiation are much greater than actually being burned.

Also, never trust party organizers or $7.50/hour lifeguards with your life.

Note: N2 doesn't set off the same warning alarms as CO2. Swimmers were unaware of the problem until they were in trouble.

[damockturtle]

Levitating Frogs

There's something wonderfully medieval about levitating frogs. Evoking images of superstitions and witchcraft, it's hardly surprising that this won the 2000 Ignobel prize. 

If nothing else, this has taught me that water and frogs are diamagnetic enough to be levitated in a 16 Tesla Bitter Solenoid... In other words, if Magneto was strong enough in X-Men II, he could have picked someone up without any metal around.

[damockturtle]

Trebuchet Physics Explained

People often assume that the medieval trebuchet was just a crude lever arm system. As a result, trebuchets and catapults are often portrayed horribly in the movies (look at the orc trebuchets in Lord of the Rings).

In addition to the fixed ratio provided by the lever, traditional hanging counterweight trebuchets rely on the sling and the rapid change in counterweight (CW) direction (which causes a counterweight pause) to transfer energy from the falling counterweight and projectile.

As shown in this video, the sling and the CW L-route are impressive energy transfer mechanisms. The CW L-Route transfers most of the CW's energy to the arm/projectile, causing the CW to stall at a predictable point. Similarly, sling will transfer a lot of energy from the arm/CW to the projectile. causing the arm/CW to stall at a point determined by sling length.

For an efficient throw, it is important that both stall points are very close together and that your release coincides with these points.

[damockturtle]

Here is a video on how to make a pencil trebuchet. It only requires pencils, floss, a bit of scotch tape, and a paperclip.

[damockturtle]

Placing an array of superconductors on a metal film was popular among physicists in the 90s. Each superconducting island would interact with the other through Josephson coupling and much of the behavior would be determined by the superconducting phase (this why they are called Josephson Junction arrays). By applying a magnetic field, they could populate their array with vortices (current loops) and, at certain magnetic fields, they could get interesting repeating vortex patterns...

The picture above is a simulation of a triangular array in a magnetic field (vortices are in red. Empty spots are blue.).  There is a repeating pattern of 6 empty sites (all in a hexagon) surrounded by 6 vortex sites.

Unfortunately, the simulation did not produce a perfect pattern and there are defects where various ordered domains meet.