1

Chemistry / Things that block x-ray imaging at airport customs ?

« on: 26/11/2011 13:00:44 »

Very dense materials generally block x-rays well.  Lead and tungsten are frequently used.  Of course, if you put these in your luggage at an airport, you'd be pulled aside for "extra screening," since having a chunk of lead in your luggage is a sign that you're trying to hide something.

2

Question of the Week / Re: QotW - 07.12.09 - Power Source for Magnets?

« on: 07/12/2007 04:09:21 »

Every electron in the metal acts as a magnetic dipole.  Dipoles are the simplest sources of magnetic fields.  In order to generate a permanent magnet form a metal, you force all these dipoles to line up, so the tiny fields they each generate all add together to make a big field. 

If you stick this magnet to a fridge, I think the fields from the magnet should cause the electrons in the field to line up in an opposite direction.  This means if you have the north pole of a magnet near a fridge, the fridge will start to act like a south pole, and viola--your magnet will stick to your fridge.  If you try to push two like poles of magnets together, they repel because the electrons in each magnet are already aligned to repel each other and won't change direction easily. 

Finally, in terms of energy.  In physics, change in energy is defined as a force applied over a distance.  If you hold two magnets near each other until your arms get tired, you're not actually putting any energy into the magnets.  Similarly, by holding the magnets near each other, you're not getting energy out.  To get energy out, you'd probably have to let the magnets fly apart and use that motion to generate electricity or move something.  The simple reason why that isn't practical is that you have to put the same amount of energy into the system in the first place in order to put the magnets near each other!