Dr. Mike's bags are packed!
aveeguy: drmike, quit teasing us! Are your bags packed?I also found this post enlightening from Dr. Mike. He's given us a layman's explanation of Magnetic Viscosity:
drmike: YES! My wife never let's me do anything "just in time".
OK, let's drop down to the atomic level. The atoms are in a lattice, and several atoms form a magnetic unit, like Fe_3O_4. 1000's of units make a domain. Millions of domains make a magnet. (a very small one!)He goes on to say:
So let's take all the units to saturation so all the domains align. Now, let the external field go away.Most of the domains stay aligned, but some don't because they are not oriented along their "easy" axis, and the magnet slips back to Br. The time it takes for all the domians to flip depends on the temperature of the magnet and the "anisotropy energy". If you kill the external field instantly (impossible in reality, but let's just pretend) then the instantaneous Br will be higher than at some time 5 to 10 decay constants later.
That is magnetic viscosity.
Where does the energy go? It goes into the atomic lattice - the atoms oscillate a touch slower in the domains that don't align. They have to give up some thermal energy to go to a higher energy state of mis-alignment with their neighbors. It doesn't take long for them all to come back to equilibrium with the rest of the magnet though - the speed of sound in the material helps with that.
This seems counter intuitive to me - you'd think the misalinged domains would heat up - but that doesn't conserve energy. The fully aligned saturated system is at the lowest possible entropy state. From basic thermodynamics
T*dS = dE + dW
I guess I'll give Steorn an out here. If they can maintain T and increase dS, then dE has to increase and they can do work with it. I'll have to think about that a lot more!