Re: Forces on a small diamagnetic sphere in an AC/DC field

["nortonsm" <nortonsm@xxxxxxxxx>]

Dave,

You bring up a question I had when you mentioned that the 
boundary conditions would take care of themselves. As a newbie to 
FEMM, I'm currently demo'ing Femlab (a finite element analysis 
program that works with Matlab). One thing not covered well in the 
documentation (and not answered yet in an email to support) is when 
certain boundary conditions are applicable such as the "special" 
boundary conditions below.

> > 1) Electric insulation/continuity: n(dot)J=0, n(curl)H=0
> > or
> > 2) Magnetic discontinuity: n(curl)H1=n(curl)H2
3) Magnetic Potential: Aphi0=Aphi1
4) Surface current: -n (curl) H = Js

Will eddy currents (and resulting magnetic polarization) set up in 
the diamagnetic sphere "automatically" be taken care of in femm once 
I switch to AC fields? And off hand, do you know an example for each 
of the above boundary conditions? For instance, doesn't magnetic 
discontinuity above just mean that there is no surface current? So 
far it seems that setting up correct boundary conditions is the more 
difficult part of femm modeling. It doesn't seem clear for different 
materials and situations where one boundary condition is prefered 
over another. 

Anyways, thanks in advance for any help...
-Scott




--- In femm@xxxx, David Meeker <dmeeker@xxxx> wrote:
> This ought not to be a big deal to do in FEMM. You wouldn't have 
to 
> apply any special boundary conditions on the surface of the 
diamagnetic 
> particle--it takes care of things itself. Just make sure that you 
> prescribe the right permeability and conductivity for the material 
of 
> the sphere.
> 
> The one problem that you might run into is a problem of scale. It 
may 
> be difficult to get a good result if you are modeling a coil that 
is 
> several inchs across and simultaneously modeling micron-sized 
particles 
> in the same geometry. You'd be better off to break this into 2 
models: 
> Model just the coil to find the field and the field gradients at 
the 
> point of interest. The size of the domain would be in inches. 
Then, 
> using this info, you could make a second, small problem with the 
> particle and the air around it. The size of the domain would be on 
the 
> order of microns. You'd carefully choose the boundary conditions 
on the 
> edges of the region to give you the "right" field and gradients in 
the 
> absence of the particle.
> 
> Dave.
> 
> nortonsm wrote:
> 
> > I'm trying to work out the forces experienced by a very small
> > diamagnetic metallic sphere in a nonuniform magnetic field. In one
> > case the magnetic field is DC, in the other AC. Now what I do 
know
> > (which might not be much!):
> >
> > 1) the small (micron size range) particle should have complete 
field
> > penetration at the frequencies of interest
> >
> > 2) A static magnetic non-uniform field (like that above a current
> > ring) will suspend a diamagnetic particle (Diagmagnetic 
Suspension:
> > "Magnetic Fields" Knoepfel, H.E.John Wiley and Sons, 2000)
> >
> > 3) An AC field will induce oscillating currents in the particle. 
Due
> > to its small size and moderate frequencies, the quasistatic limit
> > should hold. These induced "eddy currents" will generate an 
induced
> > magnetic field which should change the forces on the suspended
> > particle.
> >
> > My question: I'm more interested in the AC case since as 
referenced
> > above, the DC case is covered in Knoepfel's book. What will be 
the
> > result in terms of forces of an AC magnetic field on the 
suspension
> > of the particle? How do I go about doing this calculation? 
> > Basically,what I don't know is what limitations 
or "simplifications"
> > I can do when dealing with VERY small metal diamagnetic 
particles? 
> > Are the induced eddy currents significant at all in particles with
> > such small size?
> >
> > I'm currently demo'ing Femlab to model this problem. This problem
> > obviously has azimuthal symmetry if one imagines a diamagnetic
> > sphere hovering above the center of a coil. I understand how to
> > model the coil, but Im not sure how to specify the boundary
> > conditions on the diamagnetic sphere (say its made of silver). 
Some
> > choices I have on boundary conditions include:
> > 1) Electric insulation/continuity: n(dot)J=0, n(curl)H=0
> > or
> > 2) Magnetic discontinuity: n(curl)H1=n(curl)H2
> >
> > Any thoughts or references would be appreciated. Thanks!!!
> >
> > -Scott
> >
> 
> -- 
> David Meeker <http://femm.berlios.de/dmeeker>