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

[David Meeker <dmeeker@xxxxxxxx>]

nortonsm wrote:

> Dave,
>
> Thanks again for the help... This program is really fun! I had a
> question about calculating forces in axisymmetric problems. By
> looking over the past archives I found a couple references to this
> (how one should start and end the contour on the r=0 axis). With
> respect to your "ball" example. Let us say there is an
> axisymmetric "r" component to the magnetic force (rather than being
> uniform). I'm interested in calculating the forces in both z- and r-
> on the ball. i.e. in the case where the ball is diamagnetic there
> should be an "r-" restoring force and a positive "z-" suspension
> force on the ball. However, in the axisymmetric problem the r- force
> would (and should (I havent checked)) turn out to be zero. Is there
> a way to determine the "r- component" restoring force? (even though
> its zero right at the r=0 position). I guess I want the second
> derivative of the radial force (d2F/d2r). How do I calculate that in
> femm and are there "things to watch out for" in terms of the
> calculation?
>
> Thanks!!!
> -Scott

The program should report zero as the net radial force in an 
axisymmetric problem. People who have been after internal stresses on 
an axisymmetric structure have been interested in the force that you get 
on, say, a segment of the geometry--since this isn't one of the standard 
integrals, you have to evaluate it yourself via a lua script. However, 
this isn't the same as the "r-component restoring force." Except in 
some special cases (e.g. configurations of just permanent magnets), you 
generally can't infer centering force from the axisymmetric solution in 
a straightforward way.

Dave.
--
David Meeker <http://femm.berlios.de/dmeeker>