Re: [femm] Modelling the new discovery of electrostatic rotation

[Dave Squires <djsquires@xxxxxxxx>]

I would agree with Mr. Meeker. FEMM will not show
anything mainly because it is a static tool and probably
will not take into account those aspects shown in the
analysis in the paper quoted.

In order to get rotation normally this needs an initial push
to set up a slight asymmetry in the charge distribution.
Then it takes off (if free to rotate).

The paper with the spheres is interesting in that a static
torque is maintained. Their analysis is probably reasonable
and seems to match the results. The only thing new I see
is that the spheres are held against torsional stress of
the mountings.

But I still doubt that FEMM or any other tool will
show this torque because,
1. It is a 3D problem and integration.
2. Possibly the equations are not in the software to
handle this particular type of analysis. I could
be wrong, but it is doubtful.

DRS

David Meeker wrote:
> wigstonmagna wrote:
>
> > Apparently if you charge up a sphere to a modest few kilovolts,
> > fixed in place, and then suspend two other spheres close by, the
> > other two spheres rotate. In the experiments described in the
> > references below, they didn't
> > actually rotate, but instead torqued up their suspension filaments
> > until the restoring force equalled the torque.
>
> FEMM shouldn't predict a torque about the center of a perfectly 
> conducting sphere (or infinite in the into-the-page direction cylinder, 
> either). If the sphere (or cylinder) is perfectly conducting, the 
> sphere (or cylinder) is at a constant voltage. This means that at the 
> surface of the sphere, the only components of E and D are directed 
> normal to the surface (analogous to flux lines having to enter 
> perpendicular to the surface of a block of highly permeable iron). 
> Consider the electrostatic stress tensor at any point on the surface of 
> the sphere (or cylinder). If E and D at a given point are directed 
> normal to the surface, the force at this point must also be directed 
> normal to the surface. The torque about the center of the sphere due to 
> the force at the point of interest is r cross dF, where r is the vector 
> from the center of the sphere to the point of interest, and dF is the 
> force at the point of interest. Since dF is directed normal to the 
> surface of the sphere, r and dF are pointing in the same 
> direction,--their cross-product, and therefore the torque, is zero.
>
> If you actually tried this (say on the cylinders) in FEMM, you'd 
> doubtless get some small torque--a finite element solution is only 
> approximate, after all. However, with increasingly fine meshing, the 
> computed torque ought to converge to zero.
>
> --
> David Meeker
> dmeeker@xxxxxxxx
> http://femm.berlios.de/dmeeker
>
>
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