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

- To: femm@yahoogroups.com
- Subject: Re: [femm] Modelling the new discovery of electrostatic rotation
- From: Dave Squires <djsquires@plix.com>
- Date: Tue, 12 Aug 2003 08:41:25 -0700
- References: <bh91qd+cbgu@eGroups.com> <3F384AF3.8080009@ieee.org>
- User-agent: Mozilla/5.0 (Windows; U; Win 9x 4.90; en-US; rv:1.0.2) Gecko/20030208 Netscape/7.02

    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@ieee.org
>     http://femm.berlios.de/dmeeker

> `
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``
``

- References:
  - Modelling the new discovery of electrostatic rotation
    - From: "wigstonmagna" <andrew_ppp@hotmail.com>
  - Re: [femm] Modelling the new discovery of electrostatic rotation
    - From: David Meeker <dmeeker@ieee.org>

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