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# Re: [femm] electrostatics and FEMM

- To: femm@yahoogroups.com
- Subject: Re: [femm] electrostatics and FEMM
- From: David Meeker <dmeeker@foster-miller.com>
- Date: Tue, 05 Jun 2001 11:57:30 -0400
- Organization: Foster-Miller, Inc.
- References: <9fh3df+ku2u@eGroups.com>

frank.e.lenning@boeing.com wrote:

> I've been playing a little trying to do electrostatics problems within
> FEMM in a similar way suggested by Roberto Brambilla in message 180.
> After some thought, though, I can't figure out why you can't do
> axisymmetric electrostatic in FEMM. It seems like both satisfy a
> Poisson's equation and so you should be able to equate the proper
> quantities to do it.

Although axisymmetric magnetostatic and eletrostatics both satisfy a Poisson equation, it turns out that the equations aren't exactly the same. This difference pops up because the magnetics uses a vector potential, in which 3 of the three components get chucked for the axisymmetric case, versus electrostatics, where the potential is a scalar.

Here's the version for people who have an HTML mail reader:

For the axisymmetric magnetic case:

Ñ2 A º | ¶ ¶r |  | æ ç è | 1 r |  | ¶ ¶r | ( r Aq ) | ö ÷ ø | + | ¶2 Aq ¶z2 |

For the axisymmetric electrostatic case:

Ñ2 F º | 1 r |  | ¶ ¶r |  | æ ç è | r | ¶F ¶r | ö ÷ ø | + | ¶2 F ¶z2 |

If you don't have html mail,
del^2 A = D[(1/r) D[r A,r],r] + D[A,{z,2}] for the magnetic problem
del^2 phi = (1/r) D[r D[phi,r],r] + D[phi,{z,2}] for the electrostatic problem

> Trying this out though I have run into some subtle problems with the
> way FEMM handles axisymmetric. First, I can't seem to be able to set
> A on a boundary internal to a problem space. I have tried prescibing
> A on a boundary and I get something that looks more like FEMM has set
> A/r to some constant (when I set A0=1). Second, FEMM seems to default
> the axis of symmetry boundary to A=0 and will not allow me to change
> it. For axisymmetric electrostatics problems, I'd like to set dV/dn
> to zero along this boundary but it doesn't seem to let me. The
> specific problem I'm trying to do is two cylindrical conductors
> (aligned end-on on the r=0 axis) of finite length at opposite
> potentials. Any suggestions??

Again, the trick here is just that femm is fundamentally solving a different equation in the axisymmetric case than what you need to solve for electrostatics.

Anyhow, it wouldn't be too hard for someone to make an electrostatics version of femm--most of the work would be in ripping out parts of the code for the stuff that is required for magnetics problems. And, of course, putting in different element matrices for the axisymmetric problems....

Dave.

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    tel;work:781-684-4070
    x-mozilla-html:TRUE
    url:http://members.aol.com/dcm3c
    org:Foster-Miller, Inc.;Electrical and Electronic Systems Group
    version:2.1
    email;internet:dmeeker@foster-miller.com
    title:Senior Engineer
    adr;quoted-printable:;;350 Second Avenue=0D=0A;Waltham;MA;02451-1196;USA
    fn:David Meeker, Ph.D.
    end:vcard

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