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# Re: various

- To: femm@egroups.com
- Subject: Re: various
- From: dcm3c@aol.com
- Date: Thu, 1 Jun 2000 17:06:21 EDT

    In a message dated 5/31/00 6:43:48 AM Eastern Daylight Time,
    tecnico@sincro.it writes:

    > I have observed the following while I was creating a geometry:
    > I already have some geometry (points, lines & arcs).
    > I switch to line (or arc) drawing mode and select 1 point.
    > Now I switch to selection mode, select some lines or arcs, copy them
    > somewhere. As I see femm copies the last selection and not the first
    > point (even if it was highlighted).
    > Now nothing is highlighted, but if I click another point fem will draw a
    > line (or arc).
    >
    > I think that when I switch to the selection tool femm should forget any
    > previous selection,
    > or copy it also. Anyway, after the copy operation the first point should
    be
    > completely forgotten.

    Yes, I can reproduce this behavior, and it does seem a bit inconsistent with
    what one would expect to happen. When you are selecting points in either
    segment or arc segment mode, these are for the endpoints of a new line or
    arc. The program remembers that you are in the middle of a line drawing
    operation after the move that you described is over, hence a line is drawn
    when a point is clicked after the move. I'll change it so that it does
    something more consistent.

    > There are two small things that I would like in femmview:
    > In harmonic problems I find often necessary to convert line and block
    > integral result in polar complex values (R and theta) and not only in
    Real+Imaginary.

    I find myself doing the same thing a lot of times. I've been thinking about
    letting the user select between complex and polar notation for these results
    (as well as the point values)

    > I like very much the new waj for block selection, is it possible to have
    the
    > same for line selection
    > (choose arcs and segments instead of points)?

    The problem is that the line integrals don't lend themselves as well to this
    approach. For me, most of the time the contours of interest
    for intergration or plotting don't coincide with lines or arcs drawn in the
    geometry.

    In a message dated 5/31/00 5:55:49 AM Eastern Daylight Time,
    rlbrambilla@cesi.it writes:

    > Dear David and FEMM-friends,
    >
    > I'd like to stress you that FEMM can be used to
    > compute electric fields in the case of plane
    > problems (xy-cartesian or polar problems, NOT axisymmetric),
    > if you identify elctric potential (scalar) U(x,y) to the
    > z-component of vector magnetic potential A(x,y).
    > You will have
    >
    > Ex = -dU/dx Ey = -dU/dy
    > Bx = +dA/dy By = -dA/dx
    >
    > So you have to identify U = A, Ex = By and Ey = -Bx.
    > Boundary values for A now are simply the boundary values
    > you will use for U.
    > In the case of different materials you have to insert
    > in properties mu = 1/er, where er is the relative
    > (adimensional) permittivity of material to satisfy the
    > boundary discontinuities of electric field (normal component).
    > The analogy ceases in axysimmetric problems since grad
    > and rot in cylindrical coordinates (r,z) involve different
    > derivatives.

    Yes, by analog, it is possible to trick femm into solving other types of
    problems, as long as you are careful how you interpret the results. These
    problems would include potential flow of fluids, heat flow, ground water
    flow, elastic membrane deflection, and so on. I've been threatening to cook
    up a more general version of the program that is not so tied to magnetics
    (sometimes I have to solve other types of problems), but add that to the list
    of things that I haven't had time to do lately...

    > Now a little problem : how can I represent a vertical homogeneous
    > magnetic field Bz=Bo in plane and in axisymmetric problems?

    One way to do it is to assign a mixed boundary condition to the outer edge in
    which the c1 parameter is zero and the c2 parameter is equal to mu_o * Bz,
    where mu_o is the permeability of free space, and Bz is the desired magnitude
    of the homogeneous field. This is the equivalent of assigning a surface
    current to the outer boundary that forces the field distribution that you
    want. Because it is a small file, I have attached an example of applying
    this boundary condition to an axisymmetric problem. This problem is a
    cylindrical geometry filled with air. The choice of boundary condition
    yields an even 1 T field throughout the entire region.

    Dave.

Attachment: `uniform.zip`
Description: Zip archive

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