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# electric field

- To: femm@egroups.com
- Subject: electric field
- From: Roberto Brambilla <rlbrambilla@cesi.it>
- Date: Wed, 31 May 2000 11:58:33 +0200

    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.

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

    Yours, Roberto

    Roberto Brambilla
    CESI
    Via Rubattino 54
    20134 Milano
    tel +39.2.2125.875
    fax +39.2.2125.610
    rlbrambilla@cesi.it

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