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Calculating Fields etc - Another Approach

One of life's little ironies... in late December I began a job with
Integrated Engineering Software, benchmarking electromagnetic modelling
software and training customers. Between that and some stuff still on
the go with DC Tech I've been too busy for such activities as keeping up
with PHYS-L. Now that I come back I find that you're all just wrapping
up a discussion of just the sort of thing our software does.

Its a little late, but since there seems to have been a lot of interest
in this, here's my contribution - demonstrating another approach based
largely on linear algebra that you will probably find faster and more
accurate, and may find more aesthetically pleasing.

So far you have been using the Finite Element Method. This is probably
the most obvious approach. Break 2D space up into areas, or 3D space up
into volumes, perform the derivatives and use feedback to iteratively
zero in on derivatives that match some appropriate E&M equations. I did
this in 1992 with my second year E&M class, I just set up the basic
routine in Lotus and encouraged them all to take the spreadsheet and
try out their own configurations. As you work with this method however
some drawbacks become apparent: you get feedback from the boundary, so
you need to model more space than you are interested in; because you
differentiate, errors are accentuated; the number of elements you need
in your model goes as the cube of the degree of spatial resolution

Our software mainly uses a less obvious but (to me) pleasing method to
avoid these problems - the Boundary Element Method. If you can solve
the problem on the boundary, its trivial to calculate the solution
anywhere (not just nearby). The number of elements is related to
surface area of the actual physical boundaries, so the number elements
only goes as the square of the spatial resolution. Also, since the
problem is done with integration it tends to be more accurate. Here's a
brief explanation and some sample code:

I'll deal with the case of a conducting surface held at some potential
to illustrate how this method can work. Break the surface up into a
series of surface elements each of which has some charge Qi distributed
over it. Now define a geometrical matrix G which tells you the
potential on one element due to charge Qj on another element. Then
Vi = sum(Qj*Gij)
or, in pure matrix terms, V & Q can be column or row matrices, G can be
calculated geometrically, V is known, so Q = V/G - a plain linear
algebra problem. Adding as many charges as we want distributed around
space is not a problem
Vi = sum(Qj*Gij) + Vq1i + Vq2i + Vq3i + ...
= sum(Qj*Gij) + total_Vi_due_to_charges
with Vi' = Vi - total_Vi_due_to_charges
Q = G/Vi

So the trick is to illustrate the basics of the method and not give away
any company secrets... what I've done is to produce some code to run in
the matrix math software packages Matlab/Scilab/Octave (translate into a
spreadsheet or Fortran if those are more to your liking - but I would
point out that Scilab and Octave are free). I hope the approach is
fairly self-evident. The most obvious thing would be to consider each
surface element to be a point charge, then Gij = 1/Rij. The problem
with this -- the self-potential of a point charge is infinite (oops).
So, as a next approximation I treat each surface element as consisting
of a point charge Qi at the center and 4 nodes for sampling Vi. The
code as written below took about 1-2 minutes to run. The sampling of
10,000 random locations on the surface consistently produced mean values
of about V=0.999 (I was modelling V=1) with a standard deviation of
0.03. Cutting the elements down with x=-5:5 the calculation was very
fast and V_average was still 0.999, but had a standard deviation of 0.1.

These 10,000 random locations actually represent the worst case for
calculations from the surface solution. Move a little ways away and the
calculation will be more accurate. (Like the tv picture being better
when you step back a bit - the effect of graininess averages out.) One
can similarly calculate V or E anywhere in space, near or far and Qtotal
is just the sum of the surface Qi elements.

If anybody wants to try the method out, there should be enough hints
here to get you going, and a web search for "Boundary Element Method"
also turns up quite a bit of good help.

**** here's the code:

%% a way of setting up the elements with complex numbers for surface
x = -11 : 11;
elements = ones(length(x), 1)*x + i*x'*ones(1,length(x));
elements = reshape(elements, 1, length(x)^2);

%% defining the Gij's
for counter = 1:length(x)^2
for counter2=1:length(x)^2
transfer(counter, counter2) = elements(counter) -
transfer = 0.25./abs(transfer+0.25) + 0.25./abs(transfer-0.25) +
0.25./abs(transfer+0.25*i) + 0.25./abs(transfer-0.25*i);

%% defining Vi's
V = ones(1,length(x)^2);

%% calculating Qi's
Q = V/transfer;

%% verify V_average=1 for 10000 randomly selected locations
voltages = [];
for counter = 1 : 10000
x = rand;
y = rand;
voltages = [voltages; x y sum(Q.*(1./abs(elements - x -i*y)))];


Simple enhancements include smaller elements near the edges (with Gij
scaled appropriately) and trimming elements out of the square definition
to get a more interesting geometry. A 3D surface will force you to do
the Gij calculations with something besides complex numbers.


Doug Craigen