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% fieldlines.mp
% L. Nobre G.
% 2003
%input featpost3Dplus2D;
Spread := 30;
f := (10,0,0);
color posa, posb;
numeric kfac;
posa = -5*green;
posb = 5*green;
kfac = 2.5;
def vecfunc( expr posit )=
begingroup
save modula, modulb, field;
color field;
numeric modula, modulb;
modula = conorm( posit-posa );
modulb = conorm( posit-posb );
field = ( posit-posa )/(modula**3)+kfac*( posit-posb )/(modulb**3);
( field )
endgroup
enddef;
beginfig(1);
numeric i, lena, lenb, numa, numb, sa, sb, diffstep, ray;
color stp;
path oneline;
pen grossa, fina;
grossa = pencircle scaled 3pt;
fina = pencircle scaled 2pt;
lena = 105;
lenb = 145;
numa = 72;
numb = 30;
diffstep = 1;
ray = 0.5;
sa = 36;
sb = 75;
for i=sa step numa until (360+sa):
stp := posa+( 0, ray*cosd(i), ray*sind(i) );
oneline := fieldlinepath( lena, stp, diffstep, vecfunc );
draw oneline withpen grossa;
% draw oneline withpen fina withcolor background;
endfor;
for i=sb step numb until (360+sb):
stp := posb+( 0, ray*cosd(i), ray*sind(i) );
oneline := fieldlinepath( lenb, stp, diffstep, vecfunc );
draw oneline withpen grossa;
% draw oneline withpen fina withcolor background;
endfor;
draw rp(posa) withpen grossa;
draw rp(posb) withpen grossa;
endfig;
end.
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