#!\/usr\/bin\/python
\nfrom visual import *
\nimport cmath as cm
\nimport numpy<\/p>\nball1=sphere(pos=(1.0,1.0,0.0),radius=0.3,color=color.red)
\nball2=sphere(pos=(-1.0,1.0,0.0),radius=0.3,color=color.blue)
\nball3=sphere(pos=(-1.0,-1.0,0.0),radius=0.3,color=color.red)
\nball4=sphere(pos=(1.0,-1.0,0.0),radius=0.3,color=color.blue)
\nball=[ball1,ball2,ball3,ball4]
\nq=numpy.array([1.0,-1.0,1.0,-1.0])
\nground=box(pos=(0.0,0.0,0.0),size=(12.0,12.0,0.1),opacity=0.2)
\ndt=0.002
\nN=30
\nrate(5)<\/p>\n
for i in range(len(q)):
\n for j in range(N):
\n p=0.32*cm.exp(j*cm.pi*(0.0+1.0j)*2.\/real(N))
\n line=sphere(pos=(ball[i].x+p.real,ball[i].y+p.imag,0.0),
\n radius=0.02,color=color.red)
\n line.trail=curve(color=line.color)
\n line.trail.append( pos = line.pos, color = (1, 0, 0))
\n while not(abs(line.pos)>15.0 or abs(line.pos-ball[1].pos)<0.32 \n or abs(line.pos-ball[2].pos)<0.32 or abs(line.pos-ball[3].pos)<0.32 \n or abs(line.pos-ball[0].pos)<0.32):\n E=line.pos*0.0\n if i in [0,2]:\n for k in range(len(ball)):\n E+=q[k]*(line.pos-ball[k].pos)\/abs(line.pos-ball[k].pos)**3.0\n else:\n for k in range(len(ball)):\n E-=q[k]*(line.pos-ball[k].pos)\/abs(line.pos-ball[k].pos)**3.0\n \n line.pos+=dt*E\/abs(E)\n line.trail.append( pos = line.pos, color = (1, 0, 0))\n<\/code><\/p>\n
\n\t \n<\/p>\n
\n\t\u6548\u679c\u5982\u56fe\uff1a\n<\/p>\n
![\"Eline\"](\"http:\/\/www.hsyyf.me\/wp-content\/uploads\/2014\/07\/Eline.png\")
<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"