寒山烟雨

python写的动画，用的是Vpython库，需要从VTK官网下载，目前3.x的python只有3.3有这个库，3.4还没有。依旧用的是欧拉折线法绘制电场线的轨迹，和上一篇的Fortran是一样的算法。

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#!/usr/bin/python
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from visual import *
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import cmath as cm
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import numpy
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ball1=sphere(pos=(1.0,1.0,0.0),radius=0.3,color=color.red)
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ball2=sphere(pos=(-1.0,1.0,0.0),radius=0.3,color=color.blue)
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ball3=sphere(pos=(-1.0,-1.0,0.0),radius=0.3,color=color.red)
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ball4=sphere(pos=(1.0,-1.0,0.0),radius=0.3,color=color.blue)
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ball=[ball1,ball2,ball3,ball4]
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q=numpy.array([1.0,-1.0,1.0,-1.0])
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ground=box(pos=(0.0,0.0,0.0),size=(12.0,12.0,0.1),opacity=0.2)
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dt=0.002 
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N=30 
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rate(5)
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for i in range(len(q)):
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    for j in range(N):
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        p=0.32*cm.exp(j*cm.pi*(0.0+1.0j)*2./real(N))
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        line=sphere(pos=(ball[i].x+p.real,ball[i].y+p.imag,0.0),
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        radius=0.02,color=color.red)
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        line.trail=curve(color=line.color)
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        line.trail.append( pos = line.pos, color = (1, 0, 0))
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        while not(abs(line.pos)>15.0 or abs(line.pos-ball[1].pos)<0.32 
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        or abs(line.pos-ball[2].pos)<0.32 or abs(line.pos-ball[3].pos)<0.32 
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        or abs(line.pos-ball[0].pos)<0.32):
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            E=line.pos*0.0
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            if i in [0,2]:
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                for k in range(len(ball)):
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                    E+=q[k]*(line.pos-ball[k].pos)/abs(line.pos-ball[k].pos)**3.0
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            else:
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                for k in range(len(ball)):
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                    E-=q[k]*(line.pos-ball[k].pos)/abs(line.pos-ball[k].pos)**3.0
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            line.pos+=dt*E/abs(E)
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            line.trail.append( pos = line.pos, color = (1, 0, 0))
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效果如图：