# Author: Alex Gezerlis

# Numerical Methods in Physics with Python (CUP, 2020)

import numpy as np

import matplotlib.pyplot as plt

from math import sqrt

from copy import deepcopy

def makefield(xs, ys):

qtopos = {1: (-1,0), -1: (1,0)}

n = len(xs)

Exs = [[0. for k in range(n)] for j in range(n)]

Eys = deepcopy(Exs)

for j,x in enumerate(xs):

for k,y in enumerate(ys):

for q,pos in qtopos.items():

posx, posy = pos

R = sqrt((x - posx)**2 + (y - posy)**2)

Exs[k][j] += q*(x - posx)/R**3

Eys[k][j] += q*(y - posy)/R**3

return Exs, Eys

def plotfield(boxl,n):

xs = [-boxl + i*2*boxl/(n-1) for i in range(n)]

ys = xs[:]

Exs, Eys = makefield(xs, ys)

xs=np.array(xs); ys=np.array(ys)

Exs=np.array(Exs); Eys=np.array(Eys)

plt.streamplot(xs, ys, Exs, Eys, density=1.5, color='m')

plt.xlabel('$x$')

plt.ylabel('$y$')

plt.show()

plotfield(2.,20)