example 12

This chart:

was produced by this code:

import Graphics.Rendering.Chart.Easy
import Graphics.Rendering.Chart.Backend.Cairo

r' x y z = sqrt $ x^2 + y^2 + z^2
efield sign x y = ( sign*x/r,sign*y/r) where r = r' x y 10
bfield sign x y = (-sign*y/r^2,sign*x/r^2) where r = r' x y 10
square a s = [(x,y) | x <- range, y <- range] where range = [-a,-a+s..a] :: [Double]
add (x1,y1) (x2,y2) = (x1+x2,y1+y2)

ef (x,y) = efield 1 (x-20) y `add` efield (-1) (x+20) y
bf (x,y) = bfield 1 (x-20) y `add` bfield (-1) (x+20) y
grid = square 30 3

vectorField title f grid = fmap plotVectorField $ liftEC $ do
    c <- takeColor  
    plot_vectors_mapf .= f
    plot_vectors_grid .= grid
    plot_vectors_style . vector_line_style . line_color .= c
    plot_vectors_style . vector_head_style . point_color .= c
    plot_vectors_title .= title

main = toFile def "example12_big.png" $ do
    setColors [opaque black, opaque blue]

    layout_title .= "Positive and Negative Charges"
    plot $ vectorField "Electric Field" ef grid
    plot $ vectorField "B-field" bf grid