from vpython import *
scene = canvas() # This is needed in Jupyter notebook and lab to make programs easily rerunnable
scale = 4e-14/1e17
ec = 1.6e-19 # electron charge
kel = 9e9 # Coulomb constant
scene.range = 2e-13
charges = [ sphere( pos=vector(-1e-13,0,0), Q=ec, color=color.red, size=1.2e-14*vector(1,1,1) ),
sphere( pos=vector( 1e-13,0,0), Q=-ec, color=color.blue, size=1.2e-14*vector(1,1,1) )]
s = "Click or drag to plot an electric field vector produced by the two charges.\n"
s += "On a touch screen, tap, or press and hold, then drag.\n"
s += "Arrows representing the field are bluer if low magnitude, redder if high."
scene.caption = s
def getfield(p):
f = vec(0,0,0)
for c in charges:
f = f + (p-c.pos) * kel * c.Q / mag(p-c.pos)**3
return f
def mouse_to_field(a):
p = scene.mouse.pos
f = getfield(p)
m = mag(f)
red = max( 1-1e17/m, 0 )
blue = min( 1e17/m, 1 )
if red >= blue:
blue = blue/red
red = 1.0
else:
red = red/blue
blue = 1.0
a.pos = p
a.axis = scale*f
a.color = vector(red,0,blue)
a.visible = True
drag = False
a = None
def down(ev):
global a, drag
a = arrow(shaftwidth=6e-15, visible=False)
mouse_to_field(a)
drag = True
def move(ev):
global a, drag
if not drag: return
mouse_to_field(a)
def up(ev):
global a, drag
mouse_to_field(a)
drag = False
scene.bind("mousedown", down)
scene.bind("mousemove", move)
scene.bind("mouseup", up)