from visual import *
#adapted from Chabay and Sherwood


scene.width = 600
scene.height = 600

# set up params for the coil
Radius = 1.6
L = 2*pi*Radius
Ncoils = 50


#current and stuff
I = 1.0
k = 1E-7 # mu-zero/4pi
Bscale = (2.)/(4*pi*1E-7*Ncoils*I/L) #just combining terms

dphi = 2.*pi/Ncoils #use this to adjust step size - effects accuracy of simulation
phi = arange(0,Ncoils*pi+dphi,dphi)

#Create the coil using a curve object

a = 1  #a,b set scaling for circular or elliptical coils
b = 1
helix=curve(x = a*Radius*cos(phi/2),
            y = b*Radius*sin(phi/2),
            z = phi/100)
helix.color = (1,0.7,0.2)
helix.radius = 0.02

delta = helix.pos[1:] - helix.pos[:-1]
center = (helix.pos[:-1] + helix.pos[1:])/2.
scene.range = 5*(Radius)
vwidth = L/100

#Physics happens here!  This is where the Biot-Savart Law is used
def BField(obs):
    r = obs-center
    rmag = mag(r)
    rmag.shape = (-1,1)
    return sum(k*I*cross(delta, r)/rmag**3)  #notice the different way of writing the Biot-Savart Law

Bvector = arrow(axis=(0,0,0), shaftwidth=vwidth, color=(0,1,1))
drag = 0

while 1:
    if scene.mouse.clicked:
        newobs = scene.mouse.pos
        obs = newobs
        Bvector.axis = Bscale*BField(obs)
        Bvector.pos = obs