===== Project: Mini-Particle Accelerator  =====

S.P.A.R.T.A.N force, still trapped in the town of Lakeview, has been sent as part of a larger governmental team to work on developing a micro-particle accelerator on the outskirts of town. Why does a town the size of Lakeview need a micro-particle accelerator? You are not at liberty to say. Your team is tasked with modeling the initial part of the accelerator, which uses a constant electric field to accelerate the charges. The concept is that the particles will enter a tube that is encapsulated by rings of charge. Your team needs to demonstrate that this concept will produce a __constant__ electric field.

=== Part 1: ===
The first bit of code that you have received is from the previous team who were able to construct a single ring of charge and show the electric field due to that ring at some point. Your team should construct the electric field vectors for a circle inside the accelerator (smaller than the ring) at a distance of a few centimeters from the ring face.

<code>
GlowScript 2.7 VPython

display(background = color.white)

#Set up constants
R = 0.02
r_obs = 0.05

Q = 1e-9 
N = 20
dq = Q/N

scale=1e-4
oofpez = 9e9 #1/(4pi epsilon_0) in N m^2/C^2

#Defining a ring at the origin
myring = ring(pos = vector(0,0,0), radius = R, axis = vector(0,0,1), color = color.blue, thickness = 0.02*R)

#Create an empty list for the charges
ChargeList=[]

#Set up the step size and angle for creating the charges
dtheta = 2*pi/N 
theta = dtheta/2 

#Create charges in a circle and add them to the ChargeList
while theta < 2*pi:
    rpiece = R*vector(cos(theta),sin(theta),0) #location of piece
    
    particle = sphere(pos = rpiece, radius = R/20, color = color.yellow)
    ChargeList.append(particle)
        
    theta = theta + dtheta

#Create an empty list for the observation points
ObsList = []

#Set up the step size and angle for creating the observation points
phi = 0
dphi = pi/4

#Create observation points in a circle and add them to the ObsList
while phi < 2*pi:
    r_obs_piece = r_obs*vector(cos(phi),sin(phi),1) #location of piece
    
    obs_particle = sphere(pos = r_obs_piece, radius = R/20, color = color.red)
    
    ObsList.append(obs_particle)
        
    phi = phi + dphi

#Find the electric field at each observation point
for obs_point in ObsList:
        
    for charge in ChargeList:
        Enet=vec(0,0,0)     
</code>

=== Part 2 ===

After you got this initial code working, your team was able to construct a model of a tube consisting of multiple rings, all with the same charge. But, the field doesn't look quite right - it's not constant as expected. Your bosses seem to think the field can be made constant in the tube, so it's up to you to figure out how. 

<code>
display(background = color.white)

num_points = 10
num_rings = 11
N = 11
spacing = 0.02

# Set some constants and stuff
R=0.02 #radius of ring in m
ax = vector(0,0,1) # simplify things
Q=1e-9 #charge of ring in C
oofpez=9e9 #1/(4pi epsilon_0) in N m^2/C^2

#draw axis
zaxis=cylinder(pos=-2*R*ax, radius=0.015*R, axis=4*R*ax, color=color.black)

#draw points
points = []
for i in range(num_points):
    
    xr = 0.01*sin(i*2*pi/num_points)
    yr = 0.01*cos(i*2*pi/num_points)
    
    points.append(sphere(pos=vector(xr,yr,0.01), color=color.red, radius=5*zaxis.radius))

#make and draw rings
rings = []
ring_charge = [Q,Q,Q,Q,Q,Q,Q,Q,Q,Q,Q]

for i in range(num_rings):
    
    loc = i - (num_rings)//2
    rings.append(ring(pos=vector(0,0,spacing*loc), radius=R, axis=ax, color=color.blue, thickness=0.02*R))

# Find net field
for apoint in points:

    Enet = vector(0,0,0)
    for i in range(len(rings)):
        aring = rings[i] # look at one ring

        dq = ring_charge[i]/N #charge of a piece
        dtheta = 2*pi/N #theta increment for our loop
        theta=dtheta/2 #initial theta for first piece of loop
        Ering = vector(0,0,0) #net electric field for single ring

        rpoint = apoint.pos

        scale=1.2*mag(rpoint)/8000 #used to scale the arrows representing E-field

        while theta<2*pi:
            rpiece = R*vector(cos(theta),sin(theta),aring.pos.z/R) #location of piece
            r = rpoint-rpiece #vector from piece to point in space
            rmag = mag(r) #magnitude of r
            rhat = norm(r) #unit vector for r
            dE = oofpez * dq / rmag / rmag * rhat # Electric field of peice of ring
            Enet = Enet + dE
            particle=sphere(pos=rpiece, radius=apoint.radius, color=color.yellow) #draw a particlee
            theta=theta+dtheta

    Evector=arrow(pos=rpoint, axis=scale*Enet, color=color.orange, shaftwidth=apoint.radius/2)
</code>

<WRAP INFO>
===Learning Goals===
  * Review ideas of electric field, superposition, separation vectors, and distributions of charge 
  * Review ideas of interpreting code: while and for loops, lists, creating objects/shapes
  * Explain how you created a constant electric field using rings of charge
  * Explain what would happen to a charge if it were placed in the electric field
</WRAP>

Conceptual Questions:
  - (Part 1) What does it mean for a vector (like Electric Field) to be "constant"?
  - (Part 1) What would you expect the electric field to look like for an observation point at the center of the ring? What about a ring of observation points aligned with the ring (z=0) but with a radius inside the ring of charge? What about a ring of observation points with some z value away from the ring of charge? Draw these out on the whiteboard.
  - (Part 1) Comment your code line-by-line and make sure everyone understands what each line is doing.
  - (Part 2) When you have multiple rings of charge, how do you get a constant electric field?
  - (Part 2) What is a surface charge gradient?
  - (Part 2) Why does this set up count as an "accelerator"? What would you need to change to make the accelerator stronger?