VPython mock-up of the attraction between two charged pieces of tape, used in intro E&M labs, representing the tapes as charged balls hung from strings.

tape_sim_2.py

from visual import *

# Constants and other definitions
e=1.6e-19
k=9e9
g=9.80
length=0.21
height=0.3
sep=0.1524
mass=0.0003
charge=2.0e-8
scale=1
spring=100
drag=0.001

# Pivot points for the pendulums
pivot1=sphere(pos=vector(sep/2,height,0), radius=0.02, color=color.white)
pivot2=sphere(pos=vector(-sep/2,height,0), radius=0.02, color=color.white)

# The suspended point charges that stand in for charged tapes
mass1=sphere(pos=vector(sep/2, height-length,0), radius=0.02, color=color.green)
mass1.m=mass
mass1.q=charge
mass1.p=vector(0,0,0)
mass1.pos.y=mass1.pos.y-mass1.m*g/spring #stretch to account for the mass

mass2=sphere(pos=vector(-sep/2,height-length,0), radius=0.02, color=color.yellow)
mass2.m=mass
mass2.q=charge
mass2.p=vector(0,0,0)
mass2.pos.y=mass2.pos.y-mass2.m*g/spring #stretch to account for the mass

# Strings for the pendulum, to make it look pretty
string1=cylinder(pos=pivot1.pos, axis=mass1.pos-pivot1.pos, radius=0.005, color=color.white)
string2=cylinder(pos=pivot2.pos, axis=mass2.pos-pivot2.pos, radius=0.005, color=color.white)

# Gravitational forces
fgrav1=mass1.m*g*vector(0,-1,0)
fgrav2=mass2.m*g*vector(0,-1,0)

# Treating the strings as springs to get the right tension force
stretch1=string1.axis.mag-length
stretch2=string2.axis.mag-length
ft1=-(spring*stretch1)*(string1.axis/string1.axis.mag)
ft2=-(spring*stretch2)*(string2.axis/string2.axis.mag)

# Electrostatic force between the charged balls
r=mass1.pos-mass2.pos
rhat=r/r.mag
felec1=-(k*mass1.q*mass2.q/r.mag2)*rhat
felec2=-felec1

# Arrows showing the force
fgarr1=arrow(pos=mass1.pos, axis=scale*fgrav1, color=color.blue)
fgarr2=arrow(pos=mass2.pos, axis=scale*fgrav2, color=color.blue)

ftarr1=arrow(pos=mass1.pos, axis=scale*ft1, color=color.magenta)
ftarr2=arrow(pos=mass2.pos, axis=scale*ft2, color=color.magenta)

felarr1=arrow(pos=mass1.pos, axis=scale*felec1, color=color.cyan)
felarr2=arrow(pos=mass2.pos, axis=scale*felec2, color=color.cyan)

# Loop parameters
tmax=5
t=0
dt=0.001

while t<tmax:
    rate(1000)
    #print(t)

    #Recalculate forces
    stretch1=string1.axis.mag-length
    stretch2=string2.axis.mag-length
    ft1=-(spring*stretch1)*(string1.axis/string1.axis.mag)
    ft2=-(spring*stretch2)*(string2.axis/string2.axis.mag)

    r=mass1.pos-mass2.pos
    rhat=r/r.mag
    felec1=-(k*mass1.q*mass2.q/r.mag2)*rhat
    felec2=-felec1

    #Drag force to make the system settle near equlibirum point

    fdrag1=-drag*mass1.p/mass1.m
    fdrag2=-drag*mass2.p/mass2.m

    fnet1=fgrav1+ft1+felec1+fdrag1
    fnet2=fgrav2+ft2+felec2+fdrag2

    # Update momentum and position of the charged balls
    mass1.p=mass1.p+fnet1*dt
    mass1.pos=mass1.pos+mass1.p*dt/mass1.m
    mass2.p=mass2.p+fnet2*dt
    mass2.pos=mass2.pos+mass2.p*dt/mass2.m

    # Update all the miscellaneous graphics
    string1.axis=mass1.pos-pivot1.pos
    string2.axis=mass2.pos-pivot2.pos

    fgarr1.pos=mass1.pos
    fgarr2.pos=mass2.pos

    ftarr1.pos=mass1.pos
    ftarr1.axis=scale*ft1
    ftarr2.pos=mass2.pos
    ftarr2.axis=scale*ft2
    felarr1.pos=mass1.pos
    felarr1.axis=scale*felec1
    felarr2.pos=mass2.pos
    felarr2.axis=scale*felec2

    t=t+dt

    # If the balls are in contact, stop the loop.
    r=mass1.pos-mass2.pos
    if r.mag<(mass1.radius+mass2.radius):
        print(t)
        t=tmax+dt
        
        
# Print out some parameters for data recording

angle=abs(asin((mass2.pos.x-pivot2.pos.x)/string2.axis.mag))
print(t/tmax,r.mag/(mass1.radius+mass1.radius),(r.x)/(sep),angle)