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