This is a challenging problem that requires you to construct a model of a real physical situation, to make idealizations,
approximations, and assumptions, and to work through a detailed analysis based on physical principles. Be sure to allow
yourself enough time to think it through.
You may need to measure, estimate, or look up various quantities.
A clear plastic ball-point pen is rubbed thoroughly with wool. The charged plastic pen is held above a small, uncharged
diskshaped piece of aluminum foil, smaller than a hole in a sheet of three-ring binder paper.
(a) Make a clear physics diagram of the situation, showing charges, fields, forces, and distances. Refer to this physics
diagram in your analysis.
(b) Starting from fundamental physical principles, predict quantitatively how close you must move the pen to the foil in
order to pick up the foil (that is, predict an actual numerical distance). State explicitly all approximations and
assumptions that you make.
(c) Try the experiment and compare your observation to your prediction.