# Charged pen and aluminium foil

## Homework Statement

A clear plastic pen of length 14.5 cm is rubbed with wool. The charged plastic pen is held over a small uncharged disk shaped piece of aluminium foil of diameter 0.5cm. How close you must move the pen to the foil in order to pick up the foil?

## The Attempt at a Solution

Ok I thought I would let the disk be a diople and let the pen be a point charge but nothing worked. Please explain to me how I should do this question.

Thank you very much

malawi_glenn
Homework Helper
You migth want to study the forum rules. You MUST attempt the problem, and tells us what relations and formulas you know.

This is what I did?

Ok this is what I did but don't know whether it is right or not:
E = 1/(4 pi epslon) * Q/d^2 is the electric field of the pen

I said that the aluminium foil works like a capacitor therefore:
(qA)/epsilon - 1/(4 pi epslon) * Q/d^2 = 0 also Q=10 nanoC the charge of the pen

then I can figure out the charge of the foil on one side I think(I hope this is right)

Electricfield of disk = ((Q/A)*(s/R))/2epsilon where s=thickness of the disk
also m=dV is the formula for density, but is V= pi r^2 s
then if the above is right we know F = mg
F = ((Q/A)*(s/R))/2epsilon m = D pi r^2 s
therefore:
((Q/A)*(s/R))/2epsilon = D pi r^2 s g

But is this even right, I really need you guys to help me please HELP me !!!