Magnitude of the charge on each plate

1. Sep 2, 2010

kvarner83

1. The problem statement, all variables and given/known data
A small plastic ball of mass 9.51 X 10^-3kg and charge of +0.206 micro Coulombs is suspended from an insulating thread and hangs between the plates of a capacitor. The ball is in equilibrium, with the thread making an angle of 30.0 degrees with respect to the vertical. The are of the plate is 0.01352m2. What is the magnitude of the charge on each plate.

2. Relevant equations

E=q/epsilon0A

Fe+FG+T=0

Fe - TSin30=0

TCOS30=mg

3. The attempt at a solution

So I drew a FBD and decided that since it was at equilbrium the force of attraction and repulsion of the plates was zero. And that the only other forces were mg and T. I am just having trouble understanding what to do with the tension and how I am supposed to relate that to the magnitude of charge on each plate.

2. Sep 2, 2010

JaWiB

If you treat the plastic ball as a point charge, then the electric force is F = q_b*E (q_b is the charge on the ball)

Once you solve for T, you should be able to substitute your expression for the electric field into your equation for force in the horizontal direction and simply solve for q