posted by amber .
A small plastic ball with a mass of 7.00 10-3 kg and with a charge of +0.162 µC is suspended from an insulating thread and hangs between the plates of a capacitor (see the drawing). The ball is in equilibrium, with the thread making an angle of 30.0° with respect to the vertical. The area of each plate is 0.0145 m2. What is the magnitude of the charge on each plate?
The portion of weight opposing the electric force is mg*tanTheta, right? Set that equal to the electric force.
(If you don't see the force, then draw the force diagram: mg downward, Fhorizontal. Fhorizontal =Eforce, but then Fhorizontal/mg=tanTheta, so Eforce=mgTanTheta