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?

Question

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?

bobpursley · Answer

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