# Homework Help: Parallel Plate Capacitors

1. Jan 17, 2005

### figs

A small plastic ball of mass 6.95×10-3 kg and charge +0.11 mC 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° with respect to the vertical. The area of each plate is 0.0152 m2. What is the magnitude of the charge on each plate? (the picture shows the positive plate on the left, the ball leaning 30 deg toward the negative plate, and the negative plate on the right.

With this problem, I solved for E using E=(q)/(AEo). I don't know where to go from here though. I'm confused.

Last edited: Jan 17, 2005
2. Jan 18, 2005

### clive

Write the mechanical equilibrium of the ball
$$\vec{T}+\vec{G}+\vec{F_{el}}=0$$
onto the horizontal and vertical direction. You now that
$$G=mg$$
$$F_{el}=qE$$

and $$E=\frac{Q}{\epsilon_0 A}$$

.........

Last edited: Jan 18, 2005
3. Jan 18, 2005

### figs

oooo, i set up my vectors wrong in the beginning!
how silly of me, thank you very much for the help.