Figure 20-38 shows a charge q = + 6.77 (C with a mass m = 0.081 kg suspended by a thread of length L = 0.022 m between the plates of a capacitor.

Figure 20-38,

(a) Plot the electric potential energy of the system as a function of the angle u the thread makes with the vertical. (The electric field between the plates has a magnitude E = 4.16 × 104 V/m.)
(b) Repeat part (a) for the case of the gravitational potential energy of the system.
(c) Show that the total potential energy of the system (electric plus gravitational) is a minimum when the angle u satisfies the equilibrium condition for the charge, tan ( = qE/mg. This relation implies that u = 22°.

-0.025 m- 22 +6.77 C