3.A small bead of mass m carrying a charge q is attached to one end of a massless string. The other end of the string is fixed to a uniformly charged vertical wall. Under the influence of both the gravity force and the electric force exerts on the bead (the gravitational acceleration is g), the string makes an angle θ = 30° to the wall when mechanical equilibrium of the system is achieved. What is the value of the surface charge density σ?


(A)
(B)
(C)
(D)
(E)

答案:登入後查看
統計: 尚無統計資料