A little charged bead is inside a fixed and uncharged hollow sphere of radius \(R\) made of insulating material. The mass of bead is m and charge is \(q\). A charge \(Q\) when kept at bottom of the hollow sphere, the bead is in equillibrium at the vertex of the sphere. If the bead is in stable equillibrium then \(Q \geq \)
Assume \(k\) is an electrostatic constant.