Q. A capacitor is made of two circular plates of radius each, separated by a distance . The capacitor is connected to a constant voltage. A thin conducting disc of radius and thickness is placed at the centre of the bottom plate. Find the minimum voltage required to lift the disc if the mass of the disc is .

 2782  211 Electrostatic Potential and Capacitance Report Error

Solution:

image
As shown in figure, the disc is in touch with the bottom plate. The electric field on the disc is Therefore, charge transferred to the disc

Force acting on the disc,

If the disc is to be lifted, ,
i.e.,
(for minimum )