One plate of a capacitor is fixed, and the other is connected to a spring as shown in. Area of both the plates is $A$ In strady state (equilibrium), separation between the plates is $0.8d$ (spring was unstretched , and the distance between the plates was $d$ when capacitor was uncharged). The force conntant of the spring is approximately.

A parallel plate capacitor is connected across a battery. Now, keeping the battery connected, a dielectric slab is inserted between the plates. In the process,