JEE Main Physics Class 12 Electrostatic Potential and Capacitance Part 1 Questions

The potential energy of a charged capacitor is given by U=

Q2

2C

.
When a dielectric slab is introduced between the plates the energy is given by

Q2

2KC

where K is the dielectric constant.
Again, when the dielectric slab is removed slowly its energy increases to initial potential energy. Thus, work done is zero.