When we double the separation between two plates by keeping V constant, what happens to total energy stored in the capacitor?

What type of energy is stored In capacitor?

Assertion : When a dielectric slab of dielectric constant K is introduced between the plates after disconnecting the battery, energy stored in the capacitor decreases. Reason : Potential difference between two plates decrease by a factor K ad charge remains the same, so energy decreases.

A parallel plate capacitor is charged and the charging battery is then disconnected. If the plates of the capacitor are moved farther apart by means of insulating handles. .....(a)the charge on the capacitor increase (b)the voltage across the plates increases (c)the capacitance increases (d)the electrostatic energy stored in the capacitor decreases