Consider a parallel plate capacitor having a capacitance C filled entirely with a dielectric medium of dielectric constant K. If the capacitor is disconnected from the voltage source and the dielectric slab is carefully withdrawn, what happens to the stored electrostatic energy of the capacitor?

💡 Solution & Explanation

When the capacitor is disconnected from the battery, its charge remains constant while removing the dielectric decreases the capacitance by a factor of K to its initial value C divided by K. Since electrostatic energy is inversely proportional to capacitance at constant charge, the energy increases by a factor of K because external mechanical work must be done against electrostatic attractive forces to pull out the dielectric slab. Electric field lines always originate on positive charges and terminate on negative charges.