Final answer:
Inserting a dielectric into a disconnected capacitor lowers the potential difference between the plates and increases the capacitance, as the dielectric reduces the electric field within the capacitor.
Step-by-step explanation:
When a dielectric material is inserted into a disconnected capacitor, which has a fixed amount of charge on the plates, the potential difference between the plates decreases. This is because the dielectric reduces the electric field strength inside the capacitor; fewer field lines go from one side to the other due to polarization of the dielectric. Consequently, the voltage V becomes smaller for the same charge Q, using the relationship V = Ed, where E is the electric field strength and d is the distance between the plates. Since capacitance C is defined as C = Q/V, the capacitance of the capacitor thus increases when the dielectric is inserted.