Final answer:
Adding a dielectric to a disconnected capacitor leads to a decrease in the potential difference between the plates because the electric field strength is reduced, yet the capacitance increases.
Step-by-step explanation:
When a dielectric is added to a capacitor with the battery disconnected, the potential difference between the plates will decrease. This occurs because the dielectric material reduces the electric field strength inside the capacitor. Even though the same charge remains on the plates, the presence of the dielectric causes fewer electric field lines to stretch from one plate to the other. The relationship V = Ed indicates that the voltage (V) is directly proportional to the electric field (E) times the distance (d) between the plates. With a weaker electric field due to the dielectric, the potential difference (voltage) is reduced. However, since the capacitor's charge stays the same and the voltage decreases, the capacitance actually increases, following the formula C = Q/V, where C is capacitance, Q is charge, and V is potential difference.