Final answer:
When the plates of a charged capacitor are pulled apart, the potential difference between them increases because the capacitance decreases while the charge remains constant.
Step-by-step explanation:
When a parallel plate capacitor is disconnected from a battery, the amount of charge on the plates remains constant. However, if the plates are then pulled apart, the potential difference between the plates will increase. This is due to the relationship described by the equation V = Q/C, where V is the potential difference, Q is the charge, and C is the capacitance. Since capacitance C is inversely proportional to the distance between the plates (C = ε₀A/d where ε₀ is the permittivity of free space, A is the plate area, and d is the distance between the plates), increasing the distance d while keeping the charge Q constant results in a decrease in capacitance, and thereby an increase in the potential difference V.