How does inserting a dielectric material into a capacitor affect the charge on its plates?

When you put a dielectric inside a capacitor while keeping the voltage across it fixed, the capacitor’s ability to store charge grows because the dielectric reduces the electric field inside it. This means that for the same applied voltage the capacitor can hold more charge, so the charge on its plates rises. If the capacitor is isolated and its charge is already fixed, inserting a dielectric does not change the charge; instead the voltage across the plates falls because the capacitance increases. For example, a 5 V battery connected to a 10 µF capacitor stores 50 µC; inserting a dielectric that raises the capacitance to 15 µF would increase the stored charge to 75 µC while the voltage stays at 5 V. Thus, the dielectric changes the charge only when the voltage is held constant, not when the charge is already fixed.