StellarA dielectric is an insulating material inserted between the plates of a capacitor. When a dielectric is present, the molecules in the material become polarized in the electric field, which reduces the effective electric field inside the capacitor compared with the vacuum case. Because the same amount of free charge then produces a smaller potential difference, the capacitance increases. This is why adding a dielectric allows a capacitor to store more charge at the same voltage, or lowers the voltage if the capacitor is isolated.
The effect of a dielectric is often described with the dielectric constant , where
for a capacitor completely filled with the dielectric. What changes depends on whether the capacitor remains connected to a battery. If it stays connected, the voltage remains constant and the charge increases. If it is disconnected first, the charge remains constant and the voltage decreases. In both cases, the capacitance increases because the geometry is the same but the medium between the plates changes.