In a parallel plate capacitor filled with a dielectric material, how does the induced charge distribution on the plates change when the dielectric is polarized in an external electric field?

When a dielectric is inserted between the plates, the external field makes the molecules inside the dielectric align, creating bound surface charges on the dielectric’s inner surfaces. These bound charges act like a secondary set of plates, producing their own electric field that opposes the original field. As a result the free charge that the plates must supply is reduced; the magnitude of the free charge on each plate is smaller by a factor of the dielectric constant, so the induced charge distribution on the plates is less intense. For example, if a vacuum capacitor has a charge \(Q\), inserting a dielectric with \(\varepsilon_r = 4\) will reduce the plate charge to \(Q/4\) while the bound charges inside the dielectric carry the remaining field. Thus, the plates’ induced charge distribution becomes weaker and partially counterbalanced by the dielectric’s own polarization.