A parallel-plate capacitor with plate area \(A\) and plate separation \(d\) is charged by a battery until it carries charge \(Q_0\). The capacitor is then disconnected from the battery so that it remains electrically isolated. A slab of uncharged dielectric material with dielectric constant \(\kappa > 1\) is inserted, completely filling the space between the plates. Which of the following best explains why the magnitude of the net electric field between the plates decreases after the dielectric is inserted?