A parallel-plate capacitor with rectangular plates of length \(L\), width \(w\), and plate separation \(d\) is connected to a battery that maintains a constant potential difference \(V_0\). A dielectric slab with dielectric constant \(\kappa\) and thickness \(d\) is partially inserted a distance \(x\) between the plates, where \(0 < x < L\). Which of the following expressions represents the magnitude of the electrostatic force exerted on the dielectric slab by the electric field?