A parallel-plate capacitor with plates of length \(L\), width \(w\), and plate separation \(d\) is connected to a battery that maintains a constant potential difference \(V_0\). A slab of linear dielectric material with dielectric constant \(\kappa\) and thickness \(d\) is partially inserted a distance \(x\) into the capacitor along its length (\(0 < x < L\)). Neglecting edge fringing fields and friction, which of the following expressions represents the magnitude of the electrostatic force exerted on the slab pulling it into the capacitor?