A parallel plate capacitor is made of two plates of length \(l\), width \(w\) and separated by distance \(d\). A dielectric slab ( dielectric constant \(K\)) that fits exactly between the plates is held near the edge of the plates. It is pulled into the capacitor by a force \(F = -\frac{{\partial U}}{{\partial x}}\) where \(U\) is the energy of the capacitor when dielectric is inside the capacitor up to distance \(x\) (See figure). If the charge on the capacitor is \(Q\) then the force on the dielectric when it is near the edge is

1. A \(\frac{{{Q^2}d}}{{2w{l^2}{\varepsilon _0}}}K\)
2. B \(\frac{{{Q^2}w}}{{2d{l^2}{\varepsilon _0}}}\left( {K - 1} \right)\)
3. C \(\frac{{{Q^2}d}}{{2w{l^2}{\varepsilon _0}}}\left( {K - 1} \right)\)
4. D \(\frac{{{Q^2}w}}{{2d{l^2}{\varepsilon _0}}}K\)