When a dielectric slab is gradually inserted between the plates of an isolated parallel-plate capacitor, the energy of the system come out to be a linear function of xdisplacement of the slab inside capacitor measured from the center of the plate).

since x decreases, the energy of the system decreases.

We know that

Where

is the rate of change of potential energy function with x

-ve sign indicates that force is in negative direction when energy increases with respect to x)

Therefore, Force on the slab exerted by the electric field is constant and positive.