(21-66) Suppose a dipole P is placed in a nonuniform electric field E(x)=E(x)i that points along th

(21-66) Suppose a dipole P is placed in a nonuniform electric field E(x)=E(x)i that points along the x axis. If E depends only on x, show that the net force on the dipole is F=(P∙dE/dx)i, where dE/dx is the gradient of the field in the x direction.