Chapter 25, Problem 31P

A small object with electric dipole moment $\stackrel{\to }{p}$ is placed in a nonuniform electric field $\stackrel{\to }{E}=E(x)\hat{i}$. That is, the field is in the x direction, and its magnitude depends only on the coordinate x. Let θ represent the angle between the dipole moment and the x direction. Prove that the net force on the dipole is

$F=p\left(\frac{dE}{dx}\right)\cos \theta$

acting in the direction of increasing field.