Motion of a dipole in an electric field Assume that we have some non-constant electric field $E(x,t)$ and a point-dipole at a position $q$ with a constant dipole moment $\vec{p}$. How would you describe the time evolution, i.e. the motion of such a dipole?
 A: When an electric dipole is placed in a uniform electric field making an angle with the direction of the field as shown in the figure.

Force on charge $-q=-q\overrightarrow{E}$ (opposite to $\overrightarrow{E}$)  
Force on charge $+q=q\overrightarrow{E}$  (along $\overrightarrow{E}$)   
Thus, electric dipole is under the action of two equal and unlike parallel forces, which give rise to a torque on the dipole.   
So, in a uniform electric field, an electric dipole experiences only a torque. But, when the electric field is non-uniform as you say, it experiences torque as well as net force.
A: The energy of a dipole in an electric field is just the sum of the energies of it's constituents, and is easily seen to be $U = -\mathbf{p} \cdot \mathbf{E}$. From this you can derive the dynamics of the dipole through the generalized forces $-\boldsymbol\nabla_\mathbf{p} U$ and $-\boldsymbol\nabla_\mathbf{x} U$.
This assumes that the dipole's constituents are much closer together than the length scale over which the field varies.
From the above you can see that the dipole will tend to orient itself along the electric field. Furthermore, if the field is not uniform in space, the dipole will move in the direction of increasing/decreasing field strength (depending on the instantaneous orientation).
