# torque in electric dipole placed in nonuniform electric field

In the uniform electric field case, we can know the rotational axis is at the position in the middle of positive charge and negative charge.

However, if the electric field is non-uniform, or simply to say the perpendicular force (perpendicular to the electric dipole moment) acting on the positive charge and the negative charge are not the same.

I guess the rotational axis is no longer in the middle of two charges, but can I obtain the "new rotational axis", or do we have such thing? So how can we obtain the electric dipole moment?

Thank you!

• the field being non uniform the dipole can experience a net force in the direction of larger field as well as a torque . – drvrm Jul 2 '16 at 15:03

If it is a static case, the curl of E is zero, and that type of electric field can't act on a dipole. THIS ANSWER DOES NOT APPLY TO THE QUESTION ASKED. DISREGARD IT. I had thought the question was about an E field that varied perpendicular to its direction. Sorry about that chief.

• Sorry, I am not familiar with curl yet. But do you mean the dipole is not defined in nonuniform electric field case? Furthermore, assume my electric field depends on the position in space, can I argue that the "length" (or the size) of the electric dipole is small so that I can assume the the electric field passes the electric dipole is "uniform"? – tomchan516 Oct 24 '15 at 8:18
• If you put a dipole of moment ${\bf p}$ in a uniform electric field ${\bf E}$, the torque on the dipole is ${\bf \tau} = {\bf p} \times {\bf E}$? – jim Aug 4 '16 at 18:58