As I understand it, the convention for the potential energy of a dipole in a uniform electric field has the following zero point:
$U(\pi/2)=0$
I understand how this makes the calculation easier to solve, however I can't help but wonder if this doesn't go in against the law of the conservation of energy:
Imagine a electric dipole placed in a uniform field with an angle of $\pi/2$ to said field. The electric field will put a torque on the dipole, reducing the angle to $0$. According to this convention the potential energy is now at its maximum. However when we now turn off the electric field there is no restoring force on the dipole. If we now replace the dipole with a second dipole, with no potential energy stored in it, there would be no way of differentiating between the two.
Doesn't this mean, using this convention, that energy is lost when stored inside a dipole?
Best Answer
Almost. There will be a torque which will begin to rotate the dipole toward a zero angle. This results in a decrease in potential energy and an increase in kinetic energy and angular momentum. When the dipole reaches zero, however, it doesn't stop. It has angular momentum, so it will continue past zero. If the external electric field remains on, there will be a torque opposite the angular momentum which will cause the dipole to slow down and stop, then swing back toward zero again.
If you stop the electric field at any point, the dipole will continue to move. The system has whatever kinetic energy belongs to the rotation of the dipole, which of course will radiate and eventually stop.
When you turned on the field, you changed the potential energy of the system by +pE. The electrical potential energy formally was 0, and the kinetic energy was 0 for a total mechanical energy of 0. But the system was not constrained and could change position to have a lower potential energy. When the dipole reached the zero angle, the kinetic energy was equal to pE and the system potential energy, - pE for a total mechanical energy of 0.