Why Is the Potential Energy of a Dipole Zero at 90 Degrees?

[maf249]

I am a little confused as to why my book is saying that the potential energy of a dipole is 0 at 90 degrees relative to the electric field lines. When it has the stored energy to turn from 90deg to 0deg if no force is acting on it. Wouldn't it make sense to use 0deg as a reference point then?

 

[Gordianus]

Every potential energy has an arbitrary zero point. Your book chose 0 degrees so the PE will range from minus some value to plus the same value. Of course, you´re free to choose another reference. Only PE changes matter.