Potential energy and torque in a magnetic field

[BrianC12]

If you have a coil with its magnetic dipole moment perpendicular to a uniform magnetic field, μ × B says that there will be maximum torque since sin90 is 1. But at the same time, -μ dot B gives the potential energy and cos90 is 0. What I don't understand is how you can have torque at this angle, but no potential energy. If you rotate the coil to a perpendicular angle and release it, it'll automatically rotate until the angle is 0. If there were no potential energy wouldn't it just stay in that position?

 

[Muphrid]

The zero of potential energy here is not its minimum. When the magnetic field and dipole moment are aligned, you get -\mu \cdot B = -|\mu||B|. The torque is trying to align the dipole moment to this minimum energy configuration.

 

[BrianC12]

Ahh alright that makes sense, thanks.