Potential energy and magnetic torque

[Jaymo3141]

i'm having a hard time understanding why potential energy is 0 when u(mu) is perpendicular to B(mag field). Isn't the torque at max when the angle between u and B is 90? and there for has potential to do work all the way until the angle is 0. The way my mind sees it is, since there is a torque on the loop, it will move. Since it wants to move it should therefore have potential energy, just like if a box is up in the air and wants to fall. I just don't understand how potential energy goes from 0 to negative. It doesn't make sense to me. Any help would be greatly appreciated. thank you

 

[ehild]

It is arbitrary to what position you assign the zero of potential energy. Only the change counts. The change from zero to some -U value is the same as from U to zero.

ehild