# Potential energy and torque in a magnetic field

1. Jun 7, 2012

### 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?

2. Jun 7, 2012

### 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.

3. Jun 7, 2012

### BrianC12

Ahh alright that makes sense, thanks.