Magnetic moment of a current loop: $\vec{m} = NI\vec{A}$ (direction: right-hand rule with current). Torque in uniform field: $\vec{\tau} = \vec{m} \times \vec{B}$, magnitude $\tau = mB\sin\theta$. Potential energy: $U = -\vec{m} \cdot \vec{B} = -mB\cos\theta$. Stable equilibrium: $\theta = 0$ ($\vec{m} \parallel \vec{B}$). Unstable: $\theta = \pi$. Work done to rotate from $\theta_1$ to $\theta_2$: $W = mB(\cos\theta_1 - \cos\theta_2)$.