structural:expansion, information:classification
Magnetic torque on permanent magnets

In terms of the pole model, two equal and opposite magnetic charges experiencing the same H also experience equal and opposite forces. Since these equal and opposite forces are in different locations, this produces a torque proportional to the distance (perpendicular to the force) between them. With the definition of m as the pole strength times the distance between the poles, this leads to τ = μ m H sin θ, where μ is a constant called the vacuum permeability, measuring 4π × 10 V · s /(A · m) and θ is the angle between H and m.