For 10 points, name this entity most commonly associated with objects with north and south poles.
This entity's presence in a two-dimensional conductor creates Landau levels in the quantum hall effect. The Lorentz force on a particle is given by charge times velocity cross this entity, which is equal to the curl of the vector potential. They are expelled from superconductors in the Meissner effect, and Faraday's law of induction describes how a changing one produces an electrical current. They are formed when an electrical current passes through a solenoid, and Gauss showed that the divergence of these entities is zero unless monopoles exist. For 10 points, name this entity most commonly associated with objects with north and south poles.
This entity's presence in a two-dimensional conductor creates Landau levels in the quantum hall effect.
{Magnetic Field} [accept {B-field}]