PX157 - C1b - magnetic flux

ϕ_{B} = \iint_{S} \vec{B} \cdot d \vec{S}

importantly, there are no magnetic charges, monopoles
magnetic field lines are always closed, ie: no start/endpoints
solenoidal condition:

\subset \supset \iint_{S} \vec{B} \cdot d \vec{S} = 0

magnetic field lines entering a volume also leave it