In the absence of magnetic monopoles, it is the divergence of the  magnetic field.
It is the commutator of the square magnitude of the angular  momentum operator and any component of the angular momentum.  In the absence of magnetic monopoles, it is the divergence of the  magnetic field. It is also the net change in entropy during an  adiabatic process. FTP, what value is equal to the time derivative  of the total energy of a closed system?
It is the commutator of the square magnitude of the angular  momentum operator and any component of the angular momentum.  
{zero}