Relativistic wave equation and conservation laws

The conservation of charge relates to the mathematical statement of a vanishing divergence of the charge current. Study an example:  the conservation of charge for a complex wave function. Conservation of charge can be derived using the infinitesimal change of phase of a complex wave function.

By “conserved” a physicist means there exists a
o   current,
o   conserved quantity and a
o   continuity equation for the current and charge.

Isotopic spin has three components.
Study the relation between the energy and momentum of a particle.