In physics, a conserved current, J, obeys a conservation law.

Thus for the conservation of charge, for example, in the notation of Maxwell's equations,

where:

ρ is the free electric charge density (in units of C/m³)

J is the current density:

J = ρv

v is the velocity of the charges.

The equation would apply equally to masses (or other conserved quantities), where the word mass is substituted for the words electric charge above.