For 10 points, name this quantity usually symbolized rho, whose average value is given by mass divided by volume.
The commutator of the Hamiltonian and an object with this name is equal to i h-bar times the time derivative of that object, according to an equation named for von Neumann and Liouville. That matrix with this name describes mixed states in quantum mechanics. The continuity equation in fluid dynamics states that the negative partial of this quantity with respect to time is equal to the divergence of the product of this quantity and the (*) fluid velocity; that is a consequence of the conservation of mass. The conductivity times the electric field gives this quantity for electric current, according to Ohm's law. This quantity for electric current is equal to the electric current per unit area. For 10 points, name this quantity usually symbolized rho, whose average value is given by mass divided by volume.
The commutator of the Hamiltonian and an object with this name is equal to i h-bar times the time derivative of that object, according to an equation named for von Neumann and Liouville.
density