The above definition should be modified for a system in an external electromagnetic field. In SI units, a charged particle of mass m and electric charge q includes a term due to the interaction with the electromagnetic field; j = 1 2 m [ ( Ψ Ψ ∗ ∗ p ^ ^ Ψ Ψ − − Ψ Ψ p ^ ^ Ψ Ψ ∗ ∗ ) − − 2 q A | Ψ Ψ | 2 ] where A = A (r, t) is the magnetic vector potential. The term q A has dimensions of momentum. Note that p ^ ^ = − − i ℏ ℏ ∇ ∇ used here is the canonical momentum and is not gauge invariant, unlike the kinetic momentum operator P ^ ^ = − − i ℏ ℏ ∇ ∇ − − q A .