In order to solve the equations of electromagnetism given here, it is necessary to add information about how to calculate the electric current, J. Frequently, it is convenient to separate the current into two parts, the free current and the bound current, which are modeled by different equations; J ν ν = J ν ν free + J ν ν bound , where J ν ν free = ( c ρ ρ free , J free ) = ( c ∇ ∇ ⋅ ⋅ D , − − ∂ ∂ D ∂ ∂ t + ∇ ∇ × × H ) , J ν ν bound = ( c ρ ρ bound , J bound ) = ( − − c ∇ ∇ ⋅ ⋅ P , ∂ ∂ P ∂ ∂ t + ∇ ∇ × × M ) .