The Maxwell equations simplify when the charge density at each point in space does not change over time and all electric currents likewise remain constant. All of the time derivatives vanish from the equations, leaving two expressions that involve the electric field, ∇ ∇ ⋅ ⋅ E = ρ ρ ϵ ϵ 0 and ∇ ∇ × × E = 0 , along with two formulae that involve the magnetic field: ∇ ∇ ⋅ ⋅ B = 0 and ∇ ∇ × × B = μ μ 0 J . These expressions are the basic equations of electrostatics, which focuses on situations where electrical charges do not move, and magnetostatics, the corresponding area of magnetic phenomena.