Electromagnetic fields are electric and magnetic fields, which may change with time, for instance when charges are in motion. Moving charges produce a magnetic field in accordance with Ampère's circuital law (with Maxwell's addition), which, along with Maxwell's other equations, defines the magnetic field, B , in terms of its curl: ∇ ∇ × × B = μ μ 0 ( J + ε ε 0 ∂ ∂ E ∂ ∂ t ) , where J is the current density, μ μ 0 is the vacuum permeability, and ε ε 0 is the vacuum permittivity.