In general, however, the electric field cannot be described independently of the magnetic field. Given the magnetic vector potential, A, defined so that ⁠ B = ∇ ∇ × × A ⁠, one can still define an electric potential φ φ such that: E = − − ∇ ∇ φ φ − − ∂ ∂ A ∂ ∂ t , where ∇ ∇ φ φ is the gradient of the electric potential and ∂ ∂ A ∂ ∂ t is the partial derivative of A with respect to time.