φ φ ( r , t ) = 1 4 π π ε ε 0 ∫ ∫ ρ ρ ( r ′ , t ) R d 3 r ′ A ( r , t ) = ∇ ∇ × × ∫ ∫ B ( r ′ , t ) 4 π π R d 3 r ′ where ρ (r, t) is the electric charge density, R = r − − r ′ and R = | R | (where r is any position vector in space and r ′ is a point in the charge or current distribution), the ∇ ∇ operates on r and d r is the volume element at r.