The Heaviside–Feynman formula, also known as the Jefimenko–Feynman formula, can be seen as the point-like electric charge version of Jefimenko's equations. Actually, it can be (non trivially) deduced from them using Dirac functions, or using the Liénard-Wiechert potentials. It is mostly known from The Feynman Lectures on Physics, where it was used to introduce and describe the origin of electromagnetic radiation. The formula provides a natural generalization of the Coulomb's law for cases where the source charge is moving: E = − − q 4 π π ε ε 0 [ e r ′ r ′ 2 + r ′ c d d t ( e r ′ r ′ 2 ) + 1 c 2 d 2 d t 2 e r ′ ] B = − − e r ′ × × E c Here, E and B are the electric and magnetic fields respectively, q is the electric charge, ε ε 0 is the vacuum permittivity (electric field constant) and c is the speed of light. The vector e r ′ is a unit vector pointing from the observer to the charge and r ′ is the distance between observer and charge. Since the electromagnetic field propagates at the speed of light, both these quantities are evaluated at the retarded time t − − r ′ / c .