An electric charge of magnitude Q and mass m, in the form of a spherical shell of radius a, moving at time t with velocity v and acceleration dv/dt, generates a magnetic field round it and an electric field X proportional to and in the opposite direction of the acceleration. The field X acts on the self-same charge Q to produce a reactive or inertial force QX = -m(dv/dt), in accordance with Newton’s second and third laws of motion, where m is a constant. This explains the origin of inertia as electrical in nature and internal to a body, contrary to general relativity. An expression deduced for the mass m, in terms of square of Q and radius a, is compared with the electrostatic energy En of the charge to obtain En = ½ mc^2, in contrast to the mass-energy formula of special relativity, En = mc^2, where c the speed of light in a vacuum.
