in some fields of physics, such as plasma physics and the calculation of transport coefficients (conductivity, diffusivity, "etc."), the fields generated by the sources and the motion of the sources are solved self-consistently. in such cases, however, the motion of a selected source is calculated in response to fields generated by all other sources. rarely is the motion of a particle (source) due to the fields generated by that same particle calculated. the reason for this is twofold: