Some problems of special relativity

The known problem of definition of the electromagnetic energy and momentum of charged particles and of a charge in a moving current-carrying conductor is discussed. A covariant solution of these problems is presented. The volume element has been chosen basing on the earlier introduced concept of a relativistic length. Equations for Lienard-Wiechert's potentials are also analysed from the point of view of this concept. The energy-momentum tensor for a system of charges and currents is considered. It is pointed to the inaccuracy in a well-known elementary derivation of the formula for equivalence of energy and mass