Traditionally, the theory related to the spatial angular momentum has been studied completely, while the investigation in the generator of Lorentz boost is inadequate. This paper shows that the generator of Lorentz boost has a nontrivial physical significance: it endows a charged system with an electric moment, and has an important significance for the electrical manipulations of electron spin in spintronics. An alteative treatment and interpretation for the traditional Darwin term and spin-orbit coupling are given.