2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/143089We discuss the relativistic transformation of the energy of a charged spherical capacitor. The energy stored in the electromagnetic fields observed by an uniformly moving frame is related to that of the rest frame by a factor different from $γ$. Considering the energy and the stresses in the capacitor shells we show that the total energy of the system satisfies the expected relativistic transformation.8 pages, 3 figuresClassical Physicstext