&Solar Physics BRussian Ksolar flares// We investigated some processes of stretching a magnetic loop with a longitudinal current. The results are applied to an analysis of the development of solar flares. Magnetic loops with a longitudinal current are put in conformity with loops of flares. An important factor is the magnitude of a current in an emerging magnetic loop. The magnetic energy of currents which varies with time includes mechanical work of the electrodynamical force, the Joule heating in a magnetic loop, and performing work by an exterior electromotive force. The electrodynamical force work refers to the kinetic flow energy of flare plasma and the Joule losses relate to the plasma thermal energy. The kinetic energy of the most powerful flare is 10²⁵ Joule. Less than 40 magnetic loops is required for the flare. If the plasma density in a magnetic loop is equal to 10¹⁷ m^-3, the Joule losses are capable to heat the plasma to a temperature of 1.5⋅10⁷ K. The electromotive force of the induction is 10⁷ V and the exterior electromotive force potential equals approximately 10⁶ V.