The Gibbs-Johnson variational approach was applied to thermodynamic treatment of a charged liquid drop placed on a charged substrate. The line tension effect and the presence of charges on the three-phase contact line were taken into consideration. As a result a novel form of the generalized Young equation, containing an electric driving force at the three-phase contact line, was obtained. Application of this relationship to analysis of the electrowetting phenomenon enables us to conclude that the potential-induced change in the contact angle is the secondary electrocapillary effect, caused by redistribution of the charges on the three-phase contact line.