Symmetry of energy
Based on the principle of symmetry, we propose a universal expression of the energy of physical system. If we accept a generalized coordinate as an argument, we come to potential energy; if we accept a generalized velocity as an argument, we come to kinetic co-energy. Here, like in the variational principle, the concept of kinetic energy does not work. The examples of application relate to electromagnetic fields, electromagnetic circuits, and mechanics of concentrated masses.